METHOD, APPARATUS, AND MEDIUM FOR VIDEO PROCESSING

Abstract

Embodiments of the disclosure provide a solution for video processing. A method for video processing is proposed. The method includes: determining, for a conversion between a video unit of a video and a bitstream of the video, that the video unit is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; and performing the conversion based on the motion pair.

Description

FIELDS

Embodiments of the present disclosure relates generally to video processing techniques, and more particularly, to local illumination compensation (LIC), advanced motion vector prediction (AMVP) MERGE, decoder side motion vector refinement (DMVR), out-of-boundary (OOB) and bi-prediction with coding unit (CU)-level weight (BCW) in image/video coding.

BACKGROUND

In nowadays, digital video capabilities are being applied in various aspects of peoples' lives. Multiple types of video compression technologies, such as MPEG-2, MPEG-4, ITU-TH.263, ITU-TH.264/MPEG-4 Part 10 Advanced Video Coding (AVC), ITU-TH.265 high efficiency video coding (HEVC) standard, versatile video coding (VVC) standard, have been proposed for video encoding/decoding. However, coding efficiency of video coding techniques is generally expected to be further improved.

SUMMARY

Embodiments of the present disclosure provide a solution for video processing.

In a first aspect, a method for video processing is proposed. The method comprises: determining, for a conversion between a video unit of a video and a bitstream of the video, that the video unit is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; and performing the conversion based on the motion pair. The method in accordance with the first aspect of the present disclosure improves coding efficiency, when compared with conventional technologies.

In a second aspect, another method for video processing is proposed. The method comprises: applying, for a conversion between a video unit of a video and a bitstream of the video, at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to the video unit to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; and performing the conversion based on the refined motion vector. The method in accordance with the first aspect of the present disclosure improves coding efficiency, when compared with conventional technologies.

In a third aspect, another method for video processing is proposed. The method comprises: deriving, during a conversion between a video unit of a video and a bitstream of the video, at least one local illumination compensation (LIC) model parameter of a LIC model for the video unit based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; and performing the conversion based on the updated LIC model. The method in accordance with the first aspect of the present disclosure improves coding efficiency, when compared with conventional technologies.

In a fourth aspect, another method for video processing is proposed. The method comprises: applying, for a conversion between a video unit of a video and a bitstream of the video, a non-average weighting approach to the video unit, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; and performing the conversion based on the weighted video unit. The method in accordance with the first aspect of the present disclosure improves coding efficiency, when compared with conventional technologies.

In a fifth aspect, an apparatus for video processing is proposed. The apparatus comprises a processor and a non-transitory memory with instructions thereon. The instructions upon execution by the processor, cause the processor to perform a method in accordance with any one of: the first, second, third, or fourth aspect of the present disclosure.

In a sixth aspect, a non-transitory computer-readable storage medium is proposed. The non-transitory computer-readable storage medium stores instructions that cause a processor to perform a method in accordance with any one of: the first, second, third, or fourth aspect of the present disclosure.

In a seventh aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: determining that a video unit of the video is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; and generating a bitstream of the video unit based on the motion pair.

In an eighth aspect, a method for storing a bitstream of a video is proposed. The method comprises: determining that a video unit of the video is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; generating a bitstream of the video unit based on the motion pair; and storing the bitstream in a non-transitory computer-readable recording medium.

In a ninth aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: applying at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to a video unit of the video to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; and generating a bitstream of the video unit based on the refined motion vector.

In a tenth aspect, a method for storing a bitstream of a video is proposed. The method comprises: applying at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to a video unit of the video to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; generating a bitstream of the video unit based on the refined motion vector; and storing the bitstream in a non-transitory computer-readable recording medium.

In an eleventh aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: deriving at least one local illumination compensation (LIC) model parameter of a LIC model for a video unit of the video based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; and generating a bitstream of the video unit based on the updated LIC model.

In a twelfth aspect, a method for storing a bitstream of a video is proposed. The method comprises: deriving at least one local illumination compensation (LIC) model parameter of a LIC model for a video unit of the video based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; generating a bitstream of the video unit based on the updated LIC model; and storing the bitstream in a non-transitory computer-readable recording medium.

In a thirteenth aspect, another non-transitory computer-readable recording medium is proposed. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: applying a non-average weighting approach to a video unit of the video, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; and generating a bitstream of the video unit based on the weighted video unit.

In a fourteenth aspect, a method for storing a bitstream of a video is proposed. The method comprises: applying a non-average weighting approach to a video unit of the video, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; generating a bitstream of the video unit based on the weighted video unit; and storing the bitstream in a non-transitory computer-readable recording medium.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Through the following detailed description with reference to the accompanying drawings, the above and other objectives, features, and advantages of example embodiments of the present disclosure will become more apparent. In the example embodiments of the present disclosure, the same reference numerals usually refer to the same components.

FIG. 1 illustrates a block diagram that illustrates an example video coding system, in accordance with some embodiments of the present disclosure;

FIG. 2 illustrates a block diagram that illustrates a first example video encoder, in accordance with some embodiments of the present disclosure;

FIG. 3 illustrates a block diagram that illustrates an example video decoder, in accordance with some embodiments of the present disclosure;

FIG. 4 illustrates positions of spatial merge candidate;

FIG. 5 illustrates candidate pairs considered for redundancy check of spatial merge candidates;

FIG. 6 is an illustration of motion vector scaling for temporal merge candidate;

FIG. 7 shows candidate positions for temporal merge candidate, C₀ and C₁;

FIG. 8 shows MMVD search point;

FIG. 9 shows extended CU region used in BDOF;

FIG. 10 is an illustration for symmetrical MVD mode;

FIG. 11 shows a control point based affine motion model;

FIG. 12 shows an affine MVF per subblock;

FIG. 13 illustrates locations of inherited affine motion predictors;

FIG. 14 shows control point motion vector inheritance;

FIG. 15 shows locations of candidates position for constructed affine merge mode;

FIG. 16 is an illustration of motion vector usage for proposed combined method;

FIG. 17 shows Subblock MV VSB and pixel Δv(i,j);

FIGS. 18a and 18b illustrate the SbTMVP process in VVC, where FIG. 18a illustrates spatial neighboring blocks used by SbTMVP and FIG. 18b illustrates deriving sub-CU motion field by applying a motion shift from spatial neighbor and scaling the motion information from the corresponding collocated sub-CUs;

FIG. 19 shows an extended CU region used in BDOF;

FIG. 20 shows decoding side motion vector refinement;

FIG. 21 shows top and left neighboring blocks used in CIIP weight derivation;

FIG. 22 shows examples of the GPM splits grouped by identical angles;

FIG. 23 shows uni-prediction MV selection for geometric partitioning mode;

FIG. 24 illustrates exemplified generation of a bending weight w₀ using geometric partitioning mode;

FIG. 25 shows spatial neighboring blocks used to derive the spatial merge candidates;

FIG. 26 shows template matching performs on a search area around initial MV;

FIG. 27 shows diamond regions in the search area;

FIG. 28 shows frequency response of the interpolation filter and the VVC interpolation filter at half-pel phase;

FIG. 29 shows template and reference samples of the template in reference pictures;

FIG. 30 shows template and reference samples of the template for block with sub-block motion using the motion information of the subblocks of the current block;

FIG. 31 illustrates a bi-directional MC block in current ECM;

FIG. 32 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure;

FIG. 33 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure;

FIG. 34 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure;

FIG. 35 illustrates a flowchart of a method for video processing in accordance with embodiments of the present disclosure; and

FIG. 36 illustrates a block diagram of a computing device in which various embodiments of the present disclosure can be implemented.

Throughout the drawings, the same or similar reference numerals usually refer to the same or similar elements.

DETAILED DESCRIPTION

Principle of the present disclosure will now be described with reference to some embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. The disclosure described herein can be implemented in various manners other than the ones described below.

In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

References in the present disclosure to “one embodiment,” “an embodiment,” “an example embodiment,” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an example embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

It shall be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the listed terms.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and/or “including”, when used herein, specify the presence of stated features, elements, and/or components etc., but do not preclude the presence or addition of one or more other features, elements, components and/or combinations thereof.

Example Environment

FIG. 1 is a block diagram that illustrates an example video coding system 100 that may utilize the techniques of this disclosure. As shown, the video coding system 100 may include a source device 110 and a destination device 120. The source device 110 can be also referred to as a video encoding device, and the destination device 120 can be also referred to as a video decoding device. In operation, the source device 110 can be configured to generate encoded video data and the destination device 120 can be configured to decode the encoded video data generated by the source device 110. The source device 110 may include a video source 112, a video encoder 114, and an input/output (I/O) interface 116.

The video source 112 may include a source such as a video capture device. Examples of the video capture device include, but are not limited to, an interface to receive video data from a video content provider, a computer graphics system for generating video data, and/or a combination thereof.

The video data may comprise one or more pictures. The video encoder 114 encodes the video data from the video source 112 to generate a bitstream. The bitstream may include a sequence of bits that form a coded representation of the video data. The bitstream may include coded pictures and associated data. The coded picture is a coded representation of a picture. The associated data may include sequence parameter sets, picture parameter sets, and other syntax structures. The I/O interface 116 may include a modulator/demodulator and/or a transmitter. The encoded video data may be transmitted directly to destination device 120 via the I/O interface 116 through the network 130A. The encoded video data may also be stored onto a storage medium/server 130B for access by destination device 120.

The destination device 120 may include an I/O interface 126, a video decoder 124, and a display device 122. The I/O interface 126 may include a receiver and/or a modem. The I/O interface 126 may acquire encoded video data from the source device 110 or the storage medium/server 130B. The video decoder 124 may decode the encoded video data. The display device 122 may display the decoded video data to a user. The display device 122 may be integrated with the destination device 120, or may be external to the destination device 120 which is configured to interface with an external display device.

The video encoder 114 and the video decoder 124 may operate according to a video compression standard, such as the High Efficiency Video Coding (HEVC) standard, Versatile Video Coding (VVC) standard and other current and/or further standards.

FIG. 2 is a block diagram illustrating an example of a video encoder 200, which may be an example of the video encoder 114 in the system 100 illustrated in FIG. 1, in accordance with some embodiments of the present disclosure.

The video encoder 200 may be configured to implement any or all of the techniques of this disclosure. In the example of FIG. 2, the video encoder 200 includes a plurality of functional components. The techniques described in this disclosure may be shared among the various components of the video encoder 200. In some examples, a processor may be configured to perform any or all of the techniques described in this disclosure.

In some embodiments, the video encoder 200 may include a partition unit 201, a predication unit 202 which may include a mode select unit 203, a motion estimation unit 204, a motion compensation unit 205 and an intra-prediction unit 206, a residual generation unit 207, a transform unit 208, a quantization unit 209, an inverse quantization unit 210, an inverse transform unit 211, a reconstruction unit 212, a buffer 213, and an entropy encoding unit 214.

In other examples, the video encoder 200 may include more, fewer, or different functional components. In an example, the predication unit 202 may include an intra block copy (IBC) unit. The IBC unit may perform predication in an IBC mode in which at least one reference picture is a picture where the current video block is located.

Furthermore, although some components, such as the motion estimation unit 204 and the motion compensation unit 205, may be integrated, but are represented in the example of FIG. 2 separately for purposes of explanation.

The partition unit 201 may partition a picture into one or more video blocks. The video encoder 200 and the video decoder 300 may support various video block sizes.

The mode select unit 203 may select one of the coding modes, intra or inter, e.g., based on error results, and provide the resulting intra-coded or inter-coded block to a residual generation unit 207 to generate residual block data and to a reconstruction unit 212 to reconstruct the encoded block for use as a reference picture. In some examples, the mode select unit 203 may select a combination of intra and inter predication (CIIP) mode in which the predication is based on an inter predication signal and an intra predication signal. The mode select unit 203 may also select a resolution for a motion vector (e.g., a sub-pixel or integer pixel precision) for the block in the case of inter-predication.

To perform inter prediction on a current video block, the motion estimation unit 204 may generate motion information for the current video block by comparing one or more reference frames from buffer 213 to the current video block. The motion compensation unit 205 may determine a predicted video block for the current video block based on the motion information and decoded samples of pictures from the buffer 213 other than the picture associated with the current video block.

The motion estimation unit 204 and the motion compensation unit 205 may perform different operations for a current video block, for example, depending on whether the current video block is in an I-slice, a P-slice, or a B-slice. As used herein, an “I-slice” may refer to a portion of a picture composed of macroblocks, all of which are based upon macroblocks within the same picture. Further, as used herein, in some aspects, “P-slices” and “B-slices” may refer to portions of a picture composed of macroblocks that are not dependent on macroblocks in the same picture.

In some examples, the motion estimation unit 204 may perform uni-directional prediction for the current video block, and the motion estimation unit 204 may search reference pictures of list 0 or list 1 for a reference video block for the current video block. The motion estimation unit 204 may then generate a reference index that indicates the reference picture in list 0 or list 1 that contains the reference video block and a motion vector that indicates a spatial displacement between the current video block and the reference video block. The motion estimation unit 204 may output the reference index, a prediction direction indicator, and the motion vector as the motion information of the current video block. The motion compensation unit 205 may generate the predicted video block of the current video block based on the reference video block indicated by the motion information of the current video block.

Alternatively, in other examples, the motion estimation unit 204 may perform bi-directional prediction for the current video block. The motion estimation unit 204 may search the reference pictures in list 0 for a reference video block for the current video block and may also search the reference pictures in list 1 for another reference video block for the current video block. The motion estimation unit 204 may then generate reference indexes that indicate the reference pictures in list 0 and list 1 containing the reference video blocks and motion vectors that indicate spatial displacements between the reference video blocks and the current video block. The motion estimation unit 204 may output the reference indexes and the motion vectors of the current video block as the motion information of the current video block. The motion compensation unit 205 may generate the predicted video block of the current video block based on the reference video blocks indicated by the motion information of the current video block.

In some examples, the motion estimation unit 204 may output a full set of motion information for decoding processing of a decoder. Alternatively, in some embodiments, the motion estimation unit 204 may signal the motion information of the current video block with reference to the motion information of another video block. For example, the motion estimation unit 204 may determine that the motion information of the current video block is sufficiently similar to the motion information of a neighboring video block.

In one example, the motion estimation unit 204 may indicate, in a syntax structure associated with the current video block, a value that indicates to the video decoder 300 that the current video block has the same motion information as the another video block.

In another example, the motion estimation unit 204 may identify, in a syntax structure associated with the current video block, another video block and a motion vector difference (MVD). The motion vector difference indicates a difference between the motion vector of the current video block and the motion vector of the indicated video block. The video decoder 300 may use the motion vector of the indicated video block and the motion vector difference to determine the motion vector of the current video block.

As discussed above, video encoder 200 may predictively signal the motion vector. Two examples of predictive signaling techniques that may be implemented by video encoder 200 include advanced motion vector predication (AMVP) and merge mode signaling.

The intra prediction unit 206 may perform intra prediction on the current video block. When the intra prediction unit 206 performs intra prediction on the current video block, the intra prediction unit 206 may generate prediction data for the current video block based on decoded samples of other video blocks in the same picture. The prediction data for the current video block may include a predicted video block and various syntax elements.

The residual generation unit 207 may generate residual data for the current video block by subtracting (e.g., indicated by the minus sign) the predicted video block(s) of the current video block from the current video block. The residual data of the current video block may include residual video blocks that correspond to different sample components of the samples in the current video block.

In other examples, there may be no residual data for the current video block for the current video block, for example in a skip mode, and the residual generation unit 207 may not perform the subtracting operation.

The transform processing unit 208 may generate one or more transform coefficient video blocks for the current video block by applying one or more transforms to a residual video block associated with the current video block.

After the transform processing unit 208 generates a transform coefficient video block associated with the current video block, the quantization unit 209 may quantize the transform coefficient video block associated with the current video block based on one or more quantization parameter (QP) values associated with the current video block.

The inverse quantization unit 210 and the inverse transform unit 211 may apply inverse quantization and inverse transforms to the transform coefficient video block, respectively, to reconstruct a residual video block from the transform coefficient video block. The reconstruction unit 212 may add the reconstructed residual video block to corresponding samples from one or more predicted video blocks generated by the predication unit 202 to produce a reconstructed video block associated with the current video block for storage in the buffer 213.

After the reconstruction unit 212 reconstructs the video block, loop filtering operation may be performed to reduce video blocking artifacts in the video block.

The entropy encoding unit 214 may receive data from other functional components of the video encoder 200. When the entropy encoding unit 214 receives the data, the entropy encoding unit 214 may perform one or more entropy encoding operations to generate entropy encoded data and output a bitstream that includes the entropy encoded data.

FIG. 3 is a block diagram illustrating an example of a video decoder 300, which may be an example of the video decoder 124 in the system 100 illustrated in FIG. 1, in accordance with some embodiments of the present disclosure.

The video decoder 300 may be configured to perform any or all of the techniques of this disclosure. In the example of FIG. 3, the video decoder 300 includes a plurality of functional components. The techniques described in this disclosure may be shared among the various components of the video decoder 300. In some examples, a processor may be configured to perform any or all of the techniques described in this disclosure.

In the example of FIG. 3, the video decoder 300 includes an entropy decoding unit 301, a motion compensation unit 302, an intra prediction unit 303, an inverse quantization unit 304, an inverse transformation unit 305, and a reconstruction unit 306 and a buffer 307. The video decoder 300 may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder 200.

The entropy decoding unit 301 may retrieve an encoded bitstream. The encoded bitstream may include entropy coded video data (e.g., encoded blocks of video data). The entropy decoding unit 301 may decode the entropy coded video data, and from the entropy decoded video data, the motion compensation unit 302 may determine motion information including motion vectors, motion vector precision, reference picture list indexes, and other motion information. The motion compensation unit 302 may, for example, determine such information by performing the AMVP and merge mode. AMVP is used, including derivation of several most probable candidates based on data from adjacent PBs and the reference picture. Motion information typically includes the horizontal and vertical motion vector displacement values, one or two reference picture indices, and, in the case of prediction regions in B slices, an identification of which reference picture list is associated with each index. As used herein, in some aspects, a “merge mode” may refer to deriving the motion information from spatially or temporally neighboring blocks.

The motion compensation unit 302 may produce motion compensated blocks, possibly performing interpolation based on interpolation filters. Identifiers for interpolation filters to be used with sub-pixel precision may be included in the syntax elements.

The motion compensation unit 302 may use the interpolation filters as used by the video encoder 200 during encoding of the video block to calculate interpolated values for sub-integer pixels of a reference block. The motion compensation unit 302 may determine the interpolation filters used by the video encoder 200 according to the received syntax information and use the interpolation filters to produce predictive blocks.

The motion compensation unit 302 may use at least part of the syntax information to determine sizes of blocks used to encode frame(s) and/or slice(s) of the encoded video sequence, partition information that describes how each macroblock of a picture of the encoded video sequence is partitioned, modes indicating how each partition is encoded, one or more reference frames (and reference frame lists) for each inter-encoded block, and other information to decode the encoded video sequence. As used herein, in some aspects, a “slice” may refer to a data structure that can be decoded independently from other slices of the same picture, in terms of entropy coding, signal prediction, and residual signal reconstruction. A slice can either be an entire picture or a region of a picture.

The intra prediction unit 303 may use intra prediction modes for example received in the bitstream to form a prediction block from spatially adjacent blocks. The inverse quantization unit 304 inverse quantizes, i.e., de-quantizes, the quantized video block coefficients provided in the bitstream and decoded by entropy decoding unit 301. The inverse transform unit 305 applies an inverse transform.

The reconstruction unit 306 may obtain the decoded blocks, e.g., by summing the residual blocks with the corresponding prediction blocks generated by the motion compensation unit 302 or intra-prediction unit 303. If desired, a deblocking filter may also be applied to filter the decoded blocks in order to remove blockiness artifacts. The decoded video blocks are then stored in the buffer 307, which provides reference blocks for subsequent motion compensation/intra predication and also produces decoded video for presentation on a display device.

Some exemplary embodiments of the present disclosure will be described in detailed hereinafter. It should be understood that section headings are used in the present document to facilitate ease of understanding and do not limit the embodiments disclosed in a section to only that section. Furthermore, while certain embodiments are described with reference to Versatile Video Coding or other specific video codecs, the disclosed techniques are applicable to other video coding technologies also. Furthermore, while some embodiments describe video coding steps in detail, it will be understood that corresponding steps decoding that undo the coding will be implemented by a decoder. Furthermore, the term video processing encompasses video coding or compression, video decoding or decompression and video transcoding in which video pixels are represented from one compressed format into another compressed format or at a different compressed bitrate.

1. Brief Summary

The present disclosure is related to video coding technologies. Specifically, it is about DMVR/BDOF based enhancements in image/video coding. It may be applied to the existing video coding standard like HEVC, VVC, and etc. It may be also applicable to future video coding standards or video codec.

2. Introduction

Video coding standards have evolved primarily through the development of the well-known ITU-T and ISO/IEC standards. The ITU-T produced H.261 and H.263, ISO/IEC produced MPEG-1 and MPEG-4 Visual, and the two organizations jointly produced the H.262/MPEG-2 Video and H.264/MPEG-4 Advanced Video Coding (AVC) and H.265/HEVC standards. Since H.262, the video coding standards are based on the hybrid video coding structure wherein temporal prediction plus transform coding are utilized. To explore the future video coding technologies beyond HEVC, the Joint Video Exploration Team (JVET) was founded by VCEG and MPEG jointly in 2015. The JVET meeting is concurrently held once every quarter, and the new video coding standard was officially named as Versatile Video Coding (VVC) in the April 2018 JVET meeting, and the first version of VVC test model (VTM) was released at that time. The VVC working draft and test model VTM are then updated after every meeting. The VVC project achieved technical completion (FDIS) at the July 2020 meeting.

2.1. Existing Inter Prediction Coding Tools

For each inter-predicted CU, motion parameters consisting of motion vectors, reference picture indices and reference picture list usage index, and additional information needed for the new coding feature of VVC to be used for inter-predicted sample generation. The motion parameter can be signalled in an explicit or implicit manner. When a CU is coded with skip mode, the CU is associated with one PU and has no significant residual coefficients, no coded motion vector delta or reference picture index. A merge mode is specified whereby the motion parameters for the current CU are obtained from neighbouring CUs, including spatial and temporal candidates, and additional schedules introduced in VVC. The merge mode can be applied to any inter-predicted CU, not only for skip mode. The alternative to merge mode is the explicit transmission of motion parameters, where motion vector, corresponding reference picture index for each reference picture list and reference picture list usage flag and other needed information are signalled explicitly per each CU.

Beyond the inter coding features in HEVC, VVC includes a number of new and refined inter prediction coding tools listed as follows:

• • Extended merge prediction;
  • Merge mode with MVD (MMVD);
  • Symmetric MVD (SMVD) signalling;
  • Affine motion compensated prediction;
  • Subblock-based temporal motion vector prediction (SbTMVP);
  • Adaptive motion vector resolution (AMVR);
  • Motion field storage: 1/16^th luma sample MV storage and 8×8 motion field compression;
  • Bi-prediction with CU-level weight (BCW);
  • Bi-directional optical flow (BDOF);
  • Decoder side motion vector refinement (DMVR);
  • Geometric partitioning mode (GPM);
  • Combined inter and intra prediction (CIIP).

The following text provides the details on those inter prediction methods specified in VVC.

2.1.1. Extended Merge Prediction

In VVC, the merge candidate list is constructed by including the following five types of candidates in order:

• • 1) Spatial MVP from spatial neighbour CUS;
  • 2) Temporal MVP from collocated CUs;
  • 3) History-based MVP from an FIFO table;
  • 4) Pairwise average MVP;
  • 5) Zero MVs.

The size of merge list is signalled in sequence parameter set header and the maximum allowed size of merge list is 6. For each CU code in merge mode, an index of best merge candidate is encoded using truncated unary binarization (TU). The first bin of the merge index is coded with context and bypass coding is used for other bins.

The derivation process of each category of merge candidates is provided in this session. As done in HEVC, VVC also supports parallel derivation of the merging candidate lists for all CUs within a certain size of area.

2.1.1.1. Spatial Candidates Derivation

The derivation of spatial merge candidates in VVC is same to that in HEVC except the positions of first two merge candidates are swapped. FIG. 4 is a schematic diagram 400 illustrating positions of a spatial merge candidate. A maximum of four merge candidates are selected among candidates located in the positions depicted in FIG. 4. The order of derivation is B₀, A₀, B₁, A₁ and B₂. Position B₂ is considered only when one or more than one CUs of position B₀, A₀, B₁, A₁ are not available (e.g. because it belongs to another slice or tile) or is intra coded. After candidate at position A₁ is added, the addition of the remaining candidates is subject to a redundancy check which ensures that candidates with same motion information are excluded from the list so that coding efficiency is improved. To reduce computational complexity, not all possible candidate pairs are considered in the mentioned redundancy check. FIG. 5 is a schematic diagram 500 illustrating candidate pairs considered for redundancy check of spatial merge candidates. Instead only the pairs linked with an arrow in FIG. 5 are considered and a candidate is only added to the list if the corresponding candidate used for redundancy check has not the same motion information.

2.1.1.2. Temporal Candidates Derivation

In this step, only one candidate is added to the list. Particularly, in the derivation of this temporal merge candidate, a scaled motion vector is derived based on co-located CU belong-ing to the collocated reference picture. The reference picture list to be used for derivation of the co-located CU is explicitly signalled in the slice header. The scaled motion vector for temporal merge candidate is obtained as illustrated by the dotted line in the diagram 600 of FIG. 6, which is scaled from the motion vector of the co-located CU using the POC distances, tb and td, where tb is defined to be the POC difference between the reference picture of the current picture and the current picture and td is defined to be the POC difference between the reference picture of the co-located picture and the co-located picture. The reference pic-ture index of temporal merge candidate is set equal to zero.

FIG. 7 is a schematic diagram 700 illustrating candidate positions for temporal merge candi-date, C0 and C1. The position for the temporal candidate is selected between candidates C0 and C1, as depicted in FIG. 7. If CU at position C0 is not available, is intra coded, or is out-side of the current row of CTUs, position C1 is used. Otherwise, position C0 is used in the derivation of the temporal merge candidate.

2.1.1.3. History-Based Merge Candidates Derivation

The history-based MVP (HMVP) merge candidates are added to merge list after the spatial MVP and TMVP. In this method, the motion information of a previously coded block is stored in a table and used as MVP for the current CU. The table with multiple HMVP candidates is maintained during the encoding/decoding process. The table is reset (emptied) when a new CTU row is encountered. Whenever there is a non-subblock inter-coded CU, the associated motion information is added to the last entry of the table as a new HMVP candidate.

The HMVP table size S is set to be 6, which indicates up to 6 History-based MVP (HMVP) candidates may be added to the table. When inserting a new motion candidate to the table, a constrained first-in-first-out (FIFO) rule is utilized wherein redundancy check is firstly applied to find whether there is an identical HMVP in the table. If found, the identical HMVP is removed from the table and all the HMVP candidates afterwards are moved forward.

HMVP candidates could be used in the merge candidate list construction process. The latest several HMVP candidates in the table are checked in order and inserted to the candidate list after the TMVP candidate. Redundancy check is applied on the HMVP candidates to the spatial or temporal merge candidate.

To reduce the number of redundancy check operations, the following simplifications are introduced:

• • 1. Number of HMPV candidates is used for merge list generation is set as (N<=4)? M: (8−N), wherein N indicates number of existing candidates in the merge list and M indicates number of available HMVP candidates in the table.
  • 2. Once the total number of available merge candidates reaches the maximally allowed merge candidates minus 1, the merge candidate list construction process from HMVP is terminated.

2.1.1.4. Pair-Wise Average Merge Candidates Derivation

Pairwise average candidates are generated by averaging predefined pairs of candidates in the existing merge candidate list, and the predefined pairs are defined as {(0, 1), (0, 2), (1, 2), (0, 3), (1, 3), (2, 3)}, where the numbers denote the merge indices to the merge candidate list. The averaged motion vectors are calculated separately for each reference list. If both motion vectors are available in one list, these two motion vectors are averaged even when they point to different reference pictures; if only one motion vector is available, use the one directly; if no motion vector is available, keep this list invalid.

When the merge list is not full after pair-wise average merge candidates are added, the zero MVPs are inserted in the end until the maximum merge candidate number is encountered.

2.1.1.5. Merge Estimation Region

Merge estimation region (MER) allows independent derivation of merge candidate list for the CUs in the same merge estimation region (MER). A candidate block that is within the same MER to the current CU is not included for the generation of the merge candidate list of the current CU. In addition, the updating process for the history-based motion vector predictor candidate list is updated only if (xCb+cbWidth)>>Log 2ParMrgLevel is greater than xCb>>Log 2ParMrgLevel and (yCb+cbHeight)>>Log 2ParMrgLevel is great than (yCb>>Log 2ParMrgLevel) and where (xCb, yCb) is the top-left luma sample position of the current CU in the picture and (cbWidth, cbHeight) is the CU size. The MER size is selected at encoder side and signalled as log 2_parallel_merge_level_minus2 in the sequence parameter set.

2.1.2. Merge Mode with MVD (MMVD)

In addition to merge mode, where the implicitly derived motion information is directly used for prediction samples generation of the current CU, the merge mode with motion vector differences (MMVD) is introduced in VVC. A MMVD flag is signalled right after sending a skip flag and merge flag to specify whether MMVD mode is used for a CU.

In MMVD, after a merge candidate is selected, it is further refined by the signalled MVDs information. The further information includes a merge candidate flag, an index to specify motion magnitude, and an index for indication of motion direction. In MMVD mode, one for the first two candidates in the merge list is selected to be used as MV basis. The merge candidate flag is signalled to specify which one is used.

Distance index specifies motion magnitude information and indicate the pre-defined offset from the starting point. As shown in FIG. 8, an offset is added to either horizontal component or vertical component of starting MV. The relation of distance index and pre-defined offset is specified in Table 1.

TABLE 1
The relation of distance index and pre-defined offset
Distance IDX	0	1	2	3	4	5	6	7
Offset (in unit of	¼	½	1	2	4	8	16	32
luma sample)

Direction index represents the direction of the MVD relative to the starting point. The direction index can represent of the four directions as shown in Table 2. It's noted that the meaning of MVD sign could be variant according to the information of starting MVs. When the starting MVs is an un-prediction MV or bi-prediction MVs with both lists point to the same side of the current picture (i.e. POCs of two references are both larger than the POC of the current picture, or are both smaller than the POC of the current picture), the sign Table 1 specifies the sign of MV offset added to the starting MV. When the starting MVs is bi-prediction MVs with the two MVs point to the different sides of the current picture (i.e. the POC of one reference is larger than the POC of the current picture, and the POC of the other reference is smaller than the POC of the current picture), the sign in Table 2 specifies the sign of MV offset added to the list0 MV component of starting MV and the sign for the list1 MV has opposite value.

TABLE 2
Sign of MV offset specified by direction index
Direction IDX	00	01	10	11
x-axis	+	−	N/A	N/A
y-axis	N/A	N/A	+	−

2.1.2.1. Bi-Prediction with CU-Level Weight (BCW)

In HEVC, the bi-prediction signal is generated by averaging two prediction signals obtained from two different reference pictures and/or using two different motion vectors. In VVC, the bi-prediction mode is extended beyond simple averaging to allow weighted averaging of the two prediction signals:

$\begin{array}{cc}{P}_{\mathrm{bi}-\mathrm{pred}}=\left(\left(8-w\right)*P_0+w*P_1+4\right)3.& \left(2-1\right)\end{array}$

Five weights are allowed in the weighted averaging bi-prediction, w∈{−2, 3, 4, 5, 10}. For each bi-predicted CU, the weight w is determined in one of two ways: 1) for a non-merge CU, the weight index is signalled after the motion vector difference; 2) for a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. BCW is only applied to CUs with 256 or more luma samples (i.e., CU width times CU height is greater than or equal to 256). For low-delay pictures, all 5 weights are used. For non-low-delay pictures, only 3 weights (w={3, 4, 5}) are used.

• • At the encoder, fast search algorithms are applied to find the weight index without significantly increasing the encoder complexity. These algorithms are summarized as follows. When combined with AMVR, unequal weights are only conditionally checked for 1-pel and 4-pel motion vector precisions if the current picture is a low-delay picture.
  • When combined with affine, affine ME will be performed for unequal weights if and only if the affine mode is selected as the current best mode.
  • When the two reference pictures in bi-prediction are the same, unequal weights are only conditionally checked.
  • Unequal weights are not searched when certain conditions are met, depending on the POC distance between current picture and its reference pictures, the coding QP, and the temporal level.

The BCW weight index is coded using one context coded bin followed by bypass coded bins. The first context coded bin indicates if equal weight is used; and if unequal weight is used, additional bins are signalled using bypass coding to indicate which unequal weight is used.

Weighted prediction (WP) is a coding tool supported by the H.264/AVC and HEVC standards to efficiently code video content with fading. Support for WP was also added into the VVC standard. WP allows weighting parameters (weight and offset) to be signalled for each reference picture in each of the reference picture lists L0 and L1. Then, during motion compensation, the weight(s) and offset(s) of the corresponding reference picture(s) are applied. WP and BCW are designed for different types of video content. In order to avoid interactions between WP and BCW, which will complicate VVC decoder design, if a CU uses WP, then the BCW weight index is not signalled, and w is inferred to be 4 (i.e. equal weight is applied). For a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. This can be applied to both normal merge mode and inherited affine merge mode. For constructed affine merge mode, the affine motion information is constructed based on the motion information of up to 3 blocks. The BCW index for a CU using the constructed affine merge mode is simply set equal to the BCW index of the first control point MV.

In VVC, CIIP and BCW cannot be jointly applied for a CU. When a CU is coded with CHIP mode, the BCW index of the current CU is set to 2, e.g. equal weight.

2.1.2.2. Bi-Directional Optical Flow (BDOF)

The bi-directional optical flow (BDOF) tool is included in VVC. BDOF, previously referred to as BIO, was included in the JEM. Compared to the JEM version, the BDOF in VVC is a simpler version that requires much less computation, especially in terms of number of multiplications and the size of the multiplier.

BDOF is used to refine the bi-prediction signal of a CU at the 4×4 subblock level. BDOF is applied to a CU if it satisfies all the following conditions:

• • The CU is coded using “true” bi-prediction mode, i.e., one of the two reference pictures is prior to the current picture in display order and the other is after the current picture in display order;
  • The distances (i.e. POC difference) from two reference pictures to the current picture are same;
  • Both reference pictures are short-term reference pictures;
  • The CU is not coded using affine mode or the ATMVP merge mode;
  • CU has more than 64 luma samples;
  • Both CU height and CU width are larger than or equal to 8 luma samples;
  • BCW weight index indicates equal weight;
  • WP is not enabled for the current CU;
  • CIIP mode is not used for the current CU.

BDOF is only applied to the luma component. As its name indicates, the BDOF mode is based on the optical flow concept, which assumes that the motion of an object is smooth. For each 4×4 subblock, a motion refinement (v_x, v_y) is calculated by minimizing the difference between the L0 and L1 prediction samples. The motion refinement is then used to adjust the bi-predicted sample values in the 4×4 subblock. The following steps are applied in the BDOF process.

First, the horizontal and vertical gradients,

$\frac{\partial I^{\left(k\right)}}{\partial x}\left(i,j\right)\mathrm{and}\frac{\partial I^{\left(k\right)}}{\partial y}\left(i,j\right),$

k=0,1, of the two prediction signals are computed by directly calculating the difference between two neighboring samples, i.e.,

$\begin{array}{cc}\frac{\partial I^{\left(k\right)}}{\partial x}\left(i,j\right)=\left(\left(I^{\left(k\right)}\left(i+1,j\right)\right)\mathrm{shift}1\right)-\left(I^{\left(k\right)}\left(i-1,j\right)\right)\mathrm{shift}1))\frac{\partial I^{\left(k\right)}}{\partial y}\left(i,j\right)=\left(\left(I^{\left(k\right)}\left(i,j+1\right)\right)\mathrm{shift}1\right)-\left(I^{\left(k\right)}\left(i,j-1\right)\right)\mathrm{shift}1))& \left(2-2\right)\end{array}$

where I^(k) (i, j) are the sample value at coordinate (i, j) of the prediction signal in list k, k=0,1, and shift1 is calculated based on the luma bit depth, bitDepth, as shift1=max(6, bitDepth−6).

Then, the auto- and cross-correlation of the gradients, S₁, S₂, S₃, S₅ and S₆, are calculated as

$\begin{array}{cc}{S}_1={\sum }_{\left(i,j\right)\in \Omega }\mathrm{Abs}\left({\psi }_x\left(i,j\right)\right),S_3={\sum }_{\left(i,j\right)\in \Omega }\theta \left(i,j\right)·\mathrm{Sign}\left({\psi }_x\left(i,j\right)\right)S_2=\sum _{\left(i,j\right)\in \Omega }{\psi }_x\left(i,j\right)·\mathrm{Sign}\left({\psi }_y\left(i,j\right)\right)& \left(2-3\right)\end{array}$ $\begin{array}{cc}{S}_5={\sum }_{\left(i,j\right)\in \Omega }\mathrm{Abs}\left({\psi }_y\left(i,j\right)\right),S_6={\sum }_{\left(i,j\right)\in \Omega }\theta \left(i,j\right)·\mathrm{Sign}\left({\psi }_y\left(i,j\right)\right)\mathrm{where}{\psi }_x\left(i,j\right)=\left(\frac{\partial I^{\left(1\right)}}{\partial x}\left(i,j\right)+\frac{\partial I^{\left(0\right)}}{\partial x}\left(i,j\right)\right)n_a{\psi }_y\left(i,j\right)=\left(\frac{\partial I^{\left(1\right)}}{\partial y}\left(i,j\right)+\frac{\partial I^{\left(0\right)}}{\partial y}\left(i,j\right)\right)n_a\theta \left(i,j\right)=(I^{\left(1\right)}\left(i,j\right)n_b)-(I^{\left(0\right)}\left(i,j\right)n_b)& \left(2-4\right)\end{array}$

where Ω is a 6×6 window around the 4×4 subblock, and the values of n_a and n_b are set equal to min (1, bitDepth−11) and min (4, bitDepth−8), respectively.

The motion refinement (v_x, v_y) is then derived using the cross- and auto-correlation terms using the following:

$\begin{array}{cc}{v}_x=S_1>0?\mathrm{clip}3\left(-{\mathrm{th}}_{\mathrm{BIO}}^{\prime },{\mathrm{th}}_{\mathrm{BIO}}^{\prime },-\left(\left(S_3·2^{n_b-n_{\alpha }}\right)\gg \lfloor {\log }_2{S}_1\rfloor \right)\right):0& \left(2-5\right)\end{array}$ $v_y=S_5>0?\mathrm{clip}3\left(-{\mathrm{th}}_{\mathrm{BIO}}^{\prime },{\mathrm{th}}_{\mathrm{BIO}}^{\prime },-\left(\left(S_6·2^{n_b-n_a}-\left(\left(v_x{S}_{2,m}\right)\ll n_{S_2}+v_x{S}_{2,s}\right)/2\right)\gg \lfloor {\log }_2{S}_5\rfloor \right)\right):0$

where

$S_{2,m}=S_2n_{S_2},S_{2,s}=S_2\&\left(2^{n_{S_2}}-1\right),{\mathrm{th}}_{\mathrm{BIO}}^{\prime }=2^{\max \left(5,\mathrm{BD}-7\right)}.$

└·┘ is the floor function, and n_S2=12.

Based on the motion refinement and the gradients, the following adjustment is calculated for each sample in the 4×4 subblock:

$\begin{array}{cc}b\left(x,y\right)=\mathrm{rnd}\left(\left(v_x\left(\frac{\partial I^{\left(1\right)}\left(x,y\right)}{\partial x}-\frac{\partial I^{\left(0\right)}\left(x,y\right)}{\partial x}\right)+v_y\left(\frac{\partial I^{\left(1\right)}\left(x,y\right)}{\partial y}-\frac{\partial I^{\left(0\right)}\left(x,y\right)}{\partial y}\right)+1\right)/2\right).& \left(2-6\right)\end{array}$

Finally, the BDOF samples of the CU are calculated by adjusting the bi-prediction samples as follows:

$\begin{array}{cc}pre{d}_{BDOF}\left(x,y\right)=\left(I^{\left(0\right)}\left(x,y\right)+I^{\left(1\right)}\left(x,y\right)+b\left(x,y\right)+o_{\mathrm{offset}}\right)\gg \mathrm{shift}.& \left(2-7\right)\end{array}$

These values are selected such that the multipliers in the BDOF process do not exceed 15-bit, and the maximum bit-width of the intermediate parameters in the BDOF process is kept within 32-bit.

In order to derive the gradient values, some prediction samples I^(k) (i, j) in list k (k=0,1) outside of the current CU boundaries need to be generated. FIG. 9 illustrates a schematic diagram of extended CU region used in BDOF. As depicted in the diagram 900 of FIG. 9, the BDOF in VVC uses one extended row/column around the CU's boundaries. In order to control the computational complexity of generating the out-of-boundary prediction samples, prediction samples in the extended area (denoted as 910 in FIG. 9) are generated by taking the reference samples at the nearby integer positions (using floor( ) operation on the coordinates) directly without interpolation, and the normal 8-tap motion compensation interpolation filter is used to generate prediction samples within the CU (denoted as 920 in FIG. 9). These extended sample values are used in gradient calculation only. For the remaining steps in the BDOF process, if any sample and gradient values outside of the CU boundaries are needed, they are padded (i.e. repeated) from their nearest neighbors.

When the width and/or height of a CU are larger than 16 luma samples, it will be split into subblocks with width and/or height equal to 16 luma samples, and the subblock boundaries are treated as the CU boundaries in the BDOF process. The maximum unit size for BDOF process is limited to 16×16. For each subblock, the BDOF process could skipped. When the SAD of between the initial L0 and L1 prediction samples is smaller than a threshold, the BDOF process is not applied to the subblock. The threshold is set equal to (8*W*(H>>1), where W indicates the subblock width, and H indicates subblock height. To avoid the additional complexity of SAD calculation, the SAD between the initial L0 and L1 prediction samples calculated in DVMR process is re-used here.

If BCW is enabled for the current block, i.e., the BCW weight index indicates unequal weight, then bi-directional optical flow is disabled. Similarly, if WP is enabled for the current block, i.e., the luma_weight_lx_flag is 1 for either of the two reference pictures, then BDOF is also disabled. When a CU is coded with symmetric MVD mode or CIIP mode, BDOF is also disabled.

2.1.2.3. Symmetric MVD Coding (SMVD)

In VVC, besides the normal unidirectional prediction and bi-directional prediction mode MVD signalling, symmetric MVD mode for bi-predictional MVD signalling is applied. In the symmetric MVD mode, motion information including reference picture indices of both list-0 and list-1 and MVD of list-1 are not signaled but derived.

The decoding process of the symmetric MVD mode is as follows:

• • 1) At slice level, variables BiDirPredFlag, RefIdxSymL0 and RefIdxSymL1 are derived as follows:
    • If mvd_l1_zero_flag is 1, BiDirPredFlag is set equal to 0.
    • Otherwise, if the nearest reference picture in list-0 and the nearest reference picture in list-1 form a forward and backward pair of reference pictures or a backward and forward pair of reference pictures, BiDirPredFlag is set to 1, and both list-0 and list-1 reference pictures are short-term reference pictures. Otherwise BiDirPredFlag is set to 0.
  • 2) At CU level, a symmetrical mode flag indicating whether symmetrical mode is used or not is explicitly signaled if the CU is bi-prediction coded and BiDirPredFlag is equal to 1.

When the symmetrical mode flag is true, only mvp_l0_flag, mvp_l1_flag and MVD0 are explicitly signaled. The reference indices for list-0 and list-1 are set equal to the pair of reference pictures, respectively. MVD1 is set equal to (−MVD0). The final motion vectors are shown in below formula:

$\begin{array}{cc}\{\begin{array}{c}\left({\mathrm{mvx}}_0,{\mathrm{mvy}}_0\right)=\left({\mathrm{mvpx}}_0+{\mathrm{mvdx}}_0,{\mathrm{mvpy}}_0+{\mathrm{mvdy}}_0\right)\\ \left({\mathrm{mvx}}_1,{\mathrm{mvy}}_1\right)=\left({\mathrm{mvpx}}_1-{\mathrm{mvdx}}_0,{\mathrm{mvpy}}_1-{\mathrm{mvdy}}_0\right)\end{array}.& \left(2-8\right)\end{array}$

FIG. 10 is an illustration for symmetrical MVD mode. In the encoder, symmetric MVD motion estimation starts with initial MV evaluation. A set of initial MV candidates comprising of the MV obtained from uni-prediction search, the MV obtained from bi-prediction search and the MVs from the AMVP list. The one with the lowest rate-distortion cost is chosen to be the initial MV for the symmetric MVD motion search.

2.1.3. Affine Motion Compensated Prediction

In HEVC, only translation motion model is applied for motion compensation prediction (MCP). While in the real world, there are many kinds of motion, e.g. zoom in/out, rotation, perspective motions and the other irregular motions. In VVC, a block-based affine transform motion compensation prediction is applied. As shown FIG. 11, the affine motion field of the block is described by motion information of two control point (4-parameter) or three control point motion vectors (6-parameter).

For 4-parameter affine motion model 1110 in FIG. 11, motion vector at sample location (x, y) in a block is derived as:

$\begin{array}{cc}\{\begin{array}{c}{\mathrm{mv}}_x=\frac{{\mathrm{mv}}_{1x}-{\mathrm{mv}}_{0x}}{W}x+\frac{{\mathrm{mv}}_{1y}-{\mathrm{mv}}_{0y}}{W}y+{\mathrm{mv}}_{0x}\\ {\mathrm{mv}}_y=\frac{{\mathrm{mv}}_{1y}-{\mathrm{mv}}_{0y}}{W}x+\frac{{\mathrm{mv}}_{1y}-{\mathrm{mv}}_{0x}}{W}y+{\mathrm{mv}}_{0y}\end{array}.& \left(2-9\right)\end{array}$

For 6-parameter affine motion model 1120 in FIG. 11, motion vector at sample location (x, y) in a block is derived as:

$\begin{array}{cc}\{\begin{array}{c}{\mathrm{mv}}_x=\frac{{\mathrm{mv}}_{1x}-{\mathrm{mv}}_{0x}}{W}x+\frac{{\mathrm{mv}}_{2x}-{\mathrm{mv}}_{0x}}{H}y+{\mathrm{mv}}_{0x}\\ {\mathrm{mv}}_y=\frac{{\mathrm{mv}}_{1y}-{\mathrm{mv}}_{0y}}{W}x+\frac{{\mathrm{mv}}_{2y}-{\mathrm{mv}}_{0y}}{H}y+{\mathrm{mv}}_{0y}\end{array},& \left(2-10\right)\end{array}$

where (mv_0x, mv_0y) is motion vector of the top-left corner control point, (mv_1x, mv_1y) is motion vector of the top-right corner control point, and (mv_2x, mv_2y) is motion vector of the bottom-left corner control point.

In order to simplify the motion compensation prediction, block based affine transform prediction is applied. FIG. 12 illustrates a schematic diagram 1200 of affine MVF per subblock. To derive motion vector of each 4×4 luma subblock, the motion vector of the center sample of each subblock, as shown in FIG. 12, is calculated according to above equations, and rounded to 1/16 fraction accuracy. Then the motion compensation interpolation filters are applied to generate the prediction of each subblock with derived motion vector. The subblock size of chroma-components is also set to be 4×4. The MV of a 4×4 chroma subblock is calculated as the average of the MVs of the four corresponding 4×4 luma subblocks.

As done for translational motion inter prediction, there are also two affine motion inter prediction modes: affine merge mode and affine AMVP mode.

2.1.3.1. Affine Merge Prediction

AF_MERGE mode can be applied for CUs with both width and height larger than or equal to 8. In this mode the CPMVs of the current CU is generated based on the motion information of the spatial neighbouring CUs. There can be up to five CPMVP candidates and an index is signalled to indicate the one to be used for the current CU. The following three types of CPVM candidate are used to form the affine merge candidate list:

• • Inherited affine merge candidates that extrapolated from the CPMVs of the neighbour CUs;
  • Constructed affine merge candidates CPMVPs that are derived using the translational MVs of the neighbour CUs;
  • Zero MVs.

In VVC, there are maximum two inherited affine candidates, which are derived from affine motion model of the neighbouring blocks, one from left neighbouring CUs and one from above neighbouring CUs. FIG. 13 illustrates a schematic diagram 1300 of locations of inherited affine motion predictors. The candidate blocks are shown in FIG. 13. For the left predictor, the scan order is A0->A1, and for the above predictor, the scan order is B0->B1->B2. Only the first inherited candidate from each side is selected. No pruning check is performed between two inherited candidates. When a neighbouring affine CU is identified, its control point motion vectors are used to derive the CPMVP candidate in the affine merge list of the current CU. FIG. 14 illustrates a schematic diagram 1400 of control point motion vector inheritance. As shown in FIG. 14, if the neighbour left bottom block A 1410 is coded in affine mode, the motion vectors v₂, v₃ and v₄ of the top left corner, above right corner and left bottom corner of the CU 1420 which contains the block A 1410 are attained. When block A 1410 is coded with 4-parameter affine model, the two CPMVs of the current CU are calculated according to v₂, and v₃. In case that block A is coded with 6-parameter affine model, the three CPMVs of the current CU are calculated according to v₂, v₃ and v₄.

Constructed affine candidate means the candidate is constructed by combining the neighbour translational motion information of each control point. The motion information for the control points is derived from the specified spatial neighbours and temporal neighbour shown in FIG. 15 which illustrates a schematic diagram 1500 of locations of candidates position for constructed affine merge mode. CPMV_k (k=1, 2, 3, 4) represents the k-th control point. For CPMV₁, the B2->B3->A2 blocks are checked and the MV of the first available block is used. For CPMV₂, the B1->B0 blocks are checked and for CPMV₃, the A1->A0 blocks are checked. For TMVP is used as CPMV₄ if it's available.

After MVs of four control points are attained, affine merge candidates are constructed based on that motion information. The following combinations of control point MVs are used to construct in order:

• • {CPMV₁, CPMV₂, CPMV₃}, {CPMV₁, CPMV₂, CPMV₄}, {CPMV₁, CPMV₃, CPMV₄}, {CPMV₂, CPMV₃, CPMV₄}, {CPMV₁, CPMV₂}, {CPMV₁, CPMV₃}.

The combination of 3 CPMVs constructs a 6-parameter affine merge candidate and the combination of 2 CPMVs constructs a 4-parameter affine merge candidate. To avoid motion scaling process, if the reference indices of control points are different, the related combination of control point MVs is discarded.

After inherited affine merge candidates and constructed affine merge candidate are checked, if the list is still not full, zero MVs are inserted to the end of the list.

2.1.3.2. Affine AMVP Prediction

Affine AMVP mode can be applied for CUs with both width and height larger than or equal to 16. An affine flag in CU level is signalled in the bitstream to indicate whether affine AMVP mode is used and then another flag is signalled to indicate whether 4-parameter affine or 6-parameter affine. In this mode, the difference of the CPMVs of current CU and their predictors CPMVPs is signalled in the bitstream. The affine AVMP candidate list size is 2 and it is generated by using the following four types of CPVM candidate in order:

• • Inherited affine AMVP candidates that extrapolated from the CPMVs of the neighbour CUs;
  • Constructed affine AMVP candidates CPMVPs that are derived using the translational MVs of the neighbour CUs;
  • Translational MVs from neighboring CUS;
  • Zero MVs.

The checking order of inherited affine AMVP candidates is same to the checking order of inherited affine merge candidates. The only difference is that, for AVMP candidate, only the affine CU that has the same reference picture as in current block is considered. No pruning process is applied when inserting an inherited affine motion predictor into the candidate list.

Constructed AMVP candidate is derived from the specified spatial neighbors shown in FIG. 15. The same checking order is used as done in affine merge candidate construction. In addition, reference picture index of the neighboring block is also checked. The first block in the checking order that is inter coded and has the same reference picture as in current CUs is used. There is only one When the current CU is coded with 4-parameter affine mode, and mv₀ and mv₁ are both available, they are added as one candidate in the affine AMVP list. When the current CU is coded with 6-parameter affine mode, and all three CPMVs are available, they are added as one candidate in the affine AMVP list. Otherwise, constructed AMVP candidate is set as unavailable.

If affine AMVP list candidates is still less than 2 after valid inherited affine AMVP candidates and constructed AMVP candidate are inserted, mv₀, mv₁ and mv₂ will be added, in order, as the translational MVs to predict all control point MVs of the current CU, when available. Finally, zero MVs are used to fill the affine AMVP list if it is still not full.

2.1.3.3. Affine Motion Information Storage

In VVC, the CPMVs of affine CUs are stored in a separate buffer. The stored CPMVs are only used to generate the inherited CPMVPs in affine merge mode and affine AMVP mode for the lately coded CUs. The subblock MVs derived from CPMVs are used for motion compensation, MV derivation of merge/AMVP list of translational MVs and de-blocking.

To avoid the picture line buffer for the additional CPMVs, affine motion data inheritance from the CUs from above CTU is treated differently to the inheritance from the normal neighbouring CUs. If the candidate CU for affine motion data inheritance is in the above CTU line, the bottom-left and bottom-right subblock MVs in the line buffer instead of the CPMVs are used for the affine MVP derivation. In this way, the CPMVs are only stored in local buffer. If the candidate CU is 6-parameter affine coded, the affine model is degraded to 4-parameter model. As shown in FIG. 16, along the top CTU boundary, the bottom-left and bottom right subblock motion vectors of a CU are used for affine inheritance of the CUs in bottom CTUs.

2.1.3.4. Prediction Refinement with Optical Flow for Affine Mode (PROF)

Subblock based affine motion compensation can save memory access bandwidth and reduce computation complexity compared to pixel based motion compensation, at the cost of prediction accuracy penalty. To achieve a finer granularity of motion compensation, prediction refinement with optical flow (PROF) is used to refine the subblock based affine motion compensated prediction without increasing the memory access bandwidth for motion compensation. In VVC, after the subblock based affine motion compensation is performed, luma prediction sample is refined by adding a difference derived by the optical flow equation. The PROF is described as following four steps:

• • Step 1) The subblock-based affine motion compensation is performed to generate subblock prediction I(i, j).
  • Step 2) The spatial gradients g_x(i, j) and g_y(i,j) of the subblock prediction are calculated at each sample location using a 3-tap filter [−1, 0, 1]. The gradient calculation is exactly the same as gradient calculation in BDOF.

$\begin{array}{cc}{g}_x\left(i,j\right)=\left(I\left(i+1,j\right)\gg \mathrm{shift}1\right)-\left(I\left(i-1,j\right)\right)\mathrm{shift}1)& \left(2-11\right)\end{array}$ $\begin{array}{cc}{g}_y\left(i,j\right)=\left(I\left(i,j+1\right)\mathrm{shift}1\right)-\left(I\left(i,j-1\right)\right)\mathrm{shift}1)& \left(2-12\right)\end{array}$

shift1 is used to control the gradient's precision. The subblock (i.e. 4×4) prediction is extended by one sample on each side for the gradient calculation. To avoid additional memory bandwidth and additional interpolation computation, those extended samples on the extended borders are copied from the nearest integer pixel position in the reference picture.

• • Step 3) The luma prediction refinement is calculated by the following optical flow equation.

$\begin{array}{cc}\Delta I\left(i,j\right)=g_x\left(i,j\right)*\Delta v_x\left(i,j\right)+g_y\left(i,j\right)*\Delta v_y\left(i,j\right)& \left(2-13\right)\end{array}$

where the Δv(i, j) is the difference between sample MV computed for sample location (i, j), denoted by v (i, j), and the subblock MV of the subblock to which sample (i, j) belongs, as shown in FIG. 17. The Δv(i, j) (shown as arrow 1710) is quantized in the unit of 1/32 luam sample precision.

Since the affine model parameters and the sample location relative to the subblock center are not changed from subblock to subblock, Δv(i, j) can be calculated for the first subblock, and reused for other subblocks in the same CU. Let dx(i, j) and dy(i, j) be the horizontal and vertical offset from the sample location (i, j) to the center of the subblock (x_SB, y_SB), Δv(x, y) can be derived by the following equation,

$\begin{array}{cc}\{\begin{array}{c}\mathrm{dx}\left(i,j\right)=i-x_{\mathrm{SB}}\\ \mathrm{dy}\left(i,j\right)=i-y_{\mathrm{SB}}\end{array}& \left(2-14\right)\end{array}$ $\begin{array}{cc}\{\begin{array}{c}\Delta v_x\left(i,j\right)=C*\mathrm{dx}\left(i,j\right)+D*\mathrm{dy}\left(i,j\right)\\ \Delta v_y\left(i,j\right)=E*\mathrm{dx}\left(i,j\right)+F*\mathrm{dy}\left(i,j\right)\end{array}.& \left(2-15\right)\end{array}$

In order to keep accuracy, the enter of the subblock (x_SB, y_SB) is calculated as ((W_SB−1)/2, (H_SB−1)/2), where W_SB and H_SB are the subblock width and height, respectively.

For 4-parameter affine model.

$\begin{array}{cc}\{\begin{array}{c}C=F=\frac{v_{1x}-v_{0x}}{w}\\ E=-D=\frac{v_{1y}-v_{0y}}{w}\end{array};& \left(2-16\right)\end{array}$

For 6-parameter affine model,

$\begin{array}{cc}\{\begin{array}{c}C=\frac{v_{1x}-v_{0x}}{w}\\ D=\frac{v_{2x}-v_{0x}}{h}\\ E=\frac{v_{1y}-v_{0y}}{w}\\ F=\frac{v_{2y}-v_{0y}}{h}\end{array}& \left(2-17\right)\end{array}$

where (v_0x, v_0y), (v_1x, v_1y), (v_2x, v_2y) are the top-left, top-right and bottom-left control point motion vectors, w and h are the width and height of the CU.

• • Step 4) Finally, the luma prediction refinement ΔI(i, j) is added to the subblock prediction I(i, j). The final prediction I′ is generated as the following equation.

$I^{\prime }\left(i,j\right)=I\left(i,j\right)+\Delta I\left(i,j\right)$

PROF is not be applied in two cases for an affine coded CU: 1) all control point MVs are the same, which indicates the CU only has translational motion; 2) the affine motion parameters are greater than a specified limit because the subblock based affine MC is degraded to CU based MC to avoid large memory access bandwidth requirement.

A fast encoding method is applied to reduce the encoding complexity of affine motion estimation with PROF. PROF is not applied at affine motion estimation stage in following two situations: a) if this CU is not the root block and its parent block does not select the affine mode as its best mode, PROF is not applied since the possibility for current CU to select the affine mode as best mode is low; b) if the magnitude of four affine parameters (C, D, E, F) are all smaller than a predefined threshold and the current picture is not a low delay picture, PROF is not applied because the improvement introduced by PROF is small for this case. In this way, the affine motion estimation with PROF can be accelerated.

2.1.4. Subblock-Based Temporal Motion Vector Prediction (SbTMVP)

VVC supports the subblock-based temporal motion vector prediction (SbTMVP) method. Similar to the temporal motion vector prediction (TMVP) in HEVC, SbTMVP uses the motion field in the collocated picture to improve motion vector prediction and merge mode for CUs in the current picture. The same collocated picture used by TMVP is used for SbTVMP. SbTMVP differs from TMVP in the following two main aspects:

• • TMVP predicts motion at CU level but SbTMVP predicts motion at sub-CU level;
  • Whereas TMVP fetches the temporal motion vectors from the collocated block in the collocated picture (the collocated block is the bottom-right or center block relative to the current CU), SbTMVP applies a motion shift before fetching the temporal motion information from the collocated picture, where the motion shift is obtained from the motion vector from one of the spatial neighboring blocks of the current CU.

The SbTVMP process is illustrated in FIG. 18a and FIG. 18b. FIG. 18a illustrates a schematic diagram 1810 of spatial neighboring blocks used by SbTMVP. SbTMVP predicts the motion vectors of the sub-CUs within the current CU in two steps. In the first step, the spatial neighbor A1 in FIG. 18a is examined. If A1 has a motion vector that uses the collocated picture as its reference picture, this motion vector is selected to be the motion shift to be applied. If no such motion is identified, then the motion shift is set to (0, 0).

FIG. 18b illustrates a schematic diagram of driving sub-CU motion field by applying a motion shift from spatial neighbor and scaling the motion information from the corresponding collocated sub-CUs. In the second step, the motion shift identified in Step 1 is applied (i.e. added to the current block's coordinates) to obtain sub-CU level motion information (motion vectors and reference indices) from the collocated picture as shown in FIG. 18b. The example in FIG. 18b assumes the motion shift is set to block A1's motion. Then, for each sub-CU, the motion information of its corresponding block (the smallest motion grid that covers the center sample) in the collocated picture is used to derive the motion information for the sub-CU. After the motion information of the collocated sub-CU is identified, it is converted to the motion vectors and reference indices of the current sub-CU in a similar way as the TMVP process of HEVC, where temporal motion scaling is applied to align the reference pictures of the temporal motion vectors to those of the current CU.

In VVC, a combined subblock based merge list which contains both SbTVMP candidate and affine merge candidates is used for the signalling of subblock based merge mode. The SbTVMP mode is enabled/disabled by a sequence parameter set (SPS) flag. If the SbTMVP mode is enabled, the SbTMVP predictor is added as the first entry of the list of subblock based merge candidates, and followed by the affine merge candidates. The size of subblock based merge list is signalled in SPS and the maximum allowed size of the subblock based merge list is 5 in VVC.

The sub-CU size used in SbTMVP is fixed to be 8×8, and as done for affine merge mode, SbTMVP mode is only applicable to the CU with both width and height are larger than or equal to 8.

The encoding logic of the additional SbTMVP merge candidate is the same as for the other merge candidates, that is, for each CU in P or B slice, an additional RD check is performed to decide whether to use the SbTMVP candidate.

2.1.5. Adaptive Motion Vector Resolution (AMVR)

In HEVC, motion vector differences (MVDs) (between the motion vector and predicted motion vector of a CU) are signalled in units of quarter-luma-sample when use_integer_mv_flag is equal to 0 in the slice header. In VVC, a CU-level adaptive motion vector resolution (AMVR) scheme is introduced. AMVR allows MVD of the CU to be coded in different precision. Dependent on the mode (normal AMVP mode or affine AVMP mode) for the current CU, the MVDs of the current CU can be adaptively selected as follows:

• • Normal AMVP mode: quarter-luma-sample, half-luma-sample, integer-luma-sample or four-luma-sample.
  • Affine AMVP mode: quarter-luma-sample, integer-luma-sample or 1/16 luma-sample.

The CU-level MVD resolution indication is conditionally signalled if the current CU has at least one non-zero MVD component. If all MVD components (that is, both horizontal and vertical MVDs for reference list L0 and reference list L1) are zero, quarter-luma-sample MVD resolution is inferred.

For a CU that has at least one non-zero MVD component, a first flag is signalled to indicate whether quarter-luma-sample MVD precision is used for the CU. If the first flag is 0, no further signaling is needed and quarter-luma-sample MVD precision is used for the current CU. Otherwise, a second flag is signalled to indicate half-luma-sample or other MVD precisions (integer or four-luma sample) is used for normal AMVP CU. In the case of half-luma-sample, a 6-tap interpolation filter instead of the default 8-tap interpolation filter is used for the half-luma sample position. Otherwise, a third flag is signalled to indicate whether integer-luma-sample or four-luma-sample MVD precision is used for normal AMVP CU. In the case of affine AMVP CU, the second flag is used to indicate whether integer-luma-sample or 1/16 luma-sample MVD precision is used. In order to ensure the reconstructed MV has the intended precision (quarter-luma-sample, half-luma-sample, integer-luma-sample or four-luma-sample), the motion vector predictors for the CU will be rounded to the same precision as that of the MVD before being added together with the MVD. The motion vector predictors are rounded toward zero (that is, a negative motion vector predictor is rounded toward positive infinity and a positive motion vector predictor is rounded toward negative infinity).

The encoder determines the motion vector resolution for the current CU using RD check. To avoid always performing CU-level RD check four times for each MVD resolution, in VTM13, the RD check of MVD precisions other than quarter-luma-sample is only invoked conditionally. For normal AVMP mode, the RD cost of quarter-luma-sample MVD precision and integer-luma sample MV precision is computed first. Then, the RD cost of integer-luma-sample MVD precision is compared to that of quarter-luma-sample MVD precision to decide whether it is necessary to further check the RD cost of four-luma-sample MVD precision. When the RD cost for quarter-luma-sample MVD precision is much smaller than that of the integer-luma-sample MVD precision, the RD check of four-luma-sample MVD precision is skipped. Then, the check of half-luma-sample MVD precision is skipped if the RD cost of integer-luma-sample MVD precision is significantly larger than the best RD cost of previously tested MVD precisions. For affine AMVP mode, if affine inter mode is not selected after checking rate-distortion costs of affine merge/skip mode, merge/skip mode, quarter-luma-sample MVD precision normal AMVP mode and quarter-luma-sample MVD precision affine AMVP mode, then 1/16 luma-sample MV precision and 1-pel MV precision affine inter modes are not checked. Furthermore affine parameters obtained in quarter-luma-sample MV precision affine inter mode is used as starting search point in 1/16 luma-sample and quarter-luma-sample MV precision affine inter modes.

2.1.6. Bi-Prediction with CU-Level Weight (BCW)

In HEVC, the bi-prediction signal is generated by averaging two prediction signals obtained from two different reference pictures and/or using two different motion vectors. In VVC, the bi-prediction mode is extended beyond simple averaging to allow weighted averaging of the two prediction signals:

$\begin{array}{cc}{P}_{bi-pred}=\left(\left(8-w\right)*P_0+w*P_1+4\right)\gg 3.& \left(2-18\right)\end{array}$

Five weights are allowed in the weighted averaging bi-prediction, w∈{−2, 3, 4, 5, 10}. For each bi-predicted CU, the weight w is determined in one of two ways: 1) for a non-merge CU, the weight index is signalled after the motion vector difference; 2) for a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. BCW is only applied to CUs with 256 or more luma samples (i.e., CU width times CU height is greater than or equal to 256). For low-delay pictures, all 5 weights are used. For non-low-delay pictures, only 3 weights (w∈{3, 4, 5}) are used.

• • At the encoder, fast search algorithms are applied to find the weight index without significantly increasing the encoder complexity. These algorithms are summarized as follows. For further details readers are referred to the VTM software and document JVET-L0646. When combined with AMVR, unequal weights are only conditionally checked for 1-pel and 4-pel motion vector precisions if the current picture is a low-delay picture.
  • When combined with affine, affine ME will be performed for unequal weights if and only if the affine mode is selected as the current best mode.
  • When the two reference pictures in bi-prediction are the same, unequal weights are only conditionally checked.
  • Unequal weights are not searched when certain conditions are met, depending on the POC distance between current picture and its reference pictures, the coding QP, and the temporal level.

The BCW weight index is coded using one context coded bin followed by bypass coded bins. The first context coded bin indicates if equal weight is used; and if unequal weight is used, additional bins are signalled using bypass coding to indicate which unequal weight is used.

Weighted prediction (WP) is a coding tool supported by the H.264/AVC and HEVC standards to efficiently code video content with fading. Support for WP was also added into the VVC standard. WP allows weighting parameters (weight and offset) to be signalled for each reference picture in each of the reference picture lists L0 and L1. Then, during motion compensation, the weight(s) and offset(s) of the corresponding reference picture(s) are applied. WP and BCW are designed for different types of video content. In order to avoid interactions between WP and BCW, which will complicate VVC decoder design, if a CU uses WP, then the BCW weight index is not signalled, and w is inferred to be 4 (i.e. equal weight is applied). For a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. This can be applied to both normal merge mode and inherited affine merge mode. For constructed affine merge mode, the affine motion information is constructed based on the motion information of up to 3 blocks. The BCW index for a CU using the constructed affine merge mode is simply set equal to the BCW index of the first control point MV.

In VVC, CIIP and BCW cannot be jointly applied for a CU. When a CU is coded with CHIP mode, the BCW index of the current CU is set to 2, e.g. equal weight.

2.1.7. Bi-Directional Optical Flow (BDOF)

The bi-directional optical flow (BDOF) tool is included in VVC. BDOF, previously referred to as BIO, was included in the JEM. Compared to the JEM version, the BDOF in VVC is a simpler version that requires much less computation, especially in terms of number of multiplications and the size of the multiplier.

BDOF is used to refine the bi-prediction signal of a CU at the 4×4 subblock level. BDOF is applied to a CU if it satisfies all the following conditions:

• • The CU is coded using “true” bi-prediction mode, i.e., one of the two reference pictures is prior to the current picture in display order and the other is after the current picture in display order;
  • The distances (i.e. POC difference) from two reference pictures to the current picture are same;
  • Both reference pictures are short-term reference pictures;
  • The CU is not coded using affine mode or the SbTMVP merge mode;
  • CU has more than 64 luma samples;
  • Both CU height and CU width are larger than or equal to 8 luma samples;
  • BCW weight index indicates equal weight;
  • WP is not enabled for the current CU;
  • CIIP mode is not used for the current CU.

BDOF is only applied to the luma component. As its name indicates, the BDOF mode is based on the optical flow concept, which assumes that the motion of an object is smooth. For each 4×4 subblock, a motion refinement (v_x, v_y) is calculated by minimizing the difference between the L0 and L1 prediction samples. The motion refinement is then used to adjust the bi-predicted sample values in the 4×4 subblock. The following steps are applied in the BDOF process.

First, the horizontal and vertical gradients,

$\frac{\partial I^{\left(k\right)}}{\partial x}\left(i,j\right)\mathrm{and}\frac{\partial I^{\left(k\right)}}{\partial y}\left(i,j\right),$

k=0,1, of the two prediction signals are computed by directly calculating the difference between two neighboring samples, i.e.,

$\begin{array}{cc}\begin{array}{c}\frac{\partial I^{\left(k\right)}}{\partial x}\left(i,j\right)=\left(\left(I^{\left(k\right)}\left(i+1,j\right)\right)\mathrm{shift}1\right)-\left(I^{\left(k\right)}\left(i-1,j\right)\right)\mathrm{shift}1))\\ \frac{\partial I^{\left(k\right)}}{\partial y}\left(i,j\right)=\left(\left(I^{\left(k\right)}\left(i,j+1\right)\right)\mathrm{shift}1\right)-\left(I^{\left(k\right)}\left(i,j-1\right)\right)\mathrm{shift}1))\end{array}& \left(2-19\right)\end{array}$

where I^(k) (i, j) are the sample value at coordinate (i, j) of the prediction signal in list k, k=0,1, and shift1 is calculated based on the luma bit depth, bitDepth, as shift1=max(6, bitDepth−6).

Then, the auto- and cross-correlation of the gradients, S₁, S₂, S₃, S₅ and S₆, are calculated as

$\begin{array}{cc}{S}_1={\sum }_{\left(i,j\right)\in \Omega }\mathrm{Abs}\left({\psi }_x\left(i,j\right)\right),S_3={\sum }_{\left(i,j\right)\in \Omega }\theta \left(i,j\right)·\mathrm{Sign}\left({\psi }_x\left(i,j\right)\right)S_2=\sum _{\left(i,j\right)\in \Omega }{\psi }_x\left(i,j\right)·\mathrm{Sign}\left({\psi }_y\left(i,j\right)\right)& \left(2-20\right)\end{array}$ $\begin{array}{cc}{S}_5={\sum }_{\left(i,j\right)\in \Omega }\mathrm{Abs}\left({\psi }_y\left(i,j\right)\right),S_6={\sum }_{\left(i,j\right)\in \Omega }\theta \left(i,j\right)·\mathrm{Sign}\left({\psi }_y\left(i,j\right)\right)\mathrm{where}{\psi }_x\left(i,j\right)=\left(\frac{\partial I^{\left(1\right)}}{\partial x}\left(i,j\right)+\frac{\partial I^{\left(0\right)}}{\partial x}\left(i,j\right)\right)n_a{\psi }_y\left(i,j\right)=\left(\frac{\partial I^{\left(1\right)}}{\partial y}\left(i,j\right)+\frac{\partial I^{\left(0\right)}}{\partial y}\left(i,j\right)\right)n_a\theta \left(i,j\right)=(I^{\left(1\right)}\left(i,j\right)n_b)-(I^{\left(0\right)}\left(i,j\right)n_b)& \left(2-21\right)\end{array}$

where Ω is a 6×6 window around the 4×4 subblock, and the values of n_a and n_b are set equal to min (1, bitDepth−11) and min (4, bitDepth−8), respectively.

The motion refinement (v_x, v_y) is then derived using the cross- and auto-correlation terms using the following:

$\begin{array}{cc}{v}_x=S_1>0?\mathrm{clip}3(-{\mathrm{th}}_{\mathrm{BIO}}^{\prime },{\mathrm{th}}_{\mathrm{BIO}}^{\prime },-(\left(S_3·2^{n_b-n_a}\right)\lfloor {\log }_2{S}_1\rfloor )):0& \left(2-22\right)\end{array}$ $v_y=S_5>0?\mathrm{clip}3(-{\mathrm{th}}_{\mathrm{BIO}}^{\prime },{\mathrm{th}}_{\mathrm{BIO}}^{\prime },-((S_6·2^{n_b-n_a}-$ $\left(\left(v_x{S}_{2,m}\right)n_{S_2}+v_x{S}_{2,s}\right)/2)\lfloor {\log }_2{S}_5\rfloor )):0$

where

$S_{2,m}=S_2\gg n_{S_2},S_{2,s}=S_2\&\left(2^{n_{s_2}}-1\right),{\mathrm{th}}_{\mathrm{BIO}}^{\prime }=2^{\max \left(5,\mathrm{SD}-7\right)}.$

[.] is the floor function, and n_S2=12.

Based on the motion refinement and the gradients, the following adjustment is calculated for each sample in the 4×4 subblock:

$\begin{array}{cc}b\left(x,y\right)=\mathrm{rnd}((v_x\left(\frac{\partial I^{\left(1\right)}\left(x,y\right)}{\partial x}-\frac{\partial I^{\left(0\right)}\left(x,y\right)}{\partial x}\right)+& \left(2-23\right)\end{array}$ $v_y\left(\frac{\partial I^{\left(1\right)}\left(x,y\right)}{\partial y}-\frac{\partial I^{\left(0\right)}\left(x,y\right)}{\partial y}\right)+1)/2).$

Finally, the BDOF samples of the CU are calculated by adjusting the bi-prediction samples as follows:

$\begin{array}{cc}{\mathrm{pred}}_{\mathrm{BDOF}}\left(x,y\right)=\left(I^{\left(0\right)}\left(x,y\right)+I^{\left(1\right)}\left(x,y\right)+b\left(x,y\right)+o_{\mathrm{offset}}\right)\mathrm{shift}.& \left(2-24\right)\end{array}$

These values are selected such that the multipliers in the BDOF process do not exceed 15-bit, and the maximum bit-width of the intermediate parameters in the BDOF process is kept within 32-bit.

In order to derive the gradient values, some prediction samples/(k) (i, j) in list k (k=0,1) outside of the current CU boundaries need to be generated. FIG. 19 illustrates a schematic diagram of extended CU region used in BDOF. As depicted in the diagram 1900 of FIG. 19, the BDOF in VVC uses one extended row/column around the CU's boundaries. In order to control the computational complexity of generating the out-of-boundary prediction samples, prediction samples in the extended area (denoted as 1910 in FIG. 19) are generated by taking the reference samples at the nearby integer positions (using floor( ) operation on the coordinates) directly without interpolation, and the normal 8-tap motion compensation interpolation filter is used to generate prediction samples within the CU (denoted as 1920 in FIG. 19). These extended sample values are used in gradient calculation only. For the remaining steps in the BDOF process, if any sample and gradient values outside of the CU boundaries are needed, they are padded (i.e. repeated) from their nearest neighbors.

When the width and/or height of a CU are larger than 16 luma samples, it will be split into subblocks with width and/or height equal to 16 luma samples, and the subblock boundaries are treated as the CU boundaries in the BDOF process. The maximum unit size for BDOF process is limited to 16×16. For each subblock, the BDOF process could skipped. When the SAD of between the initial L0 and L1 prediction samples is smaller than a threshold, the BDOF process is not applied to the subblock. The threshold is set equal to (8*W*(H>>1), where W indicates the subblock width, and H indicates subblock height. To avoid the additional complexity of SAD calculation, the SAD between the initial L0 and L1 prediction samples calculated in DVMR process is re-used here.

If BCW is enabled for the current block, i.e., the BCW weight index indicates unequal weight, then bi-directional optical flow is disabled. Similarly, if WP is enabled for the current block, i.e., the luma_weight_lx_flag is 1 for either of the two reference pictures, then BDOF is also disabled. When a CU is coded with symmetric MVD mode or CIIP mode, BDOF is also disabled.

2.1.8. Decoder Side Motion Vector Refinement (DMVR)

In order to increase the accuracy of the MVs of the merge mode, a bilateral-matching (BM) based decoder side motion vector refinement is applied in VVC. In bi-prediction operation, a refined MV is searched around the initial MVs in the reference picture list L0 and reference picture list L1. The BM method calculates the distortion between the two candidate blocks in the reference picture list L0 and list L1. FIG. 20 is a schematic diagram illustrating the decoding side motion vector refinement. As illustrated in FIG. 20, the SAD between the blocks 2010 and 2012 based on each MV candidate around the initial MV is calculated, where the block 2010 is in a reference picture 2001 in the list L0 and the block 2012 is in a reference picture 2003 in the List L1 for the current picture 2002. The MV candidate with the lowest SAD becomes the refined MV and used to generate the bi-predicted signal.

In VVC, the application of DMVR is restricted and is only applied for the CUs which are coded with following modes and features:

• • CU level merge mode with bi-prediction MV;
  • One reference picture is in the past and another reference picture is in the future with respect to the current picture;
  • The distances (i.e. POC difference) from two reference pictures to the current picture are same;
  • Both reference pictures are short-term reference pictures;
  • CU has more than 64 luma samples;
  • Both CU height and CU width are larger than or equal to 8 luma samples;
  • BCW weight index indicates equal weight;
  • WP is not enabled for the current block;
  • CIIP mode is not used for the current block.

The refined MV derived by DMVR process is used to generate the inter prediction samples and also used in temporal motion vector prediction for future pictures coding. While the original MV is used in deblocking process and also used in spatial motion vector prediction for future CU coding.

The additional features of DMVR are mentioned in the following sub-clauses.

2.1.8.1. Searching Scheme

In DVMR, the search points are surrounding the initial MV and the MV offset obey the MV difference mirroring rule. In other words, any points that are checked by DMVR, denoted by candidate MV pair (MV0, MV1) obey the following two equations:

$\begin{array}{cc}\mathrm{MV}{0}^{\prime }=\mathrm{MV}0+\mathrm{MV\_offset}& \left(2-25\right)\end{array}$ $\begin{array}{cc}\mathrm{MV}{1}^{\prime }=\mathrm{MV}1-\mathrm{MV\_offset}& \left(2-26\right)\end{array}$

where MV_offset represents the refinement offset between the initial MV and the refined MV in one of the reference pictures. The refinement search range is two integer luma samples from the initial MV. The searching includes the integer sample offset search stage and fractional sample refinement stage.

25 points full search is applied for integer sample offset searching. The SAD of the initial MV pair is first calculated. If the SAD of the initial MV pair is smaller than a threshold, the integer sample stage of DMVR is terminated. Otherwise SADs of the remaining 24 points are calculated and checked in raster scanning order. The point with the smallest SAD is selected as the output of integer sample offset searching stage. To reduce the penalty of the uncertainty of DMVR refinement, it is proposed to favor the original MV during the DMVR process. The SAD between the reference blocks referred by the initial MV candidates is decreased by ¼ of the SAD value.

The integer sample search is followed by fractional sample refinement. To save the calculational complexity, the fractional sample refinement is derived by using parametric error surface equation, instead of additional search with SAD comparison. The fractional sample refinement is conditionally invoked based on the output of the integer sample search stage. When the integer sample search stage is terminated with center having the smallest SAD in either the first iteration or the second iteration search, the fractional sample refinement is further applied.

In parametric error surface based sub-pixel offsets estimation, the center position cost and the costs at four neighboring positions from the center are used to fit a 2-D parabolic error surface equation of the following form

$\begin{array}{cc}E\left(x,y\right)={A\left(x-x_{\min }\right)}^2+{B\left(y-y_{\min }\right)}^2+C& \left(2-27\right)\end{array}$

where (x_min, y_min) corresponds to the fractional position with the least cost and C corresponds to the minimum cost value. By solving the above equations by using the cost value of the five search points, the (x_min, y_min) is computed as:

$\begin{array}{cc}{x}_{\min }=\left(E\left(-1,0\right)-E\left(1,0\right)\right)/\left(2\left(E\left(-1,0\right)+E\left(1,0\right)-2E\left(0,0\right)\right)\right)& \left(2-28\right)\end{array}$ $\begin{array}{cc}{y}_{\min }=\left(E\left(0,-1\right)-E\left(0,1\right)\right)/(2\left(\left(E\left(0,-1\right)+E\left(0,1\right)-2E\left(0,0\right)\right)\right).& \left(2-29\right)\end{array}$

The value of x_min and y_min are automatically constrained to be between −8 and 8 since all cost values are positive and the smallest value is E(0,0). This corresponds to half peal offset with 1/16th-pel MV accuracy in VVC. The computed fractional (x_min, y_min) are added to the integer distance refinement MV to get the sub-pixel accurate refinement delta MV.

2.1.8.2. Bilinear-Interpolation and Sample Padding

In VVC, the resolution of the MVs is 1/16 luma samples. The samples at the fractional position are interpolated using a 8-tap interpolation filter. In DMVR, the search points are surrounding the initial fractional-pel MV with integer sample offset, therefore the samples of those fractional position need to be interpolated for DMVR search process. To reduce the calculation complexity, the bi-linear interpolation filter is used to generate the fractional samples for the searching process in DMVR. Another important effect is that by using bi-linear filter is that with 2-sample search range, the DVMR does not access more reference samples compared to the normal motion compensation process. After the refined MV is attained with DMVR search process, the normal 8-tap interpolation filter is applied to generate the final prediction. In order to not access more reference samples to normal MC process, the samples, which is not needed for the interpolation process based on the original MV but is needed for the interpolation process based on the refined MV, will be padded from those available samples.

2.1.8.3. Maximum DMVR Processing Unit

When the width and/or height of a CU are larger than 16 luma samples, it will be further split into subblocks with width and/or height equal to 16 luma samples. The maximum unit size for DMVR searching process is limit to 16×16.

2.1.9. Combined Inter and Intra Prediction (CIIP)

In VVC, when a CU is coded in merge mode, if the CU contains at least 64 luma samples (that is, CU width times CU height is equal to or larger than 64), and if both CU width and CU height are less than 128 luma samples, an additional flag is signalled to indicate if the combined inter/intra prediction (CIIP) mode is applied to the current CU. As its name indicates, the CIIP prediction combines an inter prediction signal with an intra prediction signal. The inter prediction signal in the CIIP mode P_inter is derived using the same inter prediction process applied to regular merge mode; and the intra prediction signal P_intra is derived following the regular intra prediction process with the planar mode. FIG. 21 shows top and left neighboring blocks used in CIIP weight derivation. Then, the intra and inter prediction signals are combined using weighted averaging, where the weight value is calculated depending on the coding modes of the top and left neighbouring blocks (depicted in FIG. 21) as follows:

• • If the top neighbor is available and intra coded, then set isIntraTop to 1, otherwise set isIntraTop to 0;
  • If the left neighbor is available and intra coded, then set isIntraLeft to 1, otherwise set isIntraLeft to 0;
  • If (isIntraLeft+isIntraTop) is equal to 2, then wt is set to 3;
  • Otherwise, if (isIntraLeft+isIntraTop) is equal to 1, then wt is set to 2;
  • Otherwise, set wt to 1.

The CIIP prediction is formed as follows:

$\begin{array}{cc}{P}_{\mathrm{CIIP}}=\left(\left(4-\mathrm{wt}\right)*P_{\mathrm{inter}}+\mathrm{wt}*P_{\mathrm{intra}}+2\right)2.& \left(2-30\right)\end{array}$

2.1.10. Geometric Partitioning Mode (GPM)

In VVC, a geometric partitioning mode is supported for inter prediction. The geometric partitioning mode is signalled using a CU-level flag as one kind of merge mode, with other merge modes including the regular merge mode, the MMVD mode, the CIIP mode and the subblock merge mode. In total 64 partitions are supported by geometric partitioning mode for each possible CU size w×h=2^m×2ⁿ with m, n∈{3 ··· 6} excluding 8×64 and 64×8.

FIG. 22 shows examples of the GPM splits grouped by identical angles When this mode is used, a CU is split into two parts by a geometrically located straight line (FIG. 22). The location of the splitting line is mathematically derived from the angle and offset parameters of a specific partition. Each part of a geometric partition in the CU is inter-predicted using its own motion; only uni-prediction is allowed for each partition, that is, each part has one motion vector and one reference index. The uni-prediction motion constraint is applied to ensure that same as the conventional bi-prediction, only two motion compensated prediction are needed for each CU.

If geometric partitioning mode is used for the current CU, then a geometric partition index indicating the partition mode of the geometric partition (angle and offset), and two merge indices (one for each partition) are further signalled. The number of maximum GPM candidate size is signalled explicitly in SPS and specifies syntax binarization for GPM merge indices. After predicting each of part of the geometric partition, the sample values along the geometric partition edge are adjusted using a blending processing with adaptive weights. This is the prediction signal for the whole CU, and transform and quantization process will be applied to the whole CU as in other prediction modes. Finally, the motion field of a CU predicted using the geometric partition modes is stored.

2.1.10.1. Uni-Prediction Candidate List Construction

The uni-prediction candidate list is derived directly from the merge candidate list constructed according to the extended merge prediction process. Denote n as the index of the uni-prediction motion in the geometric uni-prediction candidate list. The LX motion vector of the n-th extended merge candidate, with X equal to the parity of n, is used as the n-th uni-prediction motion vector for geometric partitioning mode. FIG. 23 shows uni-prediction MV selection for geometric partitioning mode. These motion vectors are marked with “x” in FIG. 23. In case a corresponding LX motion vector of the n-the extended merge candidate does not exist, the L(1−X) motion vector of the same candidate is used instead as the uni-prediction motion vector for geometric partitioning mode.

2.1.10.2. Blending Along the Geometric Partitioning Edge

After predicting each part of a geometric partition using its own motion, blending is applied to the two prediction signals to derive samples around geometric partition edge. The blending weight for each position of the CU are derived based on the distance between individual position and the partition edge.

The distance for a position (x, y) to the partition edge are derived as:

$d\left(x,y\right)=\left(2x+1-w\right)\cos \left({\phi }_i\right)+\left(2y+1-h\right)\sin \left({\phi }_i\right)-{\rho }_j{\rho }_j={\rho }_{x,j}\cos \left({\phi }_i\right)+{\rho }_{y,j}\sin \left({\phi }_i\right){\rho }_{x,j}=\{\begin{array}{cc}0& i\%16=8\mathrm{or}\left(i\%16\ne 0\mathrm{and}h\ge w\right)\\ \pm \left(j\times w\right)2& \mathrm{otherwise}\end{array}{\rho }_{y,j}=\{\begin{array}{cc}\pm \left(j\times h\right)2& i\%16=8\mathrm{or}\left(i\%16\ne 0\mathrm{and}h\ge w\right)\\ 0& \mathrm{otherwise}\end{array}$

where i, j are the indices for angle and offset of a geometric partition, which depend on the signaled geometric partition index. The sign of ρ_x,j and ρ_y,j depend on angle index i.

The weights for each part of a geometric partition are derived as following:

$\mathrm{wIdxL}\left(x,y\right)=\mathrm{partIdx}?32+d\left(x,y\right):32-d\left(x,y\right)w_0\left(x,y\right)=\frac{\mathrm{Clip}3\left(0,8,\left(\mathrm{wIdxL}\left(x,y\right)+4\right)\right)3)}{8}{w}_1\left(x,y\right)=1-w_0\left(x,y\right).$

The partIdx depends on the angle index i. FIG. 24 shows exemplified generation of a bending weight w₀ using geometric partitioning mode. One example of weigh w₀ is illustrated in FIG. 24.

2.1.10.3. Motion Field Storage for Geometric Partitioning Mode

Mv1 from the first part of the geometric partition, Mv2 from the second part of the geometric partition and a combined Mv of Mv1 and Mv2 are stored in the motion filed of a geometric partitioning mode coded CU. The stored motion vector type for each individual position in the motion filed are determined as:

sType=abs(motionIdx)<32?2:(motionIdx≤0?(1−partIdx):partIdx)

where motionIdx is equal to d(4x+2, 4y+2). The partIdx depends on the angle index i.

If sType is equal to 0 or 1, Mv0 or Mv1 are stored in the corresponding motion field, otherwise if sType is equal to 2, a combined Mv from Mv0 and Mv2 are stored. The combined Mv are generated using the following process:

• • 1) If Mv1 and Mv2 are from different reference picture lists (one from L0 and the other from L1), then Mv1 and Mv2 are simply combined to form the bi-prediction motion vectors.
  • 2) Otherwise, if Mv1 and Mv2 are from the same list, only uni-prediction motion Mv2 is stored.

2.1.11. Local Illumination Compensation (LIC)

LIC is an inter prediction technique to model local illumination variation between current block and its prediction block as a function of that between current block template and reference block template. The parameters of the function can be denoted by a scale a and an offset β, which forms a linear equation, that is, α*p [x]+β to compensate illumination changes, where p [x] is a reference sample pointed to by MV at a location x on reference picture. Since α and β can be derived based on current block template and reference block template, no signaling overhead is required for them, except that an LIC flag is signaled for AMVP mode to indicate the use of LIC. The local illumination compensation proposed in JVET-O0066 is used for uni-prediction inter CUs with the following modifications.

• • Intra neighbor samples can be used in LIC parameter derivation;
  • LIC is disabled for blocks with less than 32 luma samples;
  • For both non-subblock and affine modes, LIC parameter derivation is performed based on the template block samples corresponding to the current CU, instead of partial template block samples corresponding to first top-left 16×16 unit;
  • Samples of the reference block template are generated by using MC with the block MV without rounding it to integer-pel precision.

2.1.12. Non-Adjacent Spatial Candidate

The non-adjacent spatial merge candidates as in JVET-L0399 are inserted after the TMVP in the regular merge candidate list. FIG. 25 shows spatial neighboring blocks used to derive the spatial merge candidates. The pattern of spatial merge candidates is shown in FIG. 25. The distances between non-adjacent spatial candidates and current coding block are based on the width and height of current coding block. The line buffer restriction is not applied.

2.1.13. Template Matching (TM)

Template matching (TM) is a decoder-side MV derivation method to refine the motion information of the current CU by finding the closest match between a template (i.e., top and/or left neighbouring blocks of the current CU) in the current picture and a block (i.e., same size to the template) in a reference picture. FIG. 26 is a schematic diagram 2600 illustrating the template matching that performs on a search area around initial MV. As illustrated in FIG. 26, a better MV is to be searched around the initial motion of the current CU within a [−8, +8]-pel search range. The template matching that was previously proposed in JVET-J0021 is adopted in this contribution with two modifications: search step size is determined based on Adaptive Motion Vector Resolution (AMVR) mode and TM can be cascaded with bilateral matching process in merge modes.

In AMVP mode, an MVP candidate is determined based on template matching error to select the one which reaches the minimum difference between the current block template and the reference block template, and then TM is performed only for this particular MVP candidate for MV refinement. TM refines this MVP candidate, starting from full-pel MVD precision (or 4-pel for 4-pel AMVR mode) within a [−8, +8]-pel search range by using iterative diamond search. The AMVP candidate may be further refined by using cross search with full-pel MVD precision (or 4-pel for 4-pel AMVR mode), followed sequentially by half-pel and quarter-pel ones depending on AMVR mode as specified in Table 3. This search process ensures that the MVP candidate still keeps the same MV precision as indicated by the AMVR mode after TM process.

TABLE 3
Search patterns of AMVR and merge mode with AMVR.
	AMVR mode
	4-	Full-	Half-	Quarter-	Merge mode
Search pattern	pel	pel	pel	pel	AltIF = 0	AltIF = 1
4-pel diamond	v
4-pel cross	v
Full-pel		v	v	v	v	v
diamond
Full-pel cross		v	v	v	v	v
Half-pel cross			v	v	v	v
Quarter-pel				v	v
cross
⅛-pel cross					v

In merge mode, similar search method is applied to the merge candidate indicated by the merge index. As Table 3 shows, TM may perform all the way down to ⅛-pel MVD precision or skipping those beyond half-pel MVD precision, depending on whether the alternative interpolation filter (that is used when AMVR is of half-pel mode) is used according to merged motion information. Besides, when TM mode is enabled, template matching may work as an independent process or an extra MV refinement process between block-based and subblock-based bilateral matching (BM) methods, depending on whether BM can be enabled or not according to its enabling condition check.

2.1.14. Multi-Pass Decoder-Side Motion Vector Refinement (mpDMVR)

A multi-pass decoder-side motion vector refinement is applied. In the first pass, bilateral matching (BM) is applied to the coding block. In the second pass, BM is applied to each 16×16 subblock within the coding block. In the third pass, MV in each 8×8 subblock is refined by applying bi-directional optical flow (BDOF). The refined MVs are stored for both spatial and temporal motion vector prediction.

2.1.14.1. First Pass—Block Based Bilateral Matching MV Refinement

In the first pass, a refined MV is derived by applying BM to a coding block. Similar to decoder-side motion vector refinement (DMVR), in bi-prediction operation, a refined MV is searched around the two initial MVs (MV0 and MV1) in the reference picture lists L0 and L1. The refined MVs (MV0_pass1 and MV1_pass1) are derived around the initiate MVs based on the minimum bilateral matching cost between the two reference blocks in L0 and L1.

BM performs local search to derive integer sample precision intDeltaMV. The local search applies a 3×3 square search pattern to loop through the search range [−sHor, sHor] in horizontal direction and [−sVer, sVer] in vertical direction, wherein, the values of sHor and sVer are determined by the block dimension, and the maximum value of sHor and sVer is 8.

The bilateral matching cost is calculated as: bilCost=mvDistanceCost+sadCost. When the block size cbW*cbH is greater than 64, MRSAD cost function is applied to remove the DC effect of distortion between reference blocks. When the bilCost at the center point of the 3×3 search pattern has the minimum cost, the intDeltaMV local search is terminated. Otherwise, the current minimum cost search point becomes the new center point of the 3×3 search pattern and continue to search for the minimum cost, until it reaches the end of the search range. The existing fractional sample refinement is further applied to derive the final deltaMV. The refined MVs after the first pass is then derived as:

$\mathrm{MV0\_pass1}=\mathrm{MV}0+\mathrm{deltaMV};\mathrm{MV1\_pass1}=\mathrm{MV}1-\mathrm{deltaMV}.$

2.1.14.2. Second Pass—Subblock Based Bilateral Matching MV Refinement

In the second pass, a refined MV is derived by applying BM to a 16×16 grid subblock. For each subblock, a refined MV is searched around the two MVs (MV0_pass1 and MV1_pass1), obtained on the first pass, in the reference picture list L0 and L1. The refined MVs (MV0_pass2(sbIdx2) and MV1_pass2(sbIdx2)) are derived based on the minimum bilateral matching cost between the two reference subblocks in L0 and L1.

For each subblock, BM performs full search to derive integer sample precision intDeltaMV. The full search has a search range [−sHor, sHor] in horizontal direction and [−sVer, sVer] in vertical direction, wherein, the values of sHor and sVer are determined by the block dimension, and the maximum value of sHor and sVer is 8.

The bilateral matching cost is calculated by applying a cost factor to the SATD cost between two reference subblocks, as: bilCost=satdCost*costFactor. The search area (2*sHor+1)*(2*sVer+1) is divided up to 5 diamond shape search regions shown in in the diagram 2700 of FIG. 27. Each search region is assigned a costFactor, which is determined by the distance (intDeltaMV) between each search point and the starting MV, and each diamond region is processed in the order starting from the center of the search area. In each region, the search points are processed in the raster scan order starting from the top left going to the bottom right corner of the region. When the minimum bilCost within the current search region is less than a threshold equal to sbW*sbH, the int-pel full search is terminated, otherwise, the int-pel full search continues to the next search region until all search points are examined.

The existing VVC DMVR fractional sample refinement is further applied to derive the final deltaMV (sbIdx2). The refined MVs at second pass is then derived as:

$\mathrm{MV0\_pass2}\left(\mathrm{sbIdx}2\right)=\mathrm{MV0\_pass1}+\mathrm{deltaMV}\left(\mathrm{sbIdx}2\right),\mathrm{MV1\_pass2}\left(\mathrm{sbIdx}2\right)=\mathrm{MV1\_pass1}-\mathrm{deltaMV}\left(\mathrm{sbIdx}2\right).$

2.1.14.3. Third Pass—Subblock Based Bi-Directional Optical Flow MV Refinement

In the third pass, a refined MV is derived by applying BDOF to an 8×8 grid subblock. For each 8×8 subblock, BDOF refinement is applied to derive scaled Vx and Vy without clipping starting from the refined MV of the parent subblock of the second pass. The derived bioM_V(Vx, Vy) is rounded to 1/16 sample precision and clipped between −32 and 32.

The refined MVs (MV0_pass3(sbIdx3) and MV1_pass3(sbIdx3)) at third pass are derived as:

$\mathrm{MV0\_pass3}\left(\mathrm{sbIdx}3\right)=\mathrm{MV0\_pass2}\left(\mathrm{sbIdx}2\right)+\mathrm{bioMv},\mathrm{MV1\_pass3}\left(\mathrm{sbIdx}3\right)=\mathrm{MV0\_pass2}\left(\mathrm{sbIdx}2\right)-\mathrm{bioMv}.$

2.1.15. OBMC

When OBMC is applied, top and left boundary pixels of a CU are refined using neighboring block's motion information with a weighted prediction as described in JVET-L0101.

Conditions of not applying OBMC are as follows:

• • When OBMC is disabled at SPS level;
  • When current block has intra mode or IBC mode;
  • When current block applies LIC;
  • When current luma block area is smaller or equal to 32.

A subblock-boundary OBMC is performed by applying the same blending to the top, left, bottom, and right subblock boundary pixels using neighboring subblocks' motion information. It is enabled for the subblock based coding tools:

• • Affine AMVP modes;
  • Affine merge modes and subblock-based temporal motion vector prediction (SbTMVP);
  • Subblock-based bilateral matching.

2.1.16. Sample-Based BDOF

In the sample-based BDOF, instead of deriving motion refinement (Vx, Vy) on a block basis, it is performed per sample.

The coding block is divided into 8×8 subblocks. For each subblock, whether to apply BDOF or not is determined by checking the SAD between the two reference subblocks against a threshold. If decided to apply BDOF to a subblock, for every sample in the subblock, a sliding 5×5 window is used and the existing BDOF process is applied for every sliding window to derive Vx and Vy. The derived motion refinement (Vx, Vy) is applied to adjust the bi-predicted sample value for the center sample of the window.

2.1.17. Interpolation

The 8-tap interpolation filter used in VVC is replaced with a 12-tap filter. The interpolation filter is derived from the sinc function of which the frequency response is cut off at Nyquist frequency, and cropped by a cosine window function. Table 4 gives the filter coefficients of all 16 phases. FIG. 28 shows frequency responses of the interpolation filter and the VVC interpolation filter at half-pel phase. It compares the frequency responses of the interpolation filters with the VVC interpolation filter, all at half-pel phase.

TABLE 4
Filter coefficients of the 12-tap interpolation filter
1/16	−1	2	−3	6	−14	254	16	−7	4	−2	1	0
2/16	−1	3	−7	12	−26	249	35	−15	8	−4	2	0
3/16	−2	5	−9	17	−36	241	54	−22	12	−6	3	−1
4/16	−2	5	−11	21	−43	230	75	−29	15	−8	4	−1
5/16	−2	6	−13	24	−48	216	97	−36	19	−10	4	−1
6/16	−2	7	−14	25	−51	200	119	−42	22	−12	5	−1
7/16	−2	7	−14	26	−51	181	140	−46	24	−13	6	−2
8/16	−2	6	−13	25	−50	162	162	−50	25	−13	6	−2
9/16	−2	6	−13	24	−46	140	181	−51	26	−14	7	−2
10/16	−1	5	−12	22	−42	119	200	−51	25	−14	7	−2
11/16	−1	4	−10	19	−36	97	216	−48	24	−13	6	−2
12/16	−1	4	−8	15	−29	75	230	−43	21	−11	5	−2
13/16	−1	3	−6	12	−22	54	241	−36	17	−9	5	−2
14/16	0	2	−4	8	−15	35	249	−26	12	−7	3	−1
15/16	0	1	−2	4	−7	16	254	−14	6	−3	2	−1

2.1.18. Multi-Hypothesis Prediction (MHP)

In the multi-hypothesis inter prediction mode (JVET-M0425), one or more additional motion-compensated prediction signals are signaled, in addition to the conventional bi prediction signal. The resulting overall prediction signal is obtained by sample-wise weighted superposition. With the bi prediction signal p_bi and the first additional inter prediction signal/hypothesis h₃, the resulting prediction signal p₃ is obtained as follows:

$p_3=\left(1-\alpha \right)p_{\mathrm{bi}}+\alpha h_3.$

The weighting factor α is specified by the new syntax element add_hyp_weight_idx, according to the following mapping.

add_hyp_weight_idx α
0                  ¼
1                  −⅛

Analogously to above, more than one additional prediction signal can be used. The resulting overall prediction signal is accumulated iteratively with each additional prediction signal.

$p_{n+1}=\left(1-{\alpha }_{n+1}\right)p_n+{\alpha }_{n+1}{h}_{n+1}$

The resulting overall prediction signal is obtained as the last p_n (i.e., the p_n having the largest index n). Within this EE, up to two additional prediction signals can be used (i.e., n is limited to 2).

The motion parameters of each additional prediction hypothesis can be signaled either explicitly by specifying the reference index, the motion vector predictor index, and the motion vector difference, or implicitly by specifying a merge index. A separate multi-hypothesis merge flag distinguishes between these two signalling modes.

For inter AMVP mode, MHP is only applied if non-equal weight in BCW is selected in bi-prediction mode.

Combination of MHP and BDOF is possible, however the BDOF is only applied to the bi-prediction signal part of the prediction signal (i.e., the ordinary first two hypotheses).

2.1.19. Adaptive Reordering of Merge Candidates with Template Matching (ARMC-TM)

The merge candidates are adaptively reordered with template matching (TM). The reordering method is applied to regular merge mode, template matching (TM) merge mode, and affine merge mode (excluding the SbTMVP candidate). For the TM merge mode, merge candidates are reordered before the refinement process.

After a merge candidate list is constructed, merge candidates are divided into several subgroups. The subgroup size is set to 5 for regular merge mode and TM merge mode. The subgroup size is set to 3 for affine merge mode. Merge candidates in each subgroup are reordered ascendingly according to cost values based on template matching. For simplification, merge candidates in the last but not the first subgroup are not reordered.

The template matching cost of a merge candidate is measured by the sum of absolute differences (SAD) between samples of a template of the current block and their corresponding reference samples. The template comprises a set of reconstructed samples neighboring to the current block. Reference samples of the template are located by the motion information of the merge candidate.

FIG. 29 shows a schematic diagram 2900 of template and reference samples of the template in reference list 0 and reference list 1. When a merge candidate utilizes bi-directional prediction, the reference samples of the template of the merge candidate are also generated by bi-prediction as shown in FIG. 29. When a merge candidate utilizes bi-directional prediction, the reference samples of the template of the merge candidate are denoted by RT and RT may be generated from RT₀ which are derived from a reference picture 2920 in reference picture list 0 and RT₁ derived from a reference picture 2930 in reference picture list 1. In one example, RT₀ includes a set of reference samples on the reference picture 2920 of the current block in the current picture 2910 indicated by the reference index of the merge candidate referring to a reference picture in reference list 0 with the MV of the merge candidate referring to reference list 0, In one example, RT₁ includes a set of reference samples on the reference picture 2930 of the current block indicated by the reference index of the merge candidate referring to a reference picture in reference list 1 with the MV of the merge candidate referring to reference list 1.

For subblock-based merge candidates with subblock size equal to Wsub x Hsub, the above template comprises several sub-templates with the size of Wsub x 1, and the left template comprises several sub-templates with the size of 1×Hsub. FIG. 30 shows template and reference samples of the template for block with sub-block motion using the motion information of the subblocks of the current block. As shown in FIG. 30, the motion information of the subblocks in the first row and the first column of current block is used to derive the reference samples of each sub-template.

2.1.20. Geometric Partitioning Mode (GPM) with Merge Motion Vector Differences (MMVD)

GPM in VVC is extended by applying motion vector refinement on top of the existing GPM uni-directional MVs. A flag is first signalled for a GPM CU, to specify whether this mode is used. If the mode is used, each geometric partition of a GPM CU can further decide whether to signal MVD or not. If MVD is signalled for a geometric partition, after a GPM merge candidate is selected, the motion of the partition is further refined by the signalled MVDs information. All other procedures are kept the same as in GPM.

The MVD is signaled as a pair of distance and direction, similar as in MMVD. There are nine candidate distances (¼-pel, ½-pel, 1-pel, 2-pel, 3-pel, 4-pel, 6-pel, 8-pel, 16-pel), and eight candidate directions (four horizontal/vertical directions and four diagonal directions) involved in GPM with MMVD (GPM-MMVD). In addition, when pic_fpel_mmvd_enabled_flag is equal to 1, the MVD is left shifted by 2 as in MMVD.

2.1.21. Geometric Partitioning Mode (GPM) with Template Matching (TM)

Template matching is applied to GPM. When GPM mode is enabled for a CU, a CU-level flag is signaled to indicate whether TM is applied to both geometric partitions. Motion information for each geometric partition is refined using TM. When TM is chosen, a template is constructed using left, above or left and above neighboring samples according to partition angle, as shown in Table 5. The motion is then refined by minimizing the difference between the current template and the template in the reference picture using the same search pattern of merge mode with half-pel interpolation filter disabled.

TABLE 5
Template for the 1st and 2nd geometric partitions, where A represents using above samples,
L represents using left samples, and L + A represents using both left and above samples.
	Partition angle
	0	2	3	4	5	8	11	12	13	14
1st partition	A	A	A	A	L + A	L + A	L + A	L + A	A	A
2nd partition	L + A	L + A	L + A	L	L	L	L	L + A	L + A	L + A
	Partition angle
	16	18	19	20	21	24	27	28	29	30
1st partition	A	A	A	A	L + A	L + A	L + A	L + A	A	A
2nd partition	L + A	L + A	L + A	L	L	L	L	L + A	L + A	L + A

A GPM candidate list is constructed as follows:

• • 1. Interleaved List-0 MV candidates and List-1 MV candidates are derived directly from the regular merge candidate list, where List-0 MV candidates are higher priority than List-1 MV candidates. A pruning method with an adaptive threshold based on the current CU size is applied to remove redundant MV candidates.
  • 2. Interleaved List-1 MV candidates and List-0 MV candidates are further derived directly from the regular merge candidate list, where List-1 MV candidates are higher priority than List-0 MV candidates. The same pruning method with the adaptive threshold is also applied to remove redundant MV candidates.
  • 3. Zero MV candidates are padded until the GPM candidate list is full.

The GPM-MMVD and GPM-TM are exclusively enabled to one GPM CU. This is done by firstly signaling the GPM-MMVD syntax. When both two GPM-MMVD control flags are equal to false (i.e., the GPM-MMVD are disabled for two GPM partitions), the GPM-TM flag is signaled to indicate whether the template matching is applied to the two GPM partitions. Otherwise (at least one GPM-MMVD flag is equal to true), the value of the GPM-TM flag is inferred to be false.

2.1.22. GPM Woth Inter and Intra Prediction (GPM Inter-Intra)

With the GPM inter-intra, pre-defined intra prediction modes against geometric partitioning line can be selected in addition to merge candidates for each non-rectangular split region in the GPM-applied CU. In the proposed method, whether intra or inter prediction mode is determined for each GPM-separated region with a flag from the encoder. When the inter prediction mode, a uni-prediction signal is generated by MVs from the merge candidate list. On the other hand, when the intra prediction mode, a uni-prediction signal is generated from the neighboring pixels for the intra prediction mode specified by an index from the encoder. The variation of the possible intra prediction modes is restricted by the geometric shapes. Finally, the two uni-prediction signals are blended with the same way of ordinary GPM.

2.1.23. Adaptive Decoder Side Motion Vector Refinement (Adaptive DMVR)

Adaptive decoder side motion vector refinement method consists of the two new merge modes introduced to refine MV only in one direction, either L0 or L1, of the bi prediction for the merge candidates that meet the DMVR conditions. The multi-pass DMVR process is applied for the selected merge candidate to refine the motion vectors, however either MVD0 or MVD1 is set to zero in the 1st pass (i.e. PU level) DMVR.

Like the regular merge mode, merge candidates for the proposed merge modes are derived from the spatial neighboring coded blocks, TMVPs, non-adjacent blocks, HMVPs, and pair-wise candidate. The difference is that only those meet DMVR conditions are added into the candidate list. The same merge candidate list is used by the two proposed merge modes and merge index is coded as in regular merge mode.

2.1.24. Bilateral Matching AMVP-MERGE Mode (AMVP-MERGE)

In the AMVP-merge mode, the bi-directional predictor is composed of an AMVP predictor in one direction and a merge predictor in the other direction.

AMVP part of the proposed mode is signaled as a regular uni-directional AMVP, i.e. reference index and MVD are signaled, and it has a derived MVP index if template matching is used (TM_AMVP) or MVP index is signaled when template matching is disabled. Merge index is not signalled, and merge predictor is selected from the candidate list with smallest template or bilateral matching cost.

When the selected merge predictor and the AMVP predictor satisfy DMVR condition, which is there is at least one reference picture from the past and one reference picture from the future relatively to the current picture and the distances from two reference pictures to the current picture are the same, the bilateral matching MV refinement is applied for the merge MV candidate and AMVP MVP as a starting point. Otherwise, if template matching functionality is enabled, template matching MV refinement is applied to the merge predictor or the AMVP predictor which has a higher template matching cost.

The third pass which is 8×8 sub-PU BDOF refinement of the multi-pass DMVR is enabled to AMVP-merge mode coded block.

2.2. Out-of-Boundary Issue in the Bi-Directional Motion Compensation

In ECM-3.0, due to the reference samples padding of the reference picture, it is valid for an inter CU to have a reference block located outside the reference picture partially or totally as illustrated in FIG. 31. FIG. 31 shows bi-directional MC block in current ECM. In the FIG. 31, bi-directional motion compensation is performed to generate the inter prediction block of the current block. In this example, list0 reference block is partially out-of-boundary (OOB) while list 1 reference block is fully inside the reference picture. However, the OOB part of a motion compensated blocks usually provides less prediction efficiency because the OOB part is simply repetitive samples derived from the boundary samples within the reference picture. Yet, in the ECM, the less efficiency of the OOB part is not considered for the inter prediction.

3. Combined Inter Intra Prediction and Transform

Interaction Between Adaptive DMVR and Other Tools

• • 1. In one example, adaptive DMVR may be applied to other coding tools beyond adaptive DMVR itself.
    • a. For example, the adaptive DMVR may refer to a DMVR method that fix the motion vector in one prediction direction (such as LX), and then refine the motion vector in the other direction (such as L(1−X)), wherein the motion vector is bi-directional predicted.
    • b. For example, the motion vector in the merge candidate list may be further refined by adaptive DMVR.
    • c. For example, the motion vector of CIIP may be further refined by adaptive DMVR.
    • d. For example, the motion vector of GPM may be further refined by adaptive DMVR.
    • e. For example, the motion vector of MMVD may be further refined by adaptive DMVR.
    • f. For example, the motion vectors of subblock merge (e.g., Affine merge, subblock TMVP, etc.) may be further refined by adaptive DMVR.
    • g. For example, the motion vector of AMVP inter may be further refined by adaptive DMVR.
    • h. For example, the motion vector of SMVD may be further refined by adaptive DMVR.
    • i. For example, the motion vectors of subblock AMVP (e.g., Affine AMVP, etc.) may be further refined by adaptive DMVR.
    • j. For example, when adaptive DMVR is applied to other coding tools, the signalling of the usage of the adaptive DMVR to the block may be not necessary.
      • i. For example, if certain conditions are met, the adaptive DMVR is applied to the block without signalling.
  • 2. For example, whether to use adaptive DMVR or regular DMVR (and/or BDMVR) at a video unit level may be signalled in the bitstream.
    • a. For example, the video unit level may be a level of sequence/picture/slice/tile group/tile/sub-picture/PB/TB/CB/PU/TU/CU/VPDU/CTU/CTU row/other kinds of region contain more than one sample or pixel.

Interaction Between AMVP-MERGE and Other Tools

• • 3. In one example, AMVP-MERGE may be applied to other coding tools beyond AMVP-MERGE itself.
    • a. For example, the AMVP-MERGE may refer to an inter coding method that generates motion vector based on LX motion of an AMVP candidate and L(1−X) motion of a MERGE candidate.
    • b. For example, the motion vector generated by AMVP-MERGE may be used in CIIP.
    • c. For example, the motion vector generated by AMVP-MERGE may be used in GPM.
    • d. For example, the motion vector generated by AMVP-MERGE may be used in MMVD.
    • e. For example, the motion vector generated by AMVP-MERGE may be used in subblock merge (e.g., Affine merge, subblock TMVP, etc.).
    • f. For example, the motion vector generated by AMVP-MERGE may be used in GPM.
    • g. For example, the motion vector generated by AMVP-MERGE may be used in AMVP inter prediction (e.g., as an AMVP candidate for regular AMVP inter coding).
    • h. For example, the motion vector generated by AMVP-MERGE may be used in SMVD.
    • i. For example, when the motion vector generated by AMVP-MERGE use for other coding tools, it may be perceived as a motion candidate.

4. Decoder Side MHP Data Derivation_v0

• • 1. In one example, prediction of a hypothesis may be generated based on virtual constructed motion data, which is not identical to any motion data in the original MHP candidate lists.
    • a. For example, prediction of an additional hypothesis may be generated based on virtual constructed motion data.
    • b. For example, the virtual constructed motion may be based on an AMVP-MERGE candidate list.
      • i. For example, the AMVP-MERGE candidate list may be generated based on at least one AMVP motion candidate information and at least one MERGE motion candidate information.
      • ii. For example, the AMVP-MERGE candidate list may be reordered.
      • iii. For example, the motion candidates in the AMVP-MERGE motion candidate list may be refined.
    • c. For example, the virtual constructed motion may be based on a bi-directional virtual motion candidate.
      • i. For example, the bi-directional virtual motion candidate may be generated based on a certain AMVP candidate list.
      • ii. For example, the bi-directional virtual motion candidate may be generated based on a certain MERGE candidate list, such as regular merge candidate list, or MMVD candidate list, or TM merge list, or GEO merge list, or CIIP merge list, or subblock merge list, etc.
      • iii. For example, the bi-directional virtual motion candidate may be generated based on a reordered motion candidate list.
      • iv. For example, the bi-directional virtual motion candidate may be further refined.
    • d. For example, the virtual constructed motion may be based on a uni-directional virtual motion candidate.
      • i. For example, the uni-directional virtual motion candidate may be generated based on a certain AMVP candidate list.
      • ii. For example, the uni-directional virtual motion candidate may be generated based on a certain MERGE candidate list, such as regular merge candidate list, or MMVD candidate list, or TM merge list, or GEO merge list, or CIIP merge list, or subblock merge list, etc.
      • iii. For example, the uni-directional virtual motion candidate may be generated based on a reordered motion candidate list.
      • iv. For example, the uni-directional virtual motion candidate may be further refined.
    • e. For example, prediction of a base hypothesis may be generated based on virtual constructed motion data.
      • i. For example, an AMVP-MERGE coded block may be perceived as a base hypothesis, and at least one additional hypothesis may be applied to it. Finally, the final prediction of the AMVP-MERGE coded block is generated by blending the base hypothesis and additional hypotheses together.

5. Problems

There are several issues in the existing video coding techniques, which would be further improved for higher coding gain.

• • 1. In ECM-3.0, the existing AMVP-MERGE mode only supports ¼-pel precision MVD signalling, wherein the AMVR is not supported. This may be further improved.
  • 2. In ECM-3.0, the AMVP-MERGE prediction cannot be used as one of the MHP hypotheses, which may be suboptimal.
  • 3. In ECM-3.0, the existing AMVP-MERGE mode generates new prediction candidate list, which contains several bi-prediction motion candidates. Such motion candidates could be used to other coding modes for higher coding efficiency.
  • 4. In ECM-3.0, the existing adaptive DMVR mode generates new prediction candidate lists, which contains bi-prediction motion candidates only. Such motion candidates could be used to other coding modes for higher coding efficiency.
  • 5. In ECM-3.0, the AMVP-MERGE mode uses DMVR and TM to determine the (L0, L1) prediction candidate pair and refine the prediction candidate. However, this coding tool is not controlled by DMVR/TM/DMVR controlling flag, which may be further designed.
  • 6. In ECM-3.0, the LIC mode is designed based on a hypothesis that there is a strong linear correlation between the neighbor samples of current block and temporally collocated block. To be more specific, the LIC mode uses a least square model with two parameters (such as a and b) to map the neighbor samples of current block and the neighbor samples of temporally collocated block. However, this hypothesis may not stand well especially when the neighbors and references are noisy. In such cases, the actual prediction may fail and the process results in suboptimal coding efficiency.
  • 7. In ECM-3.0, the LIC for AMVP is signaled associated with the video unit, and the LIC for MERGE is inherited from neighboring video units. With decoder derived methods, the signalling and determination of LIC may be changed.
  • 8. In ECM-3.0, for bi-prediction, MHP prediction, GPM prediction, etc., a hypothetic prediction block is generated by weighting multiple hypothetical predictions. Due to the repetitive based picture boundary padding, the weighting process is conducted anyway, no matter whether there is any prediction sample in any hypothetical prediction is out-of-boundary. However, how to handle the out-of-boundary prediction sample may be further considered.
  • 9. In ECM-5.0, non-average weighting (e.g., BCW) is not applied to AMVP-MERGE mode and MHP mode, which may not be efficient.
  • 10. In ECM-5.0, the AMVP-MERGE mode is only allowed for pictures have true-bi-direction reference pictures (both forward and backward), and BCW is not allowed (the weight for the two reference pictures is by default set to average weighting), which may not be efficient.
  • 11. In ECM-5.0, DMVR based method is not used for sbTMVP and CPMVs for either Affine AMVP or Affine MERGE, which may not be efficient.

6. Embodiments of the Present Disclosure

The detailed embodiments below should be considered as examples to explain general concepts. These embodiments should not be interpreted in a narrow way. Furthermore, these embodiments can be combined in any manner.

The terms ‘video unit’ or ‘coding unit’ or ‘block’ may represent a coding tree block (CTB), a coding tree unit (CTU), a coding block (CB), a CU, a PU, a TU, a PB, a TB.

In the present disclosure, regarding “a block coded with mode N”, here “mode N” may be a prediction mode (e.g., MODE_INTRA, MODE_INTER, MODE_PLT, MODE_IBC, and etc.), or a coding technique (e.g., AMVP, Merge, SMVD, BDOF, PROF, DMVR, AMVR, TM, Affine, CIIP, GPM, GEO, TPM, MMVD, BCW, HMVP, SbTMVP, and etc.).

In the present disclosure, “a two-direction-DMVR” may indicate regular DMVR which refines both L0 and L1 motion vectors, as elaborated in section 2.1.14. Moreover, “a one-direction-DMVR” may indicate a DMVR process which refines either L0 or L1 motion vector only, such as adaptive DMVR elaborated in section 2.1.23. In the following discussion, LIC parameters may refer to the two parameters (such as a slope parameter “a” and a bias parameter “b”) derived based on a linear model, which is used to map the neighboring samples of current block and the neighboring samples of temporally collocated block (e.g., temporally collocated block may be pointed by the motion vector or a rounded motion vector of the current block). Furthermore, the LIC parameters may be used to estimate the prediction values of samples inside the current video unit.

In the following discussion, the AMVP mode may be regular AMVP mode, affine-AMVP mode, and/or SMVD mode, and/or AMVP-MERGE mode.

It is noted that the terminologies mentioned below are not limited to the specific ones defined in existing standards. Any variance of the coding tool is also applicable.

• • 6.1. To tackle the first problem, the following methods are proposed:
    • a. It is proposed to support multiple MVD/MV precisions for the AMVP-MERGE mode.
      • i. In one example, the supported precision candidates may be the same as those used for the normal AMVP mode, e.g., for non-affine case, half-pel, ¼-pel, 1-pel, 4-pel are applied; for affine case, 1/16-pel, ⅛-pel, ¼-pel are applied.
      • ii. In another example, at least one of the supported precision candidates may be different from that used for normal AMVP mode.
    • b. For example, the motion vector difference (e.g., MVD) of the AMVP side of an AMVP-MERGE mode may be coded with other precisions in addition to ¼ pel resolution.
      • i. For example, the MVD value may be of 4-pel precision.
      • ii. For example, the MVD value may be of 1-pel precision.
      • iii. For example, the MVD value may be of half-pel precision.
      • iv. For example, the MVD value may be of ⅛-pel precision.
      • v. For example, the MVD value may be of 1/16-pel precision.
    • c. For example, a second interpolation filter (e.g., Y-tap filter such as Y=6) in addition to a first interpolation filter (e.g., X-tap filter such as X=8 or 12) may be used for motion compensation.
      • i. For example, when to use the second interpolation filter for an AMVP-MERGE coded block may be dependent on the MVD prediction and/or the final MV precision.
      • ii. For example, when the signalled MVD is ½-pel precision and the final MV is also ½-pel precision, the second interpolation filter may be used. Otherwise, the first interpolation filter may be used.
      • iii. In one example, the second interpolation filter used in the AMVP-MERGE mode may be the same as that used for the normal AMVP mode (e.g., the one used for the ½-pel precision).
      • iv. Alternatively, the second interpolation filter used in the AMVP-MERGE mode may be different from that used for the normal AMVP mode (e.g., the one used for the ½-pel precision).
    • d. Alternatively, half-pel MVD precision may not be allowed to be used for AMVP-MERGE mode.
    • e. For example, at least one syntax element (e.g., flag and/or parameter index) may be signalled at block level to indicate which motion vector precision is used to code/signal the MVD value and/or MV of an AMVP-MERGE mode coded video unit.
      • i. Furthermore, the signalled MVD at any resolution may be converted to an internal precision (e.g., 1/16 pel resolution) for latter procedures such as motion compensation and etc.
  • 6.2. To tackle the second problem, the following methods are proposed:
    • a. In one example, the prediction unit generated based on AMVP-MERGE mode may be used as an MHP hypotheses.
      • i. For example, the AMVP-MERGE mode prediction block may be used as the base hypothesis of an MHP block.
      • ii. For example, syntax elements/structure related to MHP hypothesis data (e.g., whether there is additional hypothesis associated with the AMVP-MERGE coded video unit; if there is, AMVP or MERGE based MHP additional hypothesis is used, etc.) may be signalled in the bitstream right after a video unit is identified to be an AMVP-MERGE mode coded video unit.
      • iii. For example, an AMVP-MERGE prediction block may be used as an additional hypothesis of an MHP block.
      • iv. For example, additional hypothesis of an MHP block may be generated based on an AMVP-MERGE motion candidate.
        • 1) For example, in such case, syntax elements related to this AMVP-MERGE motion candidate (e.g., which side is AMVP/MERGE coded, reference index of the AMVP side, MVD value for the AMVP side, and/or MVP index of the AMVP side) may be signalled in the multiple hypothesis data structure.
      • v. For example, whether LIC is used for an AMVP-MERGE coded additional hypothesis may be inherited from the usage of LIC of the base hypothesis.
        • 1) For example, if the base hypothesis is LIC coded, the AMVP-MERGE coded additional hypothesis is LIC coded without signalling the usage of the LIC of such additional hypothesis.
        • 2) Alternatively, if the base hypothesis is NOT LIC coded, the AMVP-MERGE coded additional hypothesis is NOT LIC coded without signalling the usage of the LIC of such additional hypothesis.
      • vi. For example, whether LIC is used for an AMVP-MERGE coded hypothesis (base and/or additional hypothesis) may be dependent on the usage of LIC of the merge side of the AMVP-MERGE candidate.
        • 1) For example, given that a certain AMVP-MERGE candidate is composed of a uni-merge candidate in one side (L0 or L1) and a uni-AMVP candidate in the other side (L1 or L0),
        •  a. For example, the usage of LIC for such AMVP-MERGE candidate may be inherited from its merge candidate.
        •  b. For example, if the merge candidate uses LIC, then LIC may be used for the AMVP-MERGE coded prediction block.
    • vii. Alternatively, the usage of LIC for an AMVP-MERGE coded hypothesis may be signalling in the bitstream.
  • 6.3. To tackle the third problem, the following methods are proposed:
    • a. For example, an AMVP-MERGE candidate may be used for one or more of the following coding modes.
      • i. CIIP mode (and/or its variants, e.g., regular CIIP, CIIP-PDPC, CIIP-TM, etc).
      • ii. MMVD mode (and/or its variants, e.g., regular MMVD, affine MMVD, etc).
      • iii. MHP mode (and/or its variants, e.g., MHP base hypothesis, and/or MHP additional hypothesis, etc).
      • iv. GPM mode (and/or its variants, e.g., regular GPM, GPM-TM, GPM-MMVD, GPM-Inter-Intra, etc).
    • b. For example, an AMVP-MERGE candidate may be firstly refined by a decoder side motion vector refinement process (e.g., TM or DMVR based motion vector refinements), then used for a second coding mode (e.g., such as listed in the above sub-bullet).
    • c. For example, AMVP-MERGE candidates may be inserted to another candidate list.
      • i. For example, AMVP-MERGE candidates may be inserted to the regular merge candidate list.
        • 1) For example, AMVP-MERGE candidate may be used for regular merge mode and/or its variants.
        • 2) For example, AMVP-MERGE candidate may be used for MMVD mode and/or its variants.
        • 3) For example, AMVP-MERGE candidate may be used for CIIP mode and/or its variants.
        • 4) For example, AMVP-MERGE candidate may be used for MHP mode and/or its variants.
        • 5) For example, AMVP-MERGE candidate may be used for GPM mode and/or its variants.
      • ii. For example, AMVP-MERGE candidates may be inserted to the regular TM merge candidate list.
        • 1) For example, AMVP-MERGE candidate may be used for regular TM merge mode and/or its variants.
      • iii. Additionally, furthermore, the AMVP-MERGE candidate may be inserted to another prediction list after the original candidates of that prediction list.
    • d. For example, AMVP-MERGE candidates may be reordered based on a decoder derived method (through TM or DMVR based cost evaluation), then M of them would be selected to be added to a second candidate list (such as regular merge candidate list, regular TM merge candidate list).
      • i. Additionally, furthermore, more than one AMVP-MERGE candidates may be reordered together.
      • ii. Alternatively, a first candidate from the first AMVP-MERGE prediction list and a second candidate from the second prediction list may be reordered together.
    • e. For example, once an AMVP-MERGE candidate is used for a coding block, extra syntax elements may be signalled specifying the prediction direction (L0 or L1) of the AMVP part, and/or the reference picture index of the selected AMVP candidate, and/or the motion vector predictor index of the selected AMVP candidate, and/or the motion vector difference associated with the AMVP motion vector predictor.
      • i. Alternatively, the motion vector predictor index of the AMVP side of an AMVP-MERGE candidate may not be signalled (e.g., it may be selected by a decoder side method through TM or DMVR based cost evaluation).
    • f. Alternatively, once an AMVP-MERGE candidate is used, extra syntax element(s) may be signalled specifying the predictor index of the merge candidate.
      • i. Alternatively, the motion vector predictor index of the merge side of an AMVP-MERGE candidate may not be signalled (e.g., it may be selected by a decoder side method through TM or DMVR based cost evaluation).
    • g. In one example, the merge part of AMVP-MERGE mode may be firstly refined by a decoder side motion vector refinement process (such as TM or DMVR) before generating a AMVP-MERGE candidate.
    • h. In one example, the AMVP part of AMVP-MERGE mode may be firstly refined by a decoder side motion vector refinement process (such as TM or DMVR) before generating a AMVP-MERGE candidate.
  • 6.4. To tackle the fourth problem, the following methods are proposed:
    • a. For example, an adaptive DMVR motion candidate may be used for one or more of the following coding modes.
      • i. CIIP mode (and/or its variants, e.g., regular CIIP, CIIP-PDPC, CHIP-TM, etc).
      • ii. MMVD mode (and/or its variants, e.g., regular MMVD, affine MMVD, etc).
      • iii. MHP mode (and/or its variants, e.g., MHP base hypothesis, and/or MHP additional hypothesis, etc).
      • iv. GPM mode (and/or its variants, e.g., regular GPM, GPM-TM, GPM-MMVD, GPM-Inter-Intra, etc).
      • v. AMVP mode (and/or its variants, e.g., regular AMVP, SMVD, AMVP-MERGE, affine AMVP, etc).
    • b. For example, an adaptive DMVR motion candidate may be firstly refined by a decoder side motion vector refinement process (e.g., TM or DMVR based motion vector refinements), then used for a second coding mode (e.g., such as listed in the above sub-bullet).
    • c. For example, when an adaptive DMVR motion candidate is used for an AMVP mode,
      • i. The DMVR motion candidate may refer to a motion vector pair contains both L0 and L1 motion vectors, and/or, both L0 and L1 reference picture indexes.
      • ii. It may be used as an MVP candidate.
      • iii. The candidate index of the DMVR motion candidate (rather than the L0 and L1 motion vector predictor indexes, and L0 and L1 reference picture indexes) may be signalled for the AMVP mode in the bitstream.
      • iv. The reference picture index of one prediction direction (L0 or L1) may be signalled for the AMVP mode, and the reference picture index of the other prediction direction (L1 or L0) may be inferred (e.g., according to the DMVR condition).
    • d. For example, adaptive DMVR motion candidates may be inserted to another candidate list.
      • i. For example, adaptive DMVR motion candidates may be inserted to the regular merge candidate list.
        • 1) For example, adaptive DMVR motion candidate may be used for regular merge mode and/or its variants.
        • 2) For example, adaptive DMVR motion candidate may be used for MMVD mode and/or its variants.
        • 3) For example, adaptive DMVR motion candidate may be used for CIIP mode and/or its variants.
        • 4) For example, adaptive DMVR motion candidate may be used for MHP mode and/or its variants.
        • 5) For example, adaptive DMVR motion candidate may be used for GPM mode and/or its variants.
      • ii. For example, adaptive DMVR motion candidates may be inserted to the regular TM merge candidate list.
        • 1) For example, adaptive DMVR motion candidate may be used for regular TM merge mode and/or its variants.
      • iii. Additionally, furthermore, the adaptive DMVR motion candidate may be inserted to another prediction list after the original candidates of that prediction list.
    • e. For example, adaptive DMVR motion candidates may be reordered based on a decoder derived method (through TM or DMVR based cost evaluation), then M of them would be selected to be added to a second candidate list (such as regular merge candidate list, regular TM merge candidate list).
      • i. Additionally, furthermore, more than one adaptive DMVR motion candidates may be reordered together.
      • ii. Alternatively, a first candidate from the first adaptive DMVR merge list and a second candidate from the second prediction list may be reordered together.
  • 6.5. To tackle the fifth problem, the following methods are proposed:
    • a. For example, the enabling/disabling of a first coding tool may be controlled by a second syntax element signalled at a syntax level higher than coding block level.
      • i. For example, the syntax level higher than coding block level may indicate sequence level/group of pictures level/picture level/slice level/tile group level, such as in sequence header/picture header/SPS/VPS/DPS/DCI/PPS/APS/slice header/tile group header.
      • ii. For example, a single coding tool may be controlled by the second syntax element.
      • iii. For example, more than one coding tool may be controlled by the second syntax element.
      • iv. For example, the first coding tool may be a prediction mode that utilizes a decoder side motion derivation method (such as AMVP-MERGE mode, etc.).
      • v. For example, the second syntax elements may be an SPS/PPS/PH/SH flag specifying whether the decoder side motion derivation methods (such as DMVR and TM and etc.) is allowed or not.
      • vi. For example, the second syntax elements may be an SPS/PPS/PH/SH flag specifying whether the decoder side motion vector refinement (such as DMVR and etc.) is allowed or not.
      • vii. For example, the second syntax elements may be an SPS/PPS/PH/SH flag specifying whether the decoder side template matching (such as TM, and/or inter TM and etc.) is allowed or not.
  • 6.6. About the LIC parameter derivation (e.g., as illustrated in the sixth problem), suppose the linear model used for an LIC coded block is based on at least two parameters: a slope parameter “a” and a bias parameter “b”, and the relationship between the neighboring sample of the current block and the neighboring sample of the temporally collocated block can be represented by “reconTempNeigh=a*reconCurNeigh+b”, in which “reconTempNeigh” denotes the reconstruction/prediction value of the neighboring sample of the temporally collocated block, and “reconCurNeigh” denotes the reconstruction/prediction value of the neighboring sample of the current block, the following methods are proposed:
    • a. For example, at least one adjustment factor may be applied to adjust at least one LIC parameter for an LIC model derivation.
      • a. For example, the adjustment factor may be signalled/present in the bitstream.
      • b. For example, the adjustment factor may be derived at both encoder and decoder.
    • b. For example, at least one syntax element (e.g., a syntax parameter, or index, or variable, or offset value, or integer) may be signalled at a video unit level for calculating at least one LIC parameter of at least one LIC model.
      • a. For example, the video unit level may be PU/CU/block level.
        • i. For example, furthermore, the video unit level may be sequence/group of pictures/picture/slice/tile group/PB/TB/CB/PU/TU/CU/VPDU/CTU/CTU row/slice/tile/sub-picture level.
      • b. For example, the syntax element(s) may be used to adjust the value of at least one LIC parameter of at least one LIC model.
        • i. For example, the syntax element(s) may be used as indicator(s) for adjustment factor(s).
        • ii. Alternatively, the syntax element(s) may be used to represent/indicate the value of at least one LIC parameter.
        • iii. For example, one indicator may be signalled to adjust the parameters of one LIC model.
      • c. For example, the derivation of LIC parameters may be based on both decoder derived methods and the signalled syntax element.
      • d. For example, the syntax element may be an indicator of an integer.
        • i. For example, the value of the syntax element may be in the range of [−N, +N], for example, N=4.
        • ii. For example, the LIC parameter may be directly derived based the integer.
      • e. For example, the syntax element may be an indictor of an index.
        • i. For example, according to the first value of the index, a second value may be derived from a (pre-defined) look-up-table for the LIC parameter derivation.
      • f. For example, how many syntax elements are signalled may be dependent on how many linear/LIC model is used for the video unit (e.g., a coding block).
        • i. For example, if M LIC models are used for the video unit, then M syntax elements may be signalled associated with the video unit.
    • c. For example, the prediction sample value derivation of an LIC coded video unit may be conducted based on an updated model: ValueAfter=a′ *ValueBefore+b′, wherein a′=a+Delta, b′=b−Delta*funcD.
      • a. For example, the updated model may be used to estimate/derive the prediction samples inside the current block.
        • i. Alternatively, the updated model may be used to modulate the relationship between neighboring samples of the current block and neighboring samples of temporally collocated block.
      • b. For example, “Delta” may be a slope adjustment/offset value of an LIC model.
        • i. For example, at least one indictor of “Delta” may be signalled in the bitstream.
        • ii. Alternatively, the value of “Delta” may be derived based on decoded information (e.g., decoded sample values, decoded prediction modes of neighboring/reference blocks).
        • iii. For example, “Delta” may be an integer.
        • iv. For example, “Delta” may be an integer in the range of [−N, +N], for example, N=4.
        • v. For example, “Delta” may be a number/value/integer/constant/variable derived from an index on a look-up-table.
      • c. For example, “funcD” may be calculated by averaging the reconstruction/prediction values of all available/appropriate/possible neighboring samples of the current block (or, all available/appropriate/possible neighboring samples of the temporally collocated block).
        • i. For example, “funcD” may be calculated by averaging neighboring/reference samples from both Intra and Inter coded blocks.
        •  1. Alternatively, “funcD” may be calculated by averaging neighboring/reference samples from Inter coded blocks only.
        • ii. For example, “funcD” may be calculated by averaging all available neighboring/reference samples located at the left and/or above side of the current block and/or temporally collocated block.
        •  1. Alternatively, only partial (e.g., neighboring the first top-left M×M unit, such as M=16 or 8 or 4 or 32) of such samples may be considered.
        • iii. For example, (the neighboring samples of) the temporally collocated block may be retrieved/pointed by the block motion vector (or its variant).
        •  1. Alternatively, (the neighboring samples of) the temporally collocated block may be retrieved/pointed by a rounded block motion vector (e.g., rounded to integer-pel precision).
        • iv. For example, the averaging process may be processed with (or without) a rounding factor.
        • v. Alternatively, the averaging process may be replaced by other function such as summing up, etc.
      • d. For example, the updated model (e.g., with adjustment) may be used/allowed for all LIC coded blocks.
        • i. Alternatively, the updated model may be used/allowed for a certain kind of LIC coded blocks.
        •  1. For example, the “certain kind” may be determined based on the available/appropriate/possible neighboring samples (such as both left and above neighboring samples are available, or only left neighboring are available, or only above neighboring are available, etc.).
        •  2. For example, the “certain kind” may be determined based on the prediction mode (such as AMVP coded or MERGE coded, uni-prediction or bi-prediction, etc.).
      • ii. Alternatively, only if both left and above reference samples are available/appropriate/possible for the video unit, the updated model may be used/allowed.
      • iii. Alternatively, only if the video unit is uni-directional predicted, the updated model may be used/allowed.
    • d. The information of the adjustment for LIC or CCLM or MM-CCLM may be coded in a predictive way.
    • e. The information of the adjustment for LIC or CCLM or MM-CCLM may be coded with at least one context model.
      • a. The context model may depend on coding information.
      • b. Alternatively, it may be coded in a bypass way.
    • f. For example, the neighboring/reference samples used to derive the LIC model parameters may not be all available/appropriate/possible neighboring/reference samples from the left side and above side adjacent to the coding block and temporally collocated block.
      • a. For example, it may refer to neighboring/reference samples from both Intra and Inter coded blocks.
        • i. Alternatively, it may refer to neighboring/reference samples from Inter coded blocks only.
      • b. For example, it may refer to neighboring/reference samples located at the left (or above) side of the current block and/or temporally collocated block.
        • i. Alternatively, only partial (e.g., neighboring the first top-left M×M unit, such as M=16 or 8 or 4 or 32) of such samples may be considered.
      • c. For example, (the neighboring samples of) the temporally collocated block may be retrieved/pointed by the block motion vector (or its variant).
        • i. Alternatively, (the neighboring samples of) the temporally collocated block may be retrieved/pointed by a rounded block motion vector (e.g., rounded to integer-pel precision).
    • g. For example, whether to apply/allow the adjustment (e.g., the updated model) for a video unit may be dependent on coded information.
      • a. For example, both original model (without adjustment) and updated model (with adjustment) may be used/allowed.
        • i. Alternatively, only the updated model would be used/allowed.
      • b. For example, whether to allow (or apply) the adjustment based LIC model updating, may be signalled in the bitstream.
        • i. For example, it may be signalled at (at least) one video unit level such as sequence/group of pictures/picture/slice/tile group/PB/TB/CB/PU/TU/CU/VPDU/CTU/CTU row/slice/tile/sub-picture level.
      • c. For example, whether to allow (or apply) the adjustment based LIC model updating, may be derived at both encoder and decoder sides.
    • h. For example, the LIC parameters may be derived based on non-linear model.
      • a. For example, the LIC parameters may be derived based on predC=a0*((predR*predR+b)>>s)+a1*predR+a2*c, wherein predC denotes prediction sample values of samples neighboring to the current block, predR denotes prediction sample values of samples neighboring to reference block, a0, a1, a2 denotes the LIC parameters, b and c are integers, s denotes a shift factor which may be an integer.
      • b. For example, the LIC parameters may be derived based on predC=a0*((recR*recR+b)>>s)+a1*recR+a2*c, wherein recC denotes reconstruction sample values of samples neighboring to the current block, recR denotes reconstruction sample values of samples neighboring to reference block, a0, a1, a2 denotes the LIC parameters, b and c are integers, s denotes a shift factor which may be an integer.
      • c. For example, the values of a0, a1, a2 may be derived based on the LDL decomposition method.
      • d. For example, M rows and N columns of neighboring samples may be used to derive a0, a1, a2.
        • i. For example, M may be equal to an integer no greater than 6.
        • ii. For example, N may be equal to an integer no greater than 6.
        • iii. For example, the value of M and/N may be dependent on the CTU/LCU boundary (e.g., based on the distance between the current block and the CTU top boundary).
      • e. For example, the value of s may be derived based on the internal bit depth of the coded samples.
      • f. For example, the value of b and c may be derived based on the median/mid/mean/average value of the sample values neighboring to the current/reference block.
        • i. For example, b=c.
  • 6.7. Whether to and/or how to apply the Adjustment for CCLM, MM-CCLM or LIC may depend on coding information, such as block dimensions, coding mode, (transformed) residuals, transforms etc.
    • a. For example, the adjustment may not be applied if the width and/or height and/or size of the block is smaller than a threshold.
  • 6.8. About the signalling and determination of LIC (e.g., as illustrated in the seventh problem), the following methods are proposed:
    • a. For example, the LIC flag at a video unit level (e.g., CU/PU level LIC flag) may not be signalled but derived at both encoder and decoder sides.
      • a. For example, the CU/PU level LIC flag for an AMVP coded block may not be signaled.
      • b. For example, the CU/PU level LIC flag for an Affine AMVP coded block may not be signaled.
      • c. For example, the CU/PU level LIC flag for an AMVP-MERGE coded block may not be signaled.
    • b. For example, whether or to use LIC for a video unit may be dependent on coded information (e.g., decoder derived methods).
      • a. For example, LIC flag at video unit level may be implicit derived at both encoder and decoder sides.
        • i. For example, the CU/PU level LIC flag for a non-merge (such as AMVP, and/or affine AMVP) coded block may be implicit derived.
        • ii. For example, the CU/PU level LIC flag for a merge (and/or its variants such as TM, BM, MHP, ADMVR, CIIP, GPM, sbTMVP, Affine Merge, etc.) coded block may be implicit derived.
        • iii. For example, the implicit derivation may be based on decoder derived methods.
        • iv. For example, the implicit derivation may be based on template matching.
        • v. For example, the implicit derivation may be based on bilateral matching.
      • b. For example, decoder derived costs/errors/distortions may be calculated for both non-LIC and LIC cases, and the one with less cost/error/distortion is determined as the coding method used for the video unit.
        • i. For example, template (and/or bilateral) matching costs may be calculated for LIC coded video unit and non-LIC coded video unit, respectively.
        • ii. For example, the costs/errors/distortions are derived by neighboring samples and/or reference samples temporally (collocated) in the reference pictures.
        • iii. For example, the costs/errors/distortions are not derived by current block samples in the current picture.
      • c. For example, the coded information used for LIC mode may be neighboring/reference samples from both Intra and Inter coded blocks.
        • i. Alternatively, the coded information may be neighboring/reference samples from Inter coded blocks only.
      • d. For example, the coded information used for LIC mode may be all available neighboring/reference samples located at the left and/or above side of the current block and/or temporally collocated block.
        • i. Alternatively, only partial (e.g., neighboring the first top-left M×M unit, such as M=16 or 8 or 4 or 32) of such samples may be considered.
      • e. For example, the temporally collocated block may be retrieved/pointed by the block motion vector (or its variant).
        • i. Alternatively, the temporally collocated block may be retrieved/pointed by a rounded block motion vector (e.g., rounded to integer-pel precision).
    • c. For example, the merge index of an LIC coded merge block may not be signalled (e.g., derived at both encoder and decoder).
      • a. For example, the motion (e.g., motion vector, reference index, prediction direction, etc) of the LIC coded merge block may be derived at both encoder and decoder sides.
      • b. For example, multiple template (and/or bilateral) matching costs/errors/distortions may be calculated for all (or multiple, or pre-defined partial) available/possible/appropriate merge candidates, respectively. The one with minimum cost/error/distortion is determined as the motion used for the video unit.
        • i. For example, the template is constructed by neighboring samples and/or reference samples temporally (collocated) in the reference pictures.
        • ii. For example, the template is not constructed by current block samples in the current picture.
        • iii. For example, the template is constructed with samples without LIC.
        • iv. For example, the template is constructed with samples with LIC.
    • d. For example, a best LIC parameter set (such as a and b computed by least square fitting methods) may be determined from more than one set of LIC parameters.
      • a. For example, more than one set of LIC parameters may be appropriate for an LIC coded video unit.
      • b. For example, which LIC parameter set is used for the video unit may be derived at both encoder and decoder sides.
        • i. Alternatively, a syntax element (e.g., an index) may be signalled specifying the LIC parameter set used for the video unit.
      • c. For example, multiple template (and/or bilateral) matching costs/errors/distortions may be calculated for all appropriate sets of LIC parameters, respectively. The one with minimum cost/error/distortion is determined as the LIC parameter set used for the video unit.
        • i. For example, the template is constructed by neighboring samples and/or reference samples temporally (collocated) in the reference pictures.
        • ii. For example, the template is not constructed by current block samples in the current picture.
  • 6.9. About the out-of-boundary (OOB) prediction handling (e.g., as illustrated in the eighth problem), the following methods are proposed:
    • a. For example, OOB handling methods may be used to blend a first prediction and a second prediction.
      • a. For example, at least one prediction block/subblock is motion compensated prediction block/subblock.
        • i. For example, the first prediction may be non-inter (e.g., intra) predicted, and the second prediction may be motion compensated predicted (such as CIIP mode, etc).
        • ii. For example, both prediction blocks/subblocks may be motion compensated predicted.
      • b. For example, the two predictions may be generated from same inter prediction direction (e.g., MHP mode, GPM mode, etc.).
      • c. For example, the two predictions may be generated from different inter prediction directions (e.g., bi-predicted weighting).
      • d. For example, it may be used for MERGE mode and/or its variant mode (such as BM/ADMVR/CIIP/TM/MMVD/AffineMerge/sbTMVP/GPM/GPM-MMVD/GPM-TM mode, etc.).
      • e. For example, it may be used for AMVP mode and/or its variant mode (such as SMVD/AMVPmerge/AffineAMVP, etc.).
      • f. For example, it may be allowed/used for predictions based on decoder side motion vector refinements (such as template matching and/or bilateral matching based motion vector refinements, etc.).
        • i. For example, it may be allowed/used for predictions generated by decoder side motion vector refinement related coding modes.
        • ii. Alternatively, it may not be allowed/used for predictions based on decoder side motion vector refinements.
        • iii. Alternatively, it may not be allowed/used for predictions generated by decoder side motion vector refinement related coding modes.
      • g. For example, it may be allowed/used for predictions generated based on a certain kind of subblock-based prediction method (e.g., DMVR, TM, Affine merge, Affine AMVP, sbTMVP, OBMC, BDOF, AMVP-merge, GPM, etc.).
        • i. For example, it may be allowed/used for a certain kind of CU/PU based prediction method (e.g., regular merge, CIIP, MMVD, MHP, and/or regular AMVP, SMVD, etc.).
        • ii. Alternatively, it may not be allowed/used for predictions generated based on a certain kind of subblock-based prediction method.
        •  1. For example, it may not coexist with a certain kind of subblock-based prediction method.
        • iii. Alternatively, it may not be allowed/used for a certain kind of CU/PU based prediction method.
        •  1. For example, it may not coexist with a certain kind of CU/PU based prediction method.
    • b. For example, the OOB may refer to out-of-reference-picture-boundary.
      • a. Alternatively, it may refer to out-of-reference-subpicture-boundary.
        • i. For example, the reference subpicture ID is the same as the current subpicture.
      • b. Alternatively, it may refer to out-of-reference-slice-boundary.
      • c. Alternatively, it may refer to out-of-reference-tile-boundary.
        • i. For example, in case of MCTS (i.e., motion constrained tile set), the motion vector is constrained within the tile with same coordinate/location in the reference picture.
    • c. For example, the blending weights for the OOB samples may be different from the blending weights for those non-OOB samples inside the boundary.
      • a. For example, the weighting factor for the OOB samples of a motion compensated block/subblock may be set according to a rule, e.g., set to a certain value (such as zero).
      • b. For example, the weighting factor for the motion compensated samples around the junction border of OOB samples and non-OOB samples may be set according to a rule, e.g., gradual increase/decrease weighting values as far away from the junction border.
      • c. For example, the blending weights may be calculated based on the distance based on the boundary.
      • d. For example, the blending weights may be calculated based on the distance based on the junction border of OOB samples and non-OOB samples.
    • d. For example, when blending a first prediction sample with a second prediction sample, if both motion-compensated samples are OOB, the final prediction value may be generated without blending.
      • a. For example, the non-OOB sample which is closer to the boundary may be taken for final prediction sample generation.
      • b. For example, the final prediction value may be generated based on the non-OOB prediction sample inside the current blended block, according to a rule, e.g., the nearest available non-OOB prediction sample inside the current blended block.
      • c. Alternatively, in such case, the final prediction value for such sample may be generated by averaged/weighted blending, the same as usual (e.g., same behavior as blending the samples inside the boundary).
    • e. For example, the value of a OOB sample of a motion compensated block/subblock may be set according to a rule.
      • a. For example, the OOB sample may refer to the OOB sample right after the (uni-directional) motion compensation process (but before BDOF and blending/weighting process).
      • b. For example, the rule may be based on the non-OOB motion-compensated sample values inside the boundary.
        • i. For example, the non-OOB motion compensated sample values locates at the first row inside of the boundary may be copied above for the above OOB samples.
        • ii. For example, the non-OOB motion compensated sample values locates at the first column inside of the boundary may be copied left for the left OOB samples.
        • iii. For example, the non-OOB motion compensated sample values locates at the top-left corner inside of the boundary may be copied for the top-left OOB samples.
      • c. For example, the OOB sample may refer to the OOB sample after BDOF (if any) and before the blending/weighting process.
      • d. For example, the rule may be based on the non-OOB BDOF refined sample values inside the boundary.
        • i. For example, the non-OOB BDOF refined sample values locates at the first row inside of the boundary may be copied above for the above OOB samples.
        • ii. For example, the non-OOB BDOF refined sample values locates at the first column inside of the boundary may be copied left for the left OOB samples.
        • iii. For example, the non-OOB BDOF refined sample values locates at the top-left corner inside of the boundary may be copied for the top-left OOB samples.
    • f. For example, if a prediction block/subblock pointed by a motion vector is OOB, then a new prediction block/subblock may be generated according to a rule.
      • a. For example, the new prediction block/subblock may be generated based on a ZERO motion vector (e.g., (0,0)).
      • b. For example, the new prediction block/subblock may be replaced by a collocated block.
      • c. For example, the new prediction block/subblock may be replaced by a non-OOB prediction block/subblock that is nearest to the OOB block/subblock.
    • g. For example, the OOB check may be based on the motion vectors before decoder side motion refinements (such as template matching based motion refinement, and/or bilateral matching based motion refinement).
      • a. Alternatively, the OOB check may be based on the motion vectors after a certain stage (e.g., PU-level, or 16×16-subblock-level, or 8×8-subblock-level) of DMVR based motion refinement.
      • b. Alternatively, the OOB check may be based on the motion vectors after all stages (e.g., PU-level, 16×16-subblock-level, and 8×8-subblock-level) of DMVR based motion refinement.
      • c. Alternatively, the OOB check may be based on the motion vectors after TM based motion refinement.
      • d. For example, the blending weights for the OOB samples may be determined based on the OOB check.
    • h. For example, if a reference block is partially (or totally) OOB, the OOB part of a motion compensated block/subblock may not be used to generate the final prediction block.
    • i. For example, if a reference block is partially (or totally) OOB, the original motion compensated block/subblock may be modified/shifted to another motion compensated block/subblock.
    • j. For example, if a reference block is partially (or totally) OOB, the original motion vector used to generate the motion compensated block/subblock may be modified to another motion vector.
    • k. For example, whether the OOB handling method is used to a video unit, may be signaled at a video unit level.
      • a. Alternatively, the OOB handling method may be mandatorily used to a certain kind of blocks (such as multiple hypothesis, bi-predicted blocks), without video unit level signalling.
      • b. Alternatively, whether or not use OOB handling may be implicitly derived based on coded information.
  • 6.10 About non-average weighting (e.g., the 8th problem and related issues), the following methods are proposed:
    • a. For example, non-average weighting (e.g., BCW) may be applied to AMVP-MERGE mode.
      • a. For example, it may be applied to low-delay pictures and/or non-low-delay pictures.
      • b. For example, one weight index out of a pre-defined table containing M possible weighting factors may be determined.
        • i. For example, M=3, or 4, or 5, or 6.
        • ii. For example, M may be equal to different values according to the coding information of current picture (e.g., low-delay picture or not).
      • c. For example, the weight index may be signalled in the bitstream.
      • d. For example, the weight index may be implicitly derived by a decoder side derivation method (e.g., not signalled).
        • i. For example, the weight index may be determined based on costs derived based on DMVR/DMVD/mpDMVD/puDMVD (and/or its variants).
        • ii. For example, the weight index may be determined based on TM cost.
        • iii. For example, the template for the TM cost derivation may be equal to M×N samples neighboring to the current block and reference blocks.
        • iv. For example, the template for the TM cost derivation may be equal to M×N samples neighboring to a first reference block and a second reference block.
        • v. For example, a TM/DMVR cost may be scaled for the cost comparison.
        •  1. For example, a TM/DMVR cost derived by averaging weighting may be scaled (e.g., to a smaller cost value).
    • b. For example, K-entries (such as K>2, or K=2) non-average weighting may be applied to MHP mode.
      • a. For example, it may be applied for MHP AMVP mode (e.g., additional MHP hypothesis is generated based on AMVP) to weight two hypotheses.
      • b. For example, it may be applied for MHP MERGE mode (e.g., additional MHP hypothesis is generated based on MERGE) to weight two hypotheses.
      • c. For example, it may be applied to low-delay pictures and/or non-low-delay pictures.
      • d. For example, one weight index out of a pre-defined table containing M possible weighting factors may be determined.
        • i. For example, M=2, or 3, or 4, or 5, or 6.
        • ii. For example, M may be equal to different values according to the coding information of current picture (e.g., low-delay picture or not).
        • iii. For example, M may be equal to different values according to whether it is MHP AMVP or MHP MERGE.
      • e. For example, the weight index may be signalled in the bitstream.
      • f. For example, the weight index may be implicitly derived by a decoder side derivation method (e.g., not signalled).
        • i. For example, the weight index may be determined based on costs derived based on DMVR/DMVD/mpDMVD/puDMVD (and/or its variants).
        • ii. For example, the weight index may be determined based on TM cost.
        • iii. For example, the template for the TM cost derivation may be equal to M×N samples neighboring to the current block and reference blocks.
        • iv. For example, the template for the TM cost derivation may be equal to M×N samples neighboring to a first reference block and a second reference block.
        • v. For example, a TM/DMVR cost may be scaled for the cost comparison.
        •  1. For example, a TM/DMVR cost derived by averaging weighting may be scaled (e.g., to a smaller cost value).
    • c. For example, a decoder derived non-average weighting (e.g., BCW) method may be applied to one or more of the followings:
      • a. non-low-delay pictures
      • b. low-delay pictures
      • c. MHP AMVP and its variant
      • d. MHP MERGE and its variant
      • e. MMVD and its variant
      • f. AMVP-MERGE and its variant
  • 6.11 About improvements on the AMVP-MERGE mode (e.g., the 9th problem and related issues), the following methods are proposed:
    • a. In one example, the AMVP-MERGE mode may be allowed for low-delay pictures. Assume the AMVP-MERGE coded block is coded by a motion pair (AMVP-MV, MERGE-MV) from same direction relative to the current picture,
      • a. For example, the AMVP-MERGE mode may be allowed for low-delay pictures in random access configuration.
      • b. For example, the AMVP-MERGE mode may be allowed for low-delay pictures in low-delay configuration.
      • c. For example, the AMVP-MV and MERGE-MV may point to two different reference pictures with different POC value, respectively.
      • d. For example, for the AMVP-MERGE list generation, a two-step reference picture checking may be applied for a video unit.
        • i. For example, it may firstly check a first condition (e.g., whether there are available reference pictures from both forward and backward directions).
        •  1. If it meets the first condition, true-bi-direction AMVP-MERGE ode may be conducted.
        • ii. For example, if it does not meet the first condition, it may further check a second condition (e.g., whether there are available reference pictures from same direction but different POC distances).
        •  1. If it meets the second condition, same-direction AMVP-MERGE may be conducted.
      • e. For example, in case there are more than one reference picture which has different POC distance (diff-POC-dist) from that of the AMVP-REF (i.e., the reference picture pointed by the AMVP-MV of the AMVP-MERGE mode), a specific reference picture out of such diff-POC-dist reference pictures may be selected to derive the MERGE-MV.
        • i. For example, it may be based on the distance from the current pucture.
        •  1. For example, which is closer to the current picture.
        •  2. For example, which is closer to the AMVP-REF picture.
        • ii. For example, it may be based on the POC distance of the diff-POC-dist reference picture.
        •  1. For example, which has smaller POC value.
        •  2. For example, which has greater POC value.
      • f. For example, the AMVP-MV (single direction) may be determined based on a coded MVP index and/or a coded reference index.
      • g. For example, the MERGE-MV may be determined based on a certain rule.
        • i. For example, the certain rule may be based on a pre-defined position of a MERGE-MV candidate in the MERGE-MV list.
        •  1. For example, the M-th (e.g., first) available MERGE-MV candidate.
        • ii. For example, the certain rule may be based on TM cost.
        •  1. For example, multiple TM costs may be calculated regarding to multiple MERGE-MV candidates, and the one with minimum TM cost may be selected as the final MERGE-MV used for the AMVP-MERGE mode.
        •  2. For example, the TM cost may contain costs calculated from both AMVP-MV candidate and MERGE-MV candidate.
        •  3. For example, the TM cost may contain the cost calculated by MERGE-MV candidate only.
      • h. For example, TM based refinement may be used for the AMVP-MERGE motion pair from same direction.
    • b. For example, the above bullets/sub-bullet in 6.11 may be applied when DMVR is OFF.
    • c. For example, the above bullets/sub-bullet in 6.11 may be applied when RPR is ON.
  • 6.12 About the DMVD application (e.g., the 10th problem and related issues), the following methods are proposed:
    • a. For example, a DMVR/DMVR method and/or its variant may be used to refine the shift MV for sbTMVP mode.
      • a. For example, the shift MV may be bi-directional predicted.
      • b. For example, the shift MV may point to two reference pictures from both forward and backward.
      • c. For example, a shift MV which meets the DMVR/DMVD condition may be generated for sbTMVP.
      • d. For example, the shift MV may be used to derive motion vectors from corresponding PUs in reference pictures.
    • b. For example, a DMVR/DMVR method and/or its variant may be used to refine the CPMVs for Affine AMVP mode.
      • a. For example, only a PU level DMVD/DMVR may be used.
      • b. For example, it may be only applied to CPMVs meet the DMVR/DMVD condition.
      • c. For example, it may be only applied to CPMVs pointing to reference pictures from both forward and backward.
      • d. For example, it may be only applied to CPMV pairs for a bi-directional predicted Affine AMVP mode.
    • c. For example, a DMVR/DMVR method and/or its variant may be used to refine the CPMVs for Affine MERGE mode.Misc. Aspects
  • 6.13. For example, whether deblocking or not may be controlled at CTU (or CU) level.
  • 6.14. For example, the intra MTS type may be determined based on syntax elements (e.g., paramters of thresholds) signalled at PPS level.
  • 6.15. For example, adaptive blending weights may be used for an OBMC coded block, wherein the adaptive blending weights may be based on coded information other than neighboring reconstructed/predicted samples.
    • a. For example, adaptive blending weights may be determined by temporal coded information in reference pictures.
    • b. For example, adaptive blending weights may be selected based on more than one look-up-table.
      • a. For example, a weight index may be derived.
      • b. For example, a weight index may be signaled.

General Aspects

• • 6.16. Whether to and/or how to apply the disclosed methods above may be signalled at sequence level/group of pictures level/picture level/slice level/tile group level, such as in sequence header/picture header/SPS/VPS/DPS/DCI/PPS/APS/slice header/tile group header.
  • 6.17. Whether to and/or how to apply a the disclosed methods above may be signalled at PB/TB/CB/PU/TU/CU/VPDU/CTU/CTU row/slice/tile/sub-picture/other kinds of region contain more than one sample or pixel.
  • 6.18. Whether to and/or how to apply the disclosed methods above may be dependent on coded information, such as block size, colour format, single/dual tree partitioning, colour component, slice/picture type.

As used herein, the terms “video unit” or “coding unit” or “block” used herein may refer to one or more of: a color component, a sub-picture, a slice, a tile, a coding tree unit (CTU), a CTU row, a group of CTUs, a coding unit (CU), a prediction unit (PU), a transform unit (TU), a coding tree block (CTB), a coding block (CB), a prediction block (PB), a transform block (TB), a block, a sub-block of a block, a sub-region within the block, or a region that comprises more than one sample or pixel.

In this present disclosure, regarding “a block coded with mode N”, the term “mode N” may be a prediction mode (e.g., MODE_INTRA, MODE_INTER, MODE_PLT, MODE_IBC, and etc.), or a coding technique (e.g., AMVP, Merge, SMVD, BDOF, PROF, DMVR, AMVR, TM, Affine, CIIP, GPM, MMVD, BCW, HMVP, SbTMVP, and etc.).

In the following discussion, LIC parameters may refer to the two parameters (such as a slope parameter “a” and a bias parameter “b”) derived based on a linear model, which is used to map the neighboring samples of current block and the neighboring samples of temporally collocated block (e.g., temporally collocated block may be pointed by the motion vector or a rounded motion vector of the current block). Furthermore, the LIC parameters may be used to estimate the prediction values of samples inside the current video unit.

FIG. 32 illustrates a flowchart of a method 3200 for video processing in accordance with embodiments of the present disclosure. The method 3200 is implemented during a conversion between a video unit and a bitstream of the video unit.

At block 3210, for a conversion between a video unit of a video and a bitstream of the video, it is determined that the video unit is coded by a motion pair from a same direction relative to a current picture. The video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture.

At block 3220, the conversion is performed based on the motion pair. In some embodiments, the conversion may comprise ending the video unit into the bitstream. Alternatively, the conversion may comprise decoding the video unit from the bitstream. In this way, coding efficiency can be improved.

In some embodiments, the motion pair comprises an AMVP motion vector (MV) and a MERGE MV. For example, the AMVP-MERGE coded block may be coded by a motion pair (AMVP-MV, MERGE-MV) from same direction relative to the current picture.

In some embodiments, the AMVP merge mode is allowed for the low-delay picture in a random access configuration. In some embodiments, the AMVP merge mode is allowed for the low-delay picture in a low-delay configuration.

In some embodiments, the motion pair comprises an AMVP MV and a MERGE MV. The AMVP MV may point to a first reference picture with a first picture order count (POC) value and the MERGE MV points to a second reference picture with a second POC value. For example, the AMVP-MV and MERGE-MV may point to two different reference pictures with different POC value, respectively.

In some embodiments, the method 3200 further comprises: for a generation of an AMVP merge list, applying a two-step reference picture checking procedure to the video unit. In some embodiments, applying the two-step reference picture checking procedure comprises: determining whether a first condition is fulfilled. In some embodiments, the first condition comprises whether there are available reference pictures from both forward and backward directions. For example, it may firstly check a first condition (e.g., whether there are available reference pictures from both forward and backward directions). In some embodiments, if the first condition is fulfilled, a true-bi-direction AMVP merge mode is conducted.

In some embodiments, the method 3200 further comprises: if the first condition is not fulfilled, determining whether a second condition is fulfilled.

In some embodiments, the second condition comprises whether there are available reference pictures from a same direction but with different POC distances. For example, if it does not meet the first condition, it may further check a second condition (e.g., whether there are available reference pictures from same direction but different POC distances). In some embodiments, if the second condition is fulfilled, a same-direction AMVP merge mode is conducted.

In some embodiments, an AMVP MV in the motion pair is determined based on at least one of: a coded motion vector prediction (MVP) index or a coded reference index. In some embodiments, a template matching (TM) based refinement is used for the motion pair from a same direction.

In some embodiments, if there is a plurality of reference pictures that has a different POC distance from that of an AMVP reference picture, a target reference picture from the plurality of reference pictures is selected to drive a MERGE MV in the motion pair. For example, in case there are more than one reference picture which has different POC distance (diff-POC-dist) from that of the AMVP-REF (i.e., the reference picture pointed by the AMVP-MV of the AMVP-MERGE mode), a specific reference picture out of such diff-POC-dist reference pictures may be selected to derive the MERGE-MV.

In some embodiments, the target reference picture is selected based on a distance to a current picture. In some embodiments, the target reference picture is closer to the current picture than other reference pictures. In some embodiments, the target reference picture is closer to then AMVP reference picture than other reference pictures. For example, it may be based on the distance from the current picture. For example, which is closer to the current picture. For example, which is closer to the AMVP-REF picture.

In some embodiments, the target reference picture is selected based on a POC distance. In some embodiments, the target reference picture has a smaller POC value than other reference pictures in the plurality of reference pictures. In some embodiments, the target reference picture has a greater POC value than other reference pictures in the plurality of reference pictures. For example, it may be based on the POC distance of the diff-POC-dist reference picture. For example, which has smaller POC value. For example, which has greater POC value.

In some embodiments, a MERGE MV in the motion pair is determined based on a rule. In some embodiments, the rule is based on a predefined position of a MERGE MV candidate in a MERGE MV list. In some embodiments, the MERGE MV candidate is a M-th available MERGE MV candidate in the MERGE mv LIST, where M is an integer number.

In some embodiments, the rule is based on a TM cost. In some embodiments, a plurality of TM costs is determined for a plurality of MERGE MV candidates, and a MERGE MV candidate with a minimum TM cost is selected as the MERGE MV in the motion pair. In some embodiments, the TM cost comprises costs determined for both AMVP MV candidate and MERGE MV candidate. In some embodiments, the TM cost comprises a cost determined by MERGE MV candidate only.

In some embodiments, a decoder side motion vector refinement (DMVR) of the video unit is off, and/or where a reference picture resampling (RPR) of the video unit is on. For example, the above embodiments described with reference to FIG. 32 may be applied when DMVR is OFF. For example, the above embodiments may be applied when RPR is ON.

In some embodiments, an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture may be indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level. In some embodiments, an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture may be indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header. In some embodiments, an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

In some embodiments, the method 3200 further comprises: determining, based on coded information of the video unit, whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture. The coded information includes at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: determining that a video unit of the video is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; and generating a bitstream of the video unit based on the motion pair.

According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. The method comprises: determining that a video unit of the video is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; generating a bitstream of the video unit based on the motion pair; and storing the bitstream in a non-transitory computer-readable recording medium.

FIG. 33 illustrates a flowchart of a method 3300 for video processing in accordance with embodiments of the present disclosure. The method 3300 is implemented during a conversion between a video unit and a bitstream of the video unit.

At block 3310, during a conversion between a video unit of a video and a bitstream of the video, at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR is applied to the video unit to refine a motion vector of the video unit. The video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode.

At block 3320, the conversion is performed based on the refined motion vector. In some embodiments, the conversion may comprise ending the video unit into the bitstream. Alternatively, the conversion may comprise decoding the video unit from the bitstream. In this way, coding efficiency can be improved.

In some embodiments, at least one of the DMVR or the variant of DMVR is used to refine a shift motion vector (MV) for the sbTMVP mode. In some embodiments, the shift MV is bi-directional predicted. In some embodiments, the shift MV points to two reference pictures from both forward and backward. In some embodiments, the shift MV that meets a DMVR condition or a decoder side motion vector difference (DMVD) condition is generated for the sbTMVP mode. In some embodiments, the shift MV is used to derive motion vectors from corresponding prediction units (PUs) in reference pictures.

In some embodiments, at least one of the DMVR or the variant of DMVR is used to refine a control point motion vector (CPMV) for the affine merge mode. In some embodiments, at least one of the DMVR or the variant of DMVR is used to refine a control point motion vector (CPMV) for the affine AMVP mode. In some embodiments, only a PU level DMVD or DMVR is used. In some embodiments, at least one of the DMVR or the variant of DMVR is applied to the CPMV that meets a DMVR condition or a DMVD condition. In some embodiments, at least one of the DMVR or the variant of DMVR is applied to the CPMV that points to reference pictures from both forward and backward. In some embodiments, at least one of the DMVR or the variant of DMVR is applied to the CPMV pairs for a bi-directional predicted affine AMVP mode.

In some embodiments, an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level. In some embodiments, an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header. In some embodiments, an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

In some embodiments, the method 3300 further comprises: determining, based on coded information of the video unit, whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture. The coded information may include at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: applying at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to a video unit of the video to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; and generating a bitstream of the video unit based on the refined motion vector.

According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. The method comprises: applying at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to a video unit of the video to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; generating a bitstream of the video unit based on the refined motion vector; and storing the bitstream in a non-transitory computer-readable recording medium.

FIG. 34 illustrates a flowchart of a method 3400 for video processing in accordance with embodiments of the present disclosure. The method 3400 is implemented during a conversion between a video unit and a bitstream of the video unit.

At block 3410, during a conversion between a video unit of a video and a bitstream of the video, at least one local illumination compensation (LIC) model parameter of a LIC model for the video unit is determined based on a non-linear model. The video unit coded is a LIC coded block.

At block 3420, the LIC model is updated by adjusting the at least one LIC model parameter. At block 3430, the conversion is performed based on the updated LIC model. In some embodiments, the conversion may comprise ending the video unit into the bitstream. Alternatively, the conversion may comprise decoding the video unit from the bitstream. In this way, coding efficiency can be improved.

In some embodiments, the at least one LIC model parameter is derived based on predC=a0*((predR*predR+b)>>s)+a1*predR+a2*c. In this case, predC represents prediction sample values of samples neighboring to a current block, predR represents prediction sample values of samples neighboring to a reference block, a0, a1 and a2 represent LIC parameters, respectively, b and c are integers, s represents a shift factor which is an integer.

In some embodiments, the at least one LIC model parameter is derived based on predC=a0*((recR*recR+b)>>s)+a1*recR+a2*c. In this case, recC represents reconstruction sample values of samples neighboring to a current block, recR represents reconstruction sample values of samples neighboring to a reference block, a0, a1, and a2 represent LIC parameters, respectively, b and c are integers, s represents a shift factor which is an integer.

In some embodiments, the at least one LIC model parameter is derived based on a LDL decomposition approach, where L represents lower-unitriangular and D represents diagonal with positive diagonal entries. For example, the values of a0, a1, a2 may be derived based on the LDL decomposition method. Alternatively, the at least one LIC model parameter is derived based on a Gaussian Elimination based coefficient solving approach.

In some embodiments, M rows and N columns of neighboring samples are used to derive the at least one LIC model parameter, where M and N are integer numbers. For example, M rows and N columns of neighboring samples may be used to derive a0, a1, a2. In some embodiments, M is equal to an integer number and no greater than 6. In some embodiments, N is equal to an integer number and no greater than 6.

In some embodiments, a value of at least M or N is dependent on a coding tree unit (CTU) boundary or a largest coding unit (LCU) boundary. For example, the value of M and/N may be dependent on the CTU/LCU boundary (e.g., based on the distance between the current block and the CTU top boundary).

In some embodiments, a shift factor is derived based on an internal bit depth of coded samples. For example, the value of s may be derived based on the internal bit depth of the coded samples.

In some embodiments, values of b and c are derived based on at least one of: a median value of the sample values neighboring to the current block, a mid-value of the sample values neighboring to the current block, an average value of the sample values neighboring to the current block, a median value of the sample values neighboring to the refence block, a mid value of the sample values neighboring to the refence block, or an average value of the sample values neighboring to the refence block. For example, the value of b and c may be derived based on the median/mid/mean/average value of the sample values neighboring to the current/reference block. In some embodiments, b is equal to c.

In some embodiments, an indication of whether to and/or how to derive the at least one LIC model parameter based on the non-linear model is indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level. In some embodiments, an indication of whether to and/or how to derive the at least one LIC model parameter based on the non-linear model is indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header. In some embodiments, an indication of whether to and/or how to derive the at least one LIC model parameter based on the non-linear model is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

In some embodiments, the method 3400 further comprises: determining, based on coded information of the video unit, whether to and/or how to derive the at least one LIC model parameter based on the non-linear model. The coded information may include at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: deriving at least one local illumination compensation (LIC) model parameter of a LIC model for a video unit of the video based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; and generating a bitstream of the video unit based on the updated LIC model.

According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. The method comprises: deriving at least one local illumination compensation (LIC) model parameter of a LIC model for a video unit of the video based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; generating a bitstream of the video unit based on the updated LIC model; and storing the bitstream in a non-transitory computer-readable recording medium.

FIG. 35 illustrates a flowchart of a method 3500 for video processing in accordance with embodiments of the present disclosure. The method 3500 is implemented during a conversion between a video unit and a bitstream of the video unit.

At block 3510, for a conversion between a video unit of a video and a bitstream of the video, a non-average weighting approach is applied to the video unit. The video unit is applied with an advanced motion vector prediction (AMVP) merge mode. For example, non-average weighting (e.g., BCW) may be applied to AMVP-MERGE mode.

At block 3520, the conversion is performed based on the weighted video unit. In some embodiments, the conversion may comprise ending the video unit into the bitstream. Alternatively, the conversion may comprise decoding the video unit from the bitstream. In this way, coding efficiency can be improved.

In some embodiments, the non-average weighting approach is applied to at least one of: a low-delay picture or a non-low-delay picture. In some embodiments, one weight index out of a pre-defined table containing M possible weighting factors is determined, where M is an integer number. In some embodiments, M is one of: 3, 4, 5, or 6. In some embodiments, a value of M is dependent on coding information of a current picture. For example, M may be equal to different values according to the coding information of current picture (e.g., low-delay picture or not).

In some embodiments, a weight index is indicated in the bitstream. In some embodiments, a weight index is implicitly derived by a decoder side derivation approach. In some embodiments, the weight index is determined based on costs that are derived based on at least one of: a decoder side motion vector refinement (DMVR), a decoder side motion vector difference (DMVD), a multiple passes decoder side motion vector difference (mpDMVD), a prediction unit decoder side motion vector difference (puDMVD), a variant of DMVR, a variant of DMVD, a variant of mp DMVD, or a variant of puDMVd.

In some embodiments, the weight index is determined based on a template matching (TM) cost. In some embodiments, a template for a TM cost derivation is equal to M*N samples neighboring to a current block and a reference block, where M and N are integer numbers. In some embodiments, a template for a TM cost derivation is equal to M*N samples neighboring to a first reference block and a second reference block.

In some embodiments, a TM cost or a DMVR cost is scaled for a cost comparison. In some embodiments, the TM cost or the DMVR cost derived by averaging weighting is scaled. For example, a TM/DMVR cost derived by averaging weighting may be scaled (e.g., to a smaller cost value).

In some embodiments, K entries non-average weighting is applied to a multi-hypothesis prediction (MHP) mode, where K is an integer number. For example, K-entries (such as K>2, or K=2) non-average weighting may be applied to MHP mode.

In some embodiments, the K entries non-average weighting is applied to MHP AMVP mode to weight two hypotheses. For example, it may be applied for MHP AMVP mode (e.g., additional MHP hypothesis is generated based on AMVP) to weight two hypotheses.

Alternatively, the K entries non-average weighting is applied to MHP MERGE mode to weight two hypotheses. For example, it may be applied for MHP MERGE mode (e.g., additional MHP hypothesis is generated based on MERGE) to weight two hypotheses.

In some embodiments, the K entries non-average weighting is at least one of: a low-delay picture or a non-low-delay picture. In some embodiments, a weight index out of a predefined table including M weighting factors is determined, where M is an integer number. In some embodiments, M is equal to one of: 2, 3, 4, 5, or 6. In some embodiments, a value of M is dependent on coding information of a current picture. For example, M may be equal to different values according to the coding information of current picture (e.g., low-delay picture or not).

In some embodiments, a value of M is dependent on whether the video unit is coded with a MHP AMVP mode or a MHP MERGE mode. For example, M may be equal to different values according to whether it is MHP AMVP or MHP MERGE.

In some embodiments, a weight index is indicated in the bitstream. In some embodiments, a weight index is implicitly derived by a decoder side derivation approach. In some embodiments, the weight index is determined based on costs that are derived based on at least one of: a decoder side motion vector refinement (DMVR), a decoder side motion vector difference (DMVD), a mp DMVD, a puDMVD, a variant of DMVR, a variant of DMVD, a variant of mp DMVD, or a variant of puDMVd.

In some embodiments, the weight index is determined based on a template matching (TM) cost. In some embodiments, a template for a TM cost derivation is equal to M*N samples neighboring to a current block and a reference block, where M and N are integer numbers.

In some embodiments, a template for a TM cost derivation is equal to M*N samples neighboring to a first reference block and a second reference block. In some embodiments, a TM cost or a DMVR cost is scaled for a cost comparison. In some embodiments, the TM cost or the DMVR cost derived by averaging weighting is scaled. For example, a TM/DMVR cost derived by averaging weighting may be scaled (e.g., to a smaller cost value).

In some embodiments, a decoder derived non-average weighing approach is applied to at least one of the followings: a non-low-delay picture, a low-delay picture, a MHP AMVP mode, a variant of MHP AMVP mode, a MHP MERGE mode, a variant of MHP MERGE mode, a merge mode with motion vector difference (MMVD), a variant of MMVD, an AMVP-MERGE mode, or a variant of AMVP-MERGE mode.

In some embodiments, an indication of whether to and/or how to apply a non-average weighting approach to the video unit is indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.

In some embodiments, an indication of whether to and/or how to apply a non-average weighting approach to the video unit is indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header.

In some embodiments, an indication of whether to and/or how to apply a non-average weighting approach to the video unit is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

In some embodiments, the method 3500 further comprises: determining, based on coded information of the video unit, whether to and/or how to apply a non-average weighting approach to the video unit. The coded information may include at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

According to further embodiments of the present disclosure, a non-transitory computer-readable recording medium is provided. The non-transitory computer-readable recording medium stores a bitstream of a video which is generated by a method performed by an apparatus for video processing. The method comprises: applying a non-average weighting approach to a video unit of the video, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; and generating a bitstream of the video unit based on the weighted video unit.

According to still further embodiments of the present disclosure, a method for storing bitstream of a video is provided. The method comprises applying a non-average weighting approach to a video unit of the video, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; generating a bitstream of the video unit based on the weighted video unit; and storing the bitstream in a non-transitory computer-readable recording medium.

Implementations of the present disclosure can be described in view of the following clauses, the features of which can be combined in any reasonable manner.

Clause 1. A method of video processing, comprising: determining, for a conversion between a video unit of a video and a bitstream of the video, that the video unit is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; and performing the conversion based on the motion pair.

Clause 2. The method of clause 1, wherein the motion pair comprises an AMVP motion vector (MV) and a MERGE MV.

Clause 3. The method of clause 1, wherein the AMVP merge mode is allowed for the low-delay picture in a random access configuration.

Clause 4. The method of clause 1, wherein the AMVP merge mode is allowed for the low-delay picture in a low-delay configuration.

Clause 5. The method of clause 1, wherein the motion pair comprises an AMVP MV and a MERGE

MV, and wherein the AMVP MV points to a first reference picture with a first picture order count (POC) value and the MERGE MV points to a second reference picture with a second POC value.

Clause 6. The method of clause 1, further comprising: for a generation of an AMVP merge list, applying a two-step reference picture checking procedure to the video unit.

Clause 7. The method of clause 6, wherein applying the two-step reference picture checking procedure comprises: determining whether a first condition is fulfilled.

Clause 8. The method of clause 7, wherein the first condition comprises whether there are available reference pictures from both forward and backward directions.

Clause 9. The method of clause 7, wherein if the first condition is fulfilled, a true-bi-direction AMVP merge mode is conducted.

Clause 10. The method of clause 7, further comprising: if the first condition is not fulfilled, determining whether a second condition is fulfilled.

Clause 11. The method of clause 10, wherein the second condition comprises whether there are available reference pictures from a same direction but with different POC distances.

Clause 12. The method of clause 10, wherein if the second condition is fulfilled, a same-direction AMVP merge mode is conducted.

Clause 13. The method of clause 1, wherein an AMVP MV in the motion pair is determined based on at least one of: a coded motion vector prediction (MVP) index or a coded reference index.

Clause 14. The method of clause 1, wherein a template matching (TM) based refinement is used for the motion pair from a same direction.

Clause 15. The method of clause 1, wherein if there is a plurality of reference pictures that has a different POC distance from that of an AMVP reference picture, a target reference picture from the plurality of reference pictures is selected to drive a MERGE MV in the motion pair.

Clause 16. The method of clause 15, wherein the target reference picture is selected based on a distance to a current picture.

Clause 17. The method of clause 16, wherein the target reference picture is closer to the current picture than other reference pictures.

Clause 18. The method of clause 16, wherein the target reference picture is closer to then AMVP reference picture than other reference pictures.

Clause 19. The method of clause 15, wherein the target reference picture is selected based on a POC distance.

Clause 20. The method of clause 19, wherein the target reference picture has a smaller POC value than other reference pictures in the plurality of reference pictures.

Clause 21. The method of clause 19, wherein the target reference picture has a greater POC value than other reference pictures in the plurality of reference pictures.

Clause 22. The method of clause 1, wherein a MERGE MV in the motion pair is determined based on a rule.

Clause 23. The method of clause 22, wherein the rule is based on a predefined position of a MERGE MV candidate in a MERGE MV list.

Clause 24. The method of clause 23, wherein the MERGE MV candidate is a M-th available MERGE MV candidate in the MERGE mv LIST, wherein M is an integer number.

Clause 25. The method of clause 22, wherein the rule is based on a TM cost.

Clause 26. The method of clause 25, wherein a plurality of TM costs is determined for a plurality of MERGE MV candidates, and a MERGE MV candidate with a minimum TM cost is selected as the MERGE MV in the motion pair.

Clause 27. The method of clause 25, wherein the TM cost comprises costs determined for both AMVP MV candidate and MERGE MV candidate.

Clause 28. The method of clause 25, wherein the TM cost comprises a cost determined by MERGE MV candidate only.

Clause 29. The method of any of clauses 1-28, wherein a decoder side motion vector refinement (DMVR) of the video unit is off, and/or wherein a reference picture resampling (RPR) of the video unit is on.

Clause 30. The method of any of clauses 1-29, wherein an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.

Clause 31. The method of any of clauses 1-29, wherein an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header.

Clause 32. The method of any of clauses 1-29, wherein an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

Clause 33. The method of any of clauses 1-29, further comprising: determining, based on coded information of the video unit, whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture, the coded information including at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

Clause 34. A method of video processing, comprising: applying, for a conversion between a video unit of a video and a bitstream of the video, at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to the video unit to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; and performing the conversion based on the refined motion vector.

Clause 35. The method of clause 34, wherein at least one of the DMVR or the variant of DMVR is used to refine a shift motion vector (MV) for the sbTMVP mode.

Clause 36. The method of clause 35, wherein the shift MV is bi-directional predicted.

Clause 37. The method of clause 35, wherein the shift MV points to two reference pictures from both forward and backward.

Clause 38. The method of clause 35, wherein the shift MV that meets a DMVR condition or a decoder side motion vector difference (DMVD) condition is generated for the sbTMVP mode.

Clause 39. The method of clause 35, wherein the shift MV is used to derive motion vectors from corresponding prediction units (PUs) in reference pictures.

Clause 40. The method of clause 34, wherein at least one of the DMVR or the variant of DMVR is used to refine a control point motion vector (CPMV) for the affine merge mode.

Clause 41. The method of clause 34, wherein at least one of the DMVR or the variant of DMVR is used to refine a control point motion vector (CPMV) for the affine AMVP mode.

Clause 42. The method of clause 41, wherein only a PU level DMVD or DMVR is used.

Clause 43. The method of clause 41, wherein at least one of the DMVR or the variant of DMVR is applied to the CPMV that meets a DMVR condition or a DMVD condition.

Clause 44. The method of clause 41 wherein at least one of the DMVR or the variant of DMVR is applied to the CPMV that points to reference pictures from both forward and backward.

Clause 45. The method of clause 41, wherein at least one of the DMVR or the variant of DMVR is applied to the CPMV pairs for a bi-directional predicted affine AMVP mode.

Clause 46. The method of any of clauses 34-45, wherein an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.

Clause 47. The method of any of clauses 34-45, wherein an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header.

Clause 48. The method of any of clauses 34-45, wherein an indication of whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

Clause 49. The method of any of clauses 34-45, further comprising: determining, based on coded information of the video unit, whether to and/or how to determine the video unit is coded by the motion pair from the same direction relative to a current picture, the coded information including at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

Clause 50. A method of video processing, comprising: deriving, during a conversion between a video unit of a video and a bitstream of the video, at least one local illumination compensation (LIC) model parameter of a LIC model for the video unit based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; and performing the conversion based on the updated LIC model.

Clause 51. The method of clause 50, wherein the at least one LIC model parameter is derived based on predC=a0*((predR*predR+b)>>s)+a1*predR+a2*c, wherein predC represents prediction sample values of samples neighboring to a current block, predR represents prediction sample values of samples neighboring to a reference block, a0, a1 and a2 represent LIC parameters, respectively, b and c are integers, s represents a shift factor which is an integer.

Clause 52. The method of clause 50, wherein the at least one LIC model parameter is derived based on predC=a0*((recR*recR+b)>>s)+a1*recR+a2*c, wherein recC represents reconstruction sample values of samples neighboring to a current block, recR represents reconstruction sample values of samples neighboring to a reference block, a0, a1, and a2 represent LIC parameters, respectively, b and c are integers, s represents a shift factor which is an integer.

Clause 53. The method of clause 51 or 52, wherein the at least one LIC model parameter is derived based on a LDL decomposition approach, or wherein the at least one LIC model parameter is derived based on a Gaussian Elimination based coefficient solving approach.

Clause 54. The method of clause 51 or 52, wherein M rows and N columns of neighboring samples are used to derive the at least one LIC model parameter, wherein M and N are integer numbers.

Clause 55. The method of clause 54, wherein M is equal to an integer number and no greater than 6.

Clause 56. The method of clause 54, wherein N is equal to an integer number and no greater than 6.

Clause 57. The method of clause 54, wherein a value of at least M or N is dependent on a coding tree unit (CTU) boundary or a largest coding unit (LCU) boundary.

Clause 58. The method of clause 51 or 52, wherein a shift factor is derived based on an internal bit depth of coded samples.

Clause 59. The method of clause 51 or 52, wherein values of b and c are derived based on at least one of: a median value of the sample values neighboring to the current block, a mid value of the sample values neighboring to the current block, an average value of the sample values neighboring to the current block, a median value of the sample values neighboring to the refence block, a mid value of the sample values neighboring to the refence block, or an average value of the sample values neighboring to the refence block.

Clause 60. The method of clause 59, wherein b is equal to c.

Clause 61. The method of any of clauses 50-60, wherein an indication of whether to and/or how to derive the at least one LIC model parameter based on the non-linear model is indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.

Clause 62. The method of any of clauses 50-60, wherein an indication of whether to and/or how to derive the at least one LIC model parameter based on the non-linear model is indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header.

Clause 63. The method of any of clauses 50-60, wherein an indication of whether to and/or how to derive the at least one LIC model parameter based on the non-linear model is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

Clause 64. The method of any of clauses 50-60, further comprising: determining, based on coded information of the video unit, whether to and/or how to derive the at least one LIC model parameter based on the non-linear model, the coded information including at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

Clause 65. A method of video processing, comprising: applying, for a conversion between a video unit of a video and a bitstream of the video, a non-average weighting approach to the video unit, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; and performing the conversion based on the weighted video unit.

Clause 66. The method of clause 65, wherein the non-average weighting approach is applied to at least one of: a low-delay picture or a non-low-delay picture.

Clause 67. The method of clause 65, wherein one weight index out of a pre-defined table containing M possible weighting factors is determined, wherein M is an integer number.

Clause 68. The method of clause 67, wherein M is one of: 3, 4, 5, or 6.

Clause 69. The method of clause 67, wherein a value of M is dependent on coding information of a current picture.

Clause 70. The method of clause 65, wherein a weight index is indicated in the bitstream.

Clause 71. The method of clause 65, wherein a weight index is implicitly derived by a decoder side derivation approach.

Clause 72. The method of clause 71, wherein the weight index is determined based on costs that are derived based on at least one of: a decoder side motion vector refinement (DMVR), a decoder side motion vector difference (DMVD), a multiple passes decoder side motion vector difference (mpDMVD), a prediction unit decoder side motion vector difference (puDMVD), a variant of DMVR, a variant of DMVD, a variant of mp DMVD, or a variant of puDMVd.

Clause 73. The method of clause 71, wherein the weight index is determined based on a template matching (TM) cost.

Clause 74. The method of clause 71, wherein a template for a TM cost derivation is equal to M*N samples neighboring to a current block and a reference block, wherein M and N are integer numbers.

Clause 75. The method of clause 71, wherein a template for a TM cost derivation is equal to M*N samples neighboring to a first reference block and a second reference block.

Clause 76. The method of clause 71, wherein a TM cost or a DMVR cost is scaled for a cost comparison.

Clause 77. The method of clause 76, wherein the TM cost or the DMVR cost derived by averaging weighting is scaled.

Clause 78. The method of clause 64, wherein K entries non-average weighting is applied to a multi-hypothesis prediction (MHP) mode, wherein K is an integer number.

Clause 79. The method of clause 78, wherein the K entries non-average weighting is applied to MHP AMVP mode to weight two hypotheses, or wherein the K entries non-average weighting is applied to MHP MERGE mode to weight two hypotheses.

Clause 80. The method of clause 78, wherein the K entries non-average weighting is at least one of: a low-delay picture or a non-low-delay picture.

Clause 81. The method of clause 78, wherein a weight index out of a predefined table including M weighting factors is determined, wherein M is an integer number.

Clause 82. The method of clause 81, wherein M is equal to one of: 2, 3, 4, 5, or 6.

Clause 83. The method of clause 81, wherein a value of M is dependent on coding information of a current picture.

Clause 84. The method of clause 81, wherein a value of M is dependent on whether the video unit is coded with a MHP AMVP mode or a MHP MERGE mode.

Clause 85. The method of clause 78, wherein a weight index is indicated in the bitstream.

Clause 86. The method of clause 78, wherein a weight index is implicitly derived by a decoder side derivation approach.

Clause 87. The method of clause 86, wherein the weight index is determined based on costs that are derived based on at least one of: a decoder side motion vector refinement (DMVR), a decoder side motion vector difference (DMVD), a multiple passes decoder side motion vector difference (mpDMVD), a prediction unit decoder side motion vector difference (puDMVD), a variant of DMVR, a variant of DMVD, a variant of mp DMVD, or a variant of puDMVD.

Clause 88. The method of clause 86, wherein the weight index is determined based on a template matching (TM) cost.

Clause 89. The method of clause 86, wherein a template for a TM cost derivation is equal to M*N samples neighboring to a current block and a reference block, wherein M and N are integer numbers.

Clause 90. The method of clause 86, wherein a template for a TM cost derivation is equal to M*N samples neighboring to a first reference block and a second reference block.

Clause 91. The method of clause 86, wherein a TM cost or a DMVR cost is scaled for a cost comparison.

Clause 92. The method of clause 91, wherein the TM cost or the DMVR cost derived by averaging weighting is scaled.

Clause 93. The method of clause 65, wherein a decoder derived non-average weighing approach is applied to at least one of the followings: a non-low-delay picture, a low-delay picture, a MHP AMVP mode, a variant of MHP AMVP mode, a MHP MERGE mode, a variant of MHP MERGE mode, a merge mode with motion vector difference (MMVD), a variant of MMVD, an AMVP-MERGE mode, or a variant of AMVP-MERGE mode.

Clause 94. The method of any of clauses 65-93, wherein an indication of whether to and/or how to apply a non-average weighting approach to the video unit is indicated at one of the followings: a sequence level, a group of pictures level, a picture level, a slice level, or a tile group level.

Clause 95. The method of any of clauses 65-93, wherein an indication of whether to and/or how to apply a non-average weighting approach to the video unit is indicated in one of the following: a sequence header, a picture header, a sequence parameter set (SPS), a video parameter set (VPS), a dependency parameter set (DPS), a decoding capability information (DCI), a picture parameter set (PPS), an adaptation parameter sets (APS), a slice header, or a tile group header.

Clause 96. The method of any of clauses 65-93, wherein an indication of whether to and/or how to apply a non-average weighting approach to the video unit is included in one of the following: a prediction block (PB), a transform block (TB), a coding block (CB), a prediction unit (PU), a transform unit (TU), a coding unit (CU), a virtual pipeline data unit (VPDU), a coding tree unit (CTU), a CTU row, a slice, a tile, a sub-picture, or a region containing more than one sample or pixel.

Clause 97. The method of any of clauses 65-93, further comprising: determining, based on coded information of the video unit, whether to and/or how to apply a non-average weighting approach to the video unit, the coded information including at least one of: a block size, a colour format, a single and/or dual tree partitioning, a colour component, a slice type, or a picture type.

Clause 98. The method of any of clauses 1-97, wherein the conversion includes encoding the video unit into the bitstream.

Clause 99. The method of any of clauses 1-97, wherein the conversion includes decoding the video unit from the bitstream.

Clause 100. An apparatus for video processing comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to perform a method in accordance with any of clauses 1-33.

Clause 101. A non-transitory computer-readable storage medium storing instructions that cause a processor to perform a method in accordance with any of clauses 1-33.

Clause 102. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: determining that a video unit of the video is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction

(AMVP) merge mode and the video unit belongs to a low-delay picture; and generating a bitstream of the video unit based on the motion pair.

Clause 103. A method for storing a bitstream of a video, comprising: determining that a video unit of the video is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture; generating a bitstream of the video unit based on the motion pair; and storing the bitstream in a non-transitory computer-readable recording medium.

Clause 104. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: applying at least one of: a decoder side motion vector refinement (DMVR) or a variant of

DMVR to a video unit of the video to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; and generating a bitstream of the video unit based on the refined motion vector.

Clause 105. A method for storing a bitstream of a video, comprising: applying at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to a video unit of the video to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; generating a bitstream of the video unit based on the refined motion vector; and storing the bitstream in a non-transitory computer-readable recording medium.

Clause 106. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: deriving at least one local illumination compensation (LIC) model parameter of a LIC model for a video unit of the video based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; and generating a bitstream of the video unit based on the updated LIC model.

Clause 107. A method for storing a bitstream of a video, comprising: deriving at least one local illumination compensation (LIC) model parameter of a LIC model for a video unit of the video based on a non-linear model, wherein the video unit coded is a LIC coded block; updating the LIC model by adjusting the at least one LIC model parameter; generating a bitstream of the video unit based on the updated LIC model; and storing the bitstream in a non-transitory computer-readable recording medium.

Clause 108. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: applying a non-average weighting approach to a video unit of the video, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; and generating a bitstream of the video unit based on the weighted video unit.

Clause 109. A method for storing a bitstream of a video, comprising: applying a non-average weighting approach to a video unit of the video, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode; generating a bitstream of the video unit based on the weighted video unit; and storing the bitstream in a non-transitory computer-readable recording medium.

Example Device

FIG. 36 illustrates a block diagram of a computing device 3600 in which various embodiments of the present disclosure can be implemented. The computing device 3600 may be implemented as or included in the source device 110 (or the video encoder 114 or 200) or the destination device 120 (or the video decoder 124 or 300).

It would be appreciated that the computing device 3600 shown in FIG. 36 is merely for purpose of illustration, without suggesting any limitation to the functions and scopes of the embodiments of the present disclosure in any manner.

As shown in FIG. 36, the computing device 3600 includes a general-purpose computing device 3600. The computing device 3600 may at least comprise one or more processors or processing units 3610, a memory 3620, a storage unit 3630, one or more communication units 3640, one or more input devices 3650, and one or more output devices 3660.

In some embodiments, the computing device 3600 may be implemented as any user terminal or server terminal having the computing capability. The server terminal may be a server, a large-scale computing device or the like that is provided by a service provider. The user terminal may for example be any type of mobile terminal, fixed terminal, or portable terminal, including a mobile phone, station, unit, device, multimedia computer, multimedia tablet, Internet node, communicator, desktop computer, laptop computer, notebook computer, netbook computer, tablet computer, personal communication system (PCS) device, personal navigation device, personal digital assistant (PDA), audio/video player, digital camera/video camera, positioning device, television receiver, radio broadcast receiver, E-book device, gaming device, or any combination thereof, including the accessories and peripherals of these devices, or any combination thereof. It would be contemplated that the computing device 3600 can support any type of interface to a user (such as “wearable” circuitry and the like).

The processing unit 3610 may be a physical or virtual processor and can implement various processes based on programs stored in the memory 3620. In a multi-processor system, multiple processing units execute computer executable instructions in parallel so as to improve the parallel processing capability of the computing device 3600. The processing unit 3610 may also be referred to as a central processing unit (CPU), a microprocessor, a controller or a microcontroller.

The computing device 3600 typically includes various computer storage medium. Such medium can be any medium accessible by the computing device 3600, including, but not limited to, volatile and non-volatile medium, or detachable and non-detachable medium. The memory 3620 can be a volatile memory (for example, a register, cache, Random Access Memory (RAM)), a non-volatile memory (such as a Read-Only Memory (ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), or a flash memory), or any combination thereof. The storage unit 3630 may be any detachable or non-detachable medium and may include a machine-readable medium such as a memory, flash memory drive, magnetic disk or another other media, which can be used for storing information and/or data and can be accessed in the computing device 3600.

The computing device 3600 may further include additional detachable/non-detachable, volatile/non-volatile memory medium. Although not shown in FIG. 36, it is possible to provide a magnetic disk drive for reading from and/or writing into a detachable and non-volatile magnetic disk and an optical disk drive for reading from and/or writing into a detachable non-volatile optical disk. In such cases, each drive may be connected to a bus (not shown) via one or more data medium interfaces.

The communication unit 3640 communicates with a further computing device via the communication medium. In addition, the functions of the components in the computing device 3600 can be implemented by a single computing cluster or multiple computing machines that can communicate via communication connections. Therefore, the computing device 3600 can operate in a networked environment using a logical connection with one or more other servers, networked personal computers (PCs) or further general network nodes.

The input device 3650 may be one or more of a variety of input devices, such as a mouse, keyboard, tracking ball, voice-input device, and the like. The output device 3660 may be one or more of a variety of output devices, such as a display, loudspeaker, printer, and the like. By means of the communication unit 3640, the computing device 3600 can further communicate with one or more external devices (not shown) such as the storage devices and display device, with one or more devices enabling the user to interact with the computing device 3600, or any devices (such as a network card, a modem and the like) enabling the computing device 3600 to communicate with one or more other computing devices, if required. Such communication can be performed via input/output (I/O) interfaces (not shown).

In some embodiments, instead of being integrated in a single device, some or all components of the computing device 3600 may also be arranged in cloud computing architecture. In the cloud computing architecture, the components may be provided remotely and work together to implement the functionalities described in the present disclosure. In some embodiments, cloud computing provides computing, software, data access and storage service, which will not require end users to be aware of the physical locations or configurations of the systems or hardware providing these services. In various embodiments, the cloud computing provides the services via a wide area network (such as Internet) using suitable protocols. For example, a cloud computing provider provides applications over the wide area network, which can be accessed through a web browser or any other computing components. The software or components of the cloud computing architecture and corresponding data may be stored on a server at a remote position. The computing resources in the cloud computing environment may be merged or distributed at locations in a remote data center. Cloud computing infrastructures may provide the services through a shared data center, though they behave as a single access point for the users. Therefore, the cloud computing architectures may be used to provide the components and functionalities described herein from a service provider at a remote location. Alternatively, they may be provided from a conventional server or installed directly or otherwise on a client device.

The computing device 3600 may be used to implement video encoding/decoding in embodiments of the present disclosure. The memory 3620 may include one or more video coding modules 3625 having one or more program instructions. These modules are accessible and executable by the processing unit 3610 to perform the functionalities of the various embodiments described herein.

In the example embodiments of performing video encoding, the input device 3650 may receive video data as an input 3670 to be encoded. The video data may be processed, for example, by the video coding module 3625, to generate an encoded bitstream. The encoded bitstream may be provided via the output device 3660 as an output 3680.

In the example embodiments of performing video decoding, the input device 3650 may receive an encoded bitstream as the input 3670. The encoded bitstream may be processed, for example, by the video coding module 3625, to generate decoded video data. The decoded video data may be provided via the output device 3660 as the output 3680.

While this disclosure has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present application as defined by the appended claims. Such variations are intended to be covered by the scope of this present application. As such, the foregoing description of embodiments of the present application is not intended to be limiting.

Claims

1. A method of video processing, comprising: applying, for a conversion between a video unit of a video and a bitstream of the video, at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to the video unit to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; andperforming the conversion based on the refined motion vector.

2. The method of claim 1, wherein at least one of the DMVR or the variant of DMVR is used to refine a shift motion vector (MV) for the sbTMVP mode, and/or wherein at least one of the DMVR or the variant of DMVR is used to refine a control point motion vector (CPMV) for the affine merge mode, and/orwherein at least one of the DMVR or the variant of DMVR is used to refine a control point motion vector (CPMV) for the affine AMVP mode.

3. The method of claim 2, wherein the shift MV is bi-directional predicted, and/or wherein the shift MV points to two reference pictures from both forward and backward, and/orwherein the shift MV that meets a DMVR condition or a decoder side motion vector difference (DMVD) condition is generated for the sbTMVP mode, and/orwherein the shift MV is used to derive motion vectors from corresponding prediction units (PUs) in reference pictures.

4. The method of claim 2, wherein only a PU level DMVD or DMVR is used, and/or wherein at least one of the DMVR or the variant of DMVR is applied to the CPMV that meets a DMVR condition or a DMVD condition, and/orwherein at least one of the DMVR or the variant of DMVR is applied to the CPMV that points to reference pictures from both forward and backward, and/orwherein at least one of the DMVR or the variant of DMVR is applied to the CPMV pairs for a bi-directional predicted affine AMVP mode.

5. The method of claim 1, wherein the video unit is coded by a motion pair from a same direction relative to a current picture, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode and the video unit belongs to a low-delay picture.

6. The method of claim 5, wherein a decoder side motion vector refinement (DMVR) of the video unit is off, and/or wherein a reference picture resampling (RPR) of the video unit is on.

7. The method of claim 5, wherein the motion pair comprises an AMVP motion vector (MV) and a MERGE MV, and/or wherein the AMVP merge mode is allowed for the low-delay picture in a random access configuration, and/orwherein the AMVP merge mode is allowed for the low-delay picture in a low-delay configuration, and/orwherein the motion pair comprises an AMVP MV and a MERGE MV, and the AMVP MV points to a first reference picture with a first picture order count (POC) value and the MERGE MV points to a second reference picture with a second POC value.

8. The method of claim 5, further comprising: for a generation of an AMVP merge list, applying a two-step reference picture checking procedure to the video unit, andwherein applying the two-step reference picture checking procedure comprises: determining whether a first condition is fulfilled; and/orif the first condition is not fulfilled, determining whether a second condition is fulfilled.

9. The method of claim 8, wherein the first condition comprises whether there are available reference pictures from both forward and backward directions, and/or wherein if the first condition is fulfilled, a true-bi-direction AMVP merge mode is conducted, and/orwherein the second condition comprises whether there are available reference pictures from a same direction but with different POC distances, and/orwherein if the second condition is fulfilled, a same-direction AMVP merge mode is conducted.

10. The method of claim 5, wherein an AMVP MV in the motion pair is determined based on at least one of: a coded motion vector prediction (MVP) index or a coded reference index, and/or wherein a template matching (TM) based refinement is used for the motion pair from a same direction, and/orwherein if there is a plurality of reference pictures that has a different POC distance from that of an AMVP reference picture, a target reference picture from the plurality of reference pictures is selected to derive a MERGE MV in the motion pair.

11. The method of claim 10, wherein the target reference picture is selected based on a distance to a current picture, and/or wherein the target reference picture is selected based on a POC distance.

12. The method of claim 11, wherein the target reference picture is closer to the current picture than other reference pictures, and/or wherein the target reference picture is closer to then AMVP reference picture than other reference pictures, and/orwherein the target reference picture has a smaller POC value than other reference pictures in the plurality of reference pictures, and/orwherein the target reference picture has a greater POC value than other reference pictures in the plurality of reference pictures.

13. The method of claim 1, wherein a MERGE MV in the motion pair is determined based on a rule, and wherein the rule is based on a predefined position of a MERGE MV candidate in a MERGE MV list, orwherein the rule is based on a TM cost.

14. The method of claim 1, wherein at least one local illumination compensation (LIC) model parameter of a LIC model for the video unit is derived based on a non-linear model, wherein the video unit coded is a LIC coded block, and wherein the LIC model is updated by adjusting the at least one LIC model parameter.

15. The method of claim 14, wherein the at least one LIC model parameter is derived based on predC=a0*((predR*predR+b)>>s)+a1*predR+a2*c, wherein predC represents prediction sample values of samples neighboring to a current block, predR represents prediction sample values of samples neighboring to a reference block, a0, a1 and a2 represent LIC parameters, respectively, b and c are integers, s represents a shift factor which is an integer, orwherein the at least one LIC model parameter is derived based on predC=a0*((recR*recR+b)>>s)+a1*recR+a2*c,wherein recC represents reconstruction sample values of samples neighboring to a current block, recR represents reconstruction sample values of samples neighboring to a reference block, a0, a1, and a2 represent LIC parameters, respectively, b and c are integers, s represents a shift factor which is an integer.

16. The method of claim 1, wherein a non-average weighting approach to is applied the video unit, wherein the video unit is applied with an advanced motion vector prediction (AMVP) merge mode.

17. The method of claim 1, wherein the conversion includes encoding the video unit into the bitstream, or wherein the conversion includes decoding the video unit from the bitstream.

18. An apparatus for video processing comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to: apply, for a conversion between a video unit of a video and a bitstream of the video, at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to the video unit to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; andperform the conversion based on the refined motion vector.

19. A non-transitory computer-readable storage medium storing instructions that cause a processor to: apply, for a conversion between a video unit of a video and a bitstream of the video, at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to the video unit to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; andperform the conversion based on the refined motion vector.

20. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by an apparatus for video processing, wherein the method comprises: applying at least one of: a decoder side motion vector refinement (DMVR) or a variant of DMVR to a video unit of the video to refine a motion vector of the video unit, wherein the video unit is applied with one of: an affine advanced motion vector prediction (AMVP) mode, a subblock-based temporal motion vector prediction (sbTMVP) mode, or an affine merge mode; andgenerating a bitstream of the video unit based on the refined motion vector.