Patent Application
20070285285
The purposes and advantages of the present invention will be apparent from the following detailed description in conjunction with the appended tables and figures of drawings in which:
a-e are a series of depictions of a data block undergoing processing by the quantization stage of
a-e are also a series of depictions of a data block undergoing processing by the quantization stage, only here the data block includes high-frequency data.
a-c are a series of depictions of a data block, specifically that of the raw block of
a-b depict data blocks having other characteristics in the digital data than high or low frequency that may be employed by alternate embodiments of the present invention.
In the various figures of the drawings, like references are used to denote like or similar elements or steps.
A preferred embodiment of the present invention is apparatus and method for efficient compression of digital data. As illustrated in the various drawings herein, and particularly in the views of
In the context of H.264 video compression the present inventors have observed that poor compression efficiency results for high frequency residue information because the Huffman tables for syntax elements, such as “total_zeros,” are tuned for video content where these syntax elements take smaller values. As described above, in the Background Art section, these syntax elements tend to take larger values in the high frequency case, hence requiring more bits to represent them and resulting in poor compression.
Briefly, the present inventors have devised a way to counter this by adding flexibility to the interpretation of the syntax elements “total_zeros” and “run_before.” Specifically, the inventors propose adding flexibility so that these syntax elements (or corresponding elements in other compression techniques) can be handled conventionally for low-frequency data, and so that they can alternately be represented with counts of zeros done backwards from the last coefficient for high-frequency data.
a-c are a series of depictions of a data block 40, specifically that of the raw block 52 of
If the alternate quantization used here next uses the same technique that the quantization stage 32 of
Thus, where using the conventional approach for the very same high-frequency block 54 and high-frequency array 56 produced a “total_zeros” of 11 (see e.g.,
In general, depending upon the data at hand (whether it is high-frequency or low-frequency), an encoding stage in accord with the inventive compression system 100 can switch between the forward counting mode (i.e., employing the conventional linear zigzag forward scan order 46) and the backward counting mode (i.e., employing the novel zigzag linear reverse scan order 102) for enhanced compression performance across a greater range of data. Notably, both counting modes require the same computational effort for quantization and for the major part of entropy encoding.
From the decoder standpoint, which of the two modes of quantization and encoding was used can be indicated by the binary valued “counting mode” mode flag 108 which indicates whether forwards or backwards counting was done. Of course, the addition of an extra mode flag itself constitutes an overhead, but it has been the inventors' observation that the inventive compression system 100 still often provides a net benefit.
In theory, the mode flag 108 is optional, although it is expected by the inventors that few embodiments of the invention will not include it in at least some form. For example, using the novel zigzag linear reverse scan order 102 with no indication of this usage in an otherwise conventional compression process is one way to encrypt the digital data in the resulting data bit stream. Alternately, for some types of digital data the zigzag linear reverse scan order 102 might inherently be more efficient and its use by an encoder thus assumed by a decoder for data of the type.
Digressing slightly, before considering the burden of adding the mode flag 108, it can be helpful to appreciate the amount of high-frequency data that is actually present in typically video data.
In summary, even for H.264 video compression with its sophisticated motion modeling, there is a significant percentage of data that is associated with high-frequency characteristics. The present inventive compression system 100 is directly applicable to such data. Similarly, even greater applicability and benefit can be expected for previous generation video compression standards such as MPEG-2 and MPEG-4, with their relatively simple motion modeling.
With its implications about the potential realizable benefits,
With respect to (a), such the mode flag 108 can be indicated at a coarser level than at a 4×4 block. For instance, a natural granularity at which the mode flag 108 can be indicated is at the granularity of the motion block, the frame (or still image), a sub-sequence of video frames, or even some other unit basis entirely (e.g., a one second block of audio data). If a video motion block size of 16×16 is chosen, for example, the mode flag 108 then can be indicated at a 16×16 block level and the same mode would be used for all sixteen 4×4 blocks inside the 16×16 block, thus amortizing the extra syntax information.
With respect to (b), additional savings on the average bit-rate incurred by the mode flag 108 can be obtained by using context-based methods for predicting the likely value for the mode flag. Since typical video data exhibits high spatio-temporal correlations, the context information can be derived from values for the mode flag 108 of the spatial or temporal neighbors of the block in question.
In
Some particular variations of the compression process 200 from how it is represented in
Using the same position coding algorithm works well in the examples used herein, which are based on video data and the standardized forms of compression applied to it, but this should not be taken as implying a limitation or even desirability. For instance, the present invention can also be applied to audio type data, which often has multiple channels for stereo or other sophisticated effects. Using different position coding algorithms may be more efficient here, possibly by applying different ones chosen to take advantage of inherent relationship between the channels to achieve more efficient compression of the audio data.
Similarly, when the optional mode flag 108 is employed, step 210 will typically have minor differences over what would have previously been done conventionally. Additionally, however, here as well there is no particular reason that step 210 (VLC entropy encoding) has to be performed the same for the output from step 216 as for the output from step 218.
In summary, providing “counting mode flexibility” and using a mode flag 108 to indicate the particular mode used enables an efficient representation of low frequency as well as high-frequency data, as opposed to only the low-frequency data that is the target of the present-day video compression standards. Furthermore, as noted herein repeatedly and now for the last time, video data is merely one type of data that is suitable for application of the inventive compression system 100. For example, without limitation, it is relatively easy to appreciate that suitable embodiments of the inventive compression system 100 can be beneficially applied to still image and audio data. Conceptually, images can be thought of as similar to the individual frames of raw video data in the examples presented above, and compressing audio data today (e.g., MP3) especially uses most of the same principles and techniques as compressed video.
Finally, it should be noted that we have used examples based on low-frequency and high-frequency digital data, since these should readily be ones readily appreciated by skilled practitioners in this art. The spirit of the present invention, however, has broader applicability than merely to above the diagonal 49 low-frequency data (e.g.,
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and that the breadth and scope of the invention should not be limited by any of the above described exemplary embodiments, but should instead be defined only in accordance with the following claims and their equivalents.
