Patent Grant
11769464
The present disclosure relates to method and systems for image processing. In particular, but not exclusively, the present disclosure relates to processing image data to convert from one color space to another.
Electronic color displays are able to show images using arrays of pixels. In the case of LED and OLED displays, each pixel location is implemented using a red, a green, and a blue LED. The size of the LEDs used at each pixel location means that, to a viewer, the light being emitted from the red, green, and blue LEDs appear to emanate from the same point. Different colors can be produced by modifying the relative intensity of each of the red, green, and blue lights at a pixel location. In some cases, each of the red, green, and blue LED are of equal size, while in other cases, the size and/or shape of each of the LEDs for a given pixel location may differ.
A number of different standards for displaying color images using digital displays exist including, for example, ITU-R Recommendation BT.2020, more commonly known as Rec. 2020, or ITU-R Recommendation BT.709, more commonly known as Rec. 709. Each of these standards generally specify how certain colors are represented in image data. The image data may be used by a digital display to reproduce a color image. Different standards are also generally associated with different characteristics, for example, some standards are capable of representing colors which other standards are not. In particular, Rec. 2020 is capable of representing colors that cannot be shown using Rec. 709. That is to say that the Rec. 2020 color space has a wider color gamut than the Rec. 709 color space.
Image data, representing photos or videos may be used to reproduce an image on a plurality of device types which implement a variety of different standards and/or color spaces. For example, a video streaming service, implemented on the web or using an application, may be capable of streaming video on both a mobile device and laptop computer. The display included in the mobile device may be a different type of display to the display included in the laptop computer, and hence may operate using a different standard for representing color in image data. In some cases, a color space which is representable using a particular display may not correspond directly to a color space defined by a standard such as Rec. 2020. A color space which is reproduceable by a display may be similar to one or more such standards, for example, a digital display may be capable to reproducing a color gamut which is between the color gamut of two different standards.
According to a first aspect of the present disclosure there is provided a computer implemented method for processing image data, the computer implemented method comprising: obtaining input video data including a sequence of frames of input image data, the input image data comprising first image data values expressed according to an input color space; generating first processed image data comprising second image data values expressed according to an output color space, different to the input color space, by processing the input image data using a first color space conversion process; generating second processed image data comprising third image data values expressed according to the output color space by processing the input image data using a second color space conversion process, wherein the second color space conversion process uses a different color space conversion function to the first color space conversion process; and generating output image data derived from both the first processed image data and the second processed image data.
According to a second aspect of the present disclosure there is a provided a computer system comprising processing circuitry, the processing circuitry being configured to: obtain input video data including a sequence of frames of input image data, the input image data comprising first image data values expressed according to an input color space; generate first processed image data comprising second image data values expressed according to an output color space, different to the input color space, by processing the input image data using a first color space conversion process; generate second processed image data comprising third image data values expressed according to the output color space by processing the input image data using a second color space conversion process, wherein the second color space conversion process uses a different color space conversion function to the first color space conversion process; and generating output image data derived from both the first processed image data and the second processed image data.
According to a third aspect of the present disclosure there is provided a non-transitory computer-readable storage medium comprising computer-executable instructions which, when executed by one or more processors, cause the one or more processors to perform a process including: obtaining input video data including a sequence of frames of input image data, the input image data comprising first image data values expressed according to an input color space; generating first processed image data comprising second image data values expressed according to an output color space, different to the input color space, by processing the input image data using a first color space conversion process; generating second processed image data comprising third image data values expressed according to the output color space by processing the input image data using a second color space conversion process, wherein the second color space conversion process uses a different color space conversion function to the first color space conversion process; and generating output image data derived from both the first processed image data and the second processed image data.
Details of systems and methods according to examples will become apparent from the following description, with reference to the Figures. In this description, for the purpose of explanation, numerous specific details of certain examples are set forth. Reference in the specification to “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least that one example, but not necessarily other examples. It should be further noted that certain examples are described schematically with certain features omitted and/or necessarily simplified for ease of explanation and understanding of the concepts underlying the examples.
Computer systems and computer-implemented methods for processing input image data to generate output image data are described herein. In particular, the input image data may include a representation of an image according to an input color space and the output image data may include a representation of the image according to an output color space. The image data may comprise image data values, also referred to as pixel data values, which are expressed according to a particular color space. The image data may be provided in video data comprising a sequence of frames of image data, such as in a video stream. In some cases, the input color space may relate to a color space used when the input image data is generated while the output color space may relate to a color space which is used by a digital display to display the image using the output image data.
The input color space, according to which the image is represented in the input image data, may be a Red, Green, Blue (RGB) color space. RGB color spaces are generally additive color spaces based on the RGB color model. The RGB color model is an additive color model in which red, green, and blue light are added together in various combinations to reproduce a broad array of colors. The RGB color model is generally used for the display of images in electronic systems such as televisions, computers, and mobile devices, such as a phones and tablets. Digital displays generally comprise a red, green, and blue light for each pixel location in the display. In electronic systems, an RGB color value may be represented using a value for each of the red, green and blue components, the values specifying the intensity of the respective color light. The values may be represented using bits, for example each of the red, green, and blue may be represented using, for example, an 8-bit, 16-bit, or 32-bit value.
The RGB color model is not a universal model but is instead device dependent, Different devices may reproduce a given RBG value differently depending on the color elements, such as phosphors, dyes, or LEDs, used. This is because the different color elements used in different displays may respond to individual red, green, and blue values differently. In order to display the same color across a plurality of displays or display types color management is used to modify RGB color values so that the same colors can be accurately reproduced across multiple devices.
The gamut, or color gamut, of a specific color space refers to the complete subset of colors which can be displayed in that specific color space. The color gamut which can be displayed using a given digital display may be dependent on the arrangement and luminance of the color elements used to produce the RGB light for each pixel location. There is a plurality of different color spaces which may be commonly used for digital displays including, for example, sRGB, Adobe RGB, HDTV (Rec. 709), UHDTV (Rec. 2020), and so forth. Although in some examples, a color space directly associated with a given digital display may not directly correspond to a standardized color space but rather may be specific to the display. Where image data has been generated in one color space, such as sRGB, to display the image accurately, that is to say to reproduce the same appearance of colors, on a display which implements the UHDTV color space, color management is required. The digital representations used to display different color spaces are standardized. For example, the UHDTV color space is standardized in the ITU-R Recommendation BT.2020, more commonly known as Rec.2020 and in the ITU-R Recommendation BT.2100 standards.
Color management may be implemented using three-dimensional Lookup-Tables (3D LUTs) which are used to map one color space to another. A 3D LUT can be represented as a 3D lattice of output RGB values which are indexed by sets of input RGB values. When converting from a first color space, say sRGB, to a second color space, say UHDTV, a 3D LUT representing a mapping from the sRGB to UHDTV may be used. 3D LUTs may be generated by computing entries for the 3D LUT using a conversion operation, or a transformation function, from one color space to another color space for a set of primary colors. Where a color in an input color space does not directly relate to a specific entry in the 3D LUT, an interpolation from nearby entries in the 3D LUT may be used to convert the color in the input color space to a color in the output color space.
Throughout the present disclosure, specific examples will be described with respect to RGB color spaces as outlined above. However, it will be appreciated by one skilled in the art that the examples described herein are also applicable to alternative color spaces. For example, Y′UV color spaces may be used in which “Y′” defines a Luma component, and “U” and “V” define two chrominance components, where “U” is blue projection and “V” is red projection. Y′UV is also be used to describe file formats that are encoded using YCbCr which similarly defines color in terms of a Luma component, Y′, and blue, Cb, and red Cr, chroma components. Other examples include Hue, Saturation, Lightness (HSL) and Hue, Saturation, Value (HSV) representations of color. It will be appreciated by one skilled in the art that the present methods may be applied to any color space exhibiting suitably similar logic to the examples described herein.
Other approaches to color management also include using color conversion matrices (CCM) to map colors from one color space to another. Different systems for color management may vary in performance depending on the image data to which they are being applied. In particular, where the gamut of an input color space and an output color space differ there can be difficulties when attempting to make full use of the gamut available in the output color space. Certain methods of color management may perform better when converting colors which are outside of the gamut of the output color space than other methods. On the other hand, some other methods of color management may be more adept at converting colors which are inside the gamut of the output color space. The present disclosure provides methods and systems which make use of multiple color management processes when converting image data from an input color space to an output color space. In this way, it is possible to make use of the strengths of multiple different color management processes to generate output image data representing the image. This also allows the conversion of colors in an input color space to an output color space to be tuned according to content gamut, which represents the full subset of colors which are used in the input image data, rather than the gamut of the input color space.
The input color space may be any suitable input color space which can be used to represent an image in the video data 202. For example, the input color space may be any of, sRGB, Adobe RGB, Rec. 709, Apple RGB, Adobe Wide Gamut RGB, Rec. 2020, and so forth. Frames of image data in the input video data 202 may be gamma corrected, which includes applying a non-linear operation to encode luminance and color. Where the input video data 202 is gamma corrected, an inverse gamma function 203 may be applied to the input video data 202.
Returning to
In one example, the first color space conversion process 206 includes applying a color conversion matrix (CCM) to the input image data 302. The application of a CCM to image data values representing a given pixel location is illustrated in equation 1 below:
wherein the image data values representing a pixel location in the input color space are represented by PR, PG, PB, the image data values representing the pixel location in the output color space are represented by QR, QG, QB, and the color conversion matrix is represented as C.
Using a CCM to perform color space conversion provides accurate transformation of colors which are within the color gamut of the output color space, and in some cases outperforms the use of 3D LUTs for converting colors which are within the gamut of the output color space. In particular, using CCMs may allow highly saturated colors to be accurately displayed in the output color space. However, colors which are outside of the color gamut of the output color space may be clipped when transforming to the output color space, resulting in detail loss and potential hue changes in the image when displayed in the output color space.
Second processed image data 208 comprising third image data values 216 expressed according to the output color space is generated 106 using a second color space conversion process 210. The second color space conversion process 210 is different to the first color space conversion process 206. In particular, the first color space conversion process 206 uses a different color space conversion function to the second color space conversion process 210. For example, the second color space conversion process 210 may be a different type of process to the first color space conversion process 206. Where the first color space conversion process 206 includes using a CCM the second color space conversion process 210 may include using an LUT, and in particular a 3D LUT to convert image data values representing colors in the input color space to image data values 216 representing colors in the output color space. Using 3D LUTs generally provides accurate color conversion, however, where the content gamut expressed in the frame of input image data 302 is smaller than the full gamut of the input color space, there can be losses in saturation. The losses in saturation occur in these situations because the transformation, represented by the 3D LUT, acts to compress a larger gamut into a smaller gamut. When doing so, some parts of the input gamut may be over-compressed, that is to say, compressed to a greater extent than other parts of the input gamut, in order to allow more space in the output space for colors which are more saturated.
The losses in saturation described above are exhibited as certain colors appearing washed out when displayed in the output color space. This is because the entries in 3D LUTs are generally not equidistantly distributed, instead the 3D LUT represents a perceptually uniform conversion. The perception of color by the human eye is not uniform across a whole gamut, and so the 3D LUT may be configured such that differences in colors represented by image data values in the input color space are perceptually, rather than computationally, reproduced in the image data values in the output color space. Generally, the conversion from one color space to another represented by the 3D LUTs may be non-linear near the limits of the gamut, or gamut boundary, the limits of the gamut being the colors which are brightest and most saturated.
One approach to improve the conversion performed using 3D LUTs is to recompute the coefficients in the 3D LUT for each frame of image data that is processed. This would be done by determining the content gamut of the frame of image data and generating a 3D LUT which maps from the content gamut to the gamut of the output color space. However, recomputing the 3D LUT based on the gamut expressed in a given frame of image data is a costly operation in terms of processing resources. Recomputing the coefficients may also take considerable time meaning that where a large number of frames are to be processed sequentially, for example when processing video data, there is a lag in the production of output image data to be displayed. Hence, it is preferred, where possible, to use a static 3D LUT, or to use the same 3D LUT for a large number of frames, rather than recomputing a new 3D LUT for each frame of image data.
The method 100 includes generating 108 output image data 212 which is derived from both the first processed image data 204 and the second processed image data 208. Specific examples of generating the output image data 212 will be described below with respect to
In one example, generating 108 the output image data 212 comprises selecting between second image data values 214 of the first processed image data 204 and third image data values 216 the second processed image data 208. As discussed above, while the first processed image data 204 and the second processed image data 208 both represent the image according to the output color space, the representation of a given color in the image may differ between first 204 and second 208 processed image data due to the difference in the first color space conversion process 206 and the second color space conversion process 210. In this example, generating 108 the output image data 212 may include determining which of the first processed image data 204 and second processed image data 208 is able to represent colors of the input image data 302 more accurately in the output color space, and selecting the processed image data 204 or 208 based on that determination. In this way it is possible to select between a first color conversion technique and a second color conversion technique for the image based on the content of the image, and in particular the content gamut. The content gamut of the image may be same as the gamut of the input color space or may be narrower than the gamut of the input color space. Not all images represented in a given color space may make use of all colors available in the gamut of that color space. For example, where the content gamut is smaller than the gamut of the input color space, CCM may be used to generate the output image data 212, alternatively, where the content gamut makes use of the full gamut of the input color space a 3D LUT may be used to generate the output image data 212.
In another example, generating the output image data 212 comprises combining second image data values 214 of the first processed image data 206 and third image data values 216 of the second processed image data 208. As described above in relation to
In another example, shown in
In some instances, combining the first processed image data 204 and the second processed image data 208 may include blending second image data values 214 in the first processed pixel data 502 with third image data values 216 in the second processed pixel data 504 for each pixel location 304a, 304b, 304c in the image. In other examples, image data values may only be blended for a subset of the pixel locations 304a, 304b, 304c in the image. A combination of selecting between the first 204 and second 208 processed image data and blending image data values of the first 214 and second 216 image data values may be used for different pixel locations in the image when generating the output image data 212. Blending between image data values 214 of the first processed image data 204 and image data values 216 of the second processed image data 208 at regions of the frames of image data which represent a transition between two colors may improve the perceptual quality of the transition by mitigating the potential severity in the transition between the two regions.
In some examples, blending the second image data values 214 with the third image data values 216 includes using alpha blending second image data values 214 and third image data values 216 representing the same pixel locations. Alpha blending, or alpha compositing, is generally a process for combining two images, such as an image of a background and an object, to create the appearance of partial or full transparency. Where the two images being blended are the same image but represented differently, alpha blending provides a method for generating a weighted average of the representations of the same image. In the present example, alpha blending can be used to generate a weighted average between the first processed image data 204 and the second processed image data 208. Alpha blending in this way may be performed using the same weightings across the whole of the first processed image data 204 and the second processed image data 206 or may be performed differently for individual pixel locations, or subsets of pixel locations.
In some examples, the one or more parameter values 802 may be updated based on the output image data 212, for example, to modify the blending based on the accuracy of colors represented in the output image data 212. The one or more parameter values may, in some cases, also be determined based on the input image data 302 and/or the first processed image data 204 and the second processed image data 208. In some examples, generating 108 the output image data 212 includes generating a sequence of frames of output image data corresponding to the sequence of frames of input image data. In particular, the generating 108 the output image data 212 includes generating a first frame of output image data 808. The first frame of output image data 808 may be processed 804 to determine output image data statistics 806 and then a second frame of output image data 810 may be generated, wherein the second frame of output image data 810 is dependent on the output image data statistics 806. For example, the set of one or more parameter values 802 may be modified based on the statistics 806. The modified set of parameters values 802 may be used when generating the second frame of output image data 810, for example, to blend image data values generated using two different color space conversion processes. In examples where image data values are selected from either of the first processed image data 204 and the second processed image data 208, the statistics 806 may be used to determine which of the image data values are to be selected for each pixel location, or for a subset of pixel locations. In some examples, not shown, determining the statistics 806 may also be based on the input image data 302 and/or the first processed image data 204 and the second processed image data 208.
In one example, the statistics 806 may include an indication of a proportion of pixel locations in the output image data 212 which are clipped. For example, where it is determined from the statistics 806 that a predetermined proportion, such as more than 5%, of pixels represented by the fourth image data values 602 in the output image data 212 are clipped, then the one or more parameter values may be modified to increase the proportion of the third image data values 214 of the second processed image data 204, generated using a 3D LUT, which contributes to the output image data 212. The extent to which the one or more parameter values 802 are modified may be proportional to ratio of pixel locations which are clipped in the output pixel data 602. For example, where only a small proportion, say less than 0.01%, of pixel locations are clipped in the output image data 212, the parameter values 802 may only be modified by a relatively small amount. Whereas, in examples where a large proportion, say more than 5% of pixel locations are clipped in the output image data 212, the parameter vales 802 may be modified by a larger amount.
In other examples, the statistics 806 may include a comparison between the maximum saturation available in the output color space and the maximum saturation reproduced using the output image data 212, and in particular a given frame of output image data 212. In this way, the statistics 806 may indicate that a full range of the output color space is not being utilized by the output image data 212. In this case, the one or more parameter values 802 may be modified to increase the relative contribution of processed image data 208 generated using the second color space conversion process 210, which in this example includes using a 3D LUT.
In instances where there is a plurality of parameter values 802, each being associated with a different subset of pixel locations, only a subset of the plurality of parameter values 802 may be modified in response to the statistics 806. For example, where the statistics specify a proportion of pixel locations which are clipped in the output image data 212, only parameter values 802 associated with clipped pixel locations may be modified in response to the statistics 806.
Modifying the one or more parameter 802 values can improve the performance of the method 100 when processing subsequent frames of input image data to generate frames of output image data 212. For example, adjacent frames of image data in the input video data 202 may be likely to be similar in content gamut, for example, where adjacent frames of image data are captured in the same scene. By modifying the parameter values, to tune the performance of the alpha blending, based on the statistics 806 generated for the first frame of output image data 808 it becomes possible to improve the performance of the method when converting from an input color space to an output color space for subsequent frames of image data which have a similar content gamut to the first frame of image data. In particular, the second frame of output image data 810 corresponding to the subsequent frame of input image data may more accurately represent the colors of the subsequent frame input image data when reproduced on a digital display than the first frame of output image data 808 represents the colors of the first frame of input image data 302.
It will be appreciated by those skilled in the art that generating 108 output image data 212 may include further steps beyond those illustrated in
As described above, using static mapping functions for the first color space conversion process 206 and the second color space conversion process 208 provides faster, and less compute intensive methods for processing frames of input image data 302 to convert an input color space to an output color space. However, this relies on mapping functions, such as a CCM and/or a 3D LUT, between the input color space and the output color space to be available.
The color conversion matrix 904 may be selected from a set of two or more color conversion matrices. For example, a computer system, which will be described in more detail below with respect to
In some examples, not shown, the second color space conversion process 210 may include, before processing the image data 302 using the 3D LUT 1004, processing the image data 302 with a one-dimensional (1D) LUT. The 1D LUT may be referred to as an equidistant 1D LUT which is used to redistribute image data values in the input image data 302 to match a distribution of entries in the 3D LUT 1004. As described above, the 3D LUT may include higher densities of entries around certain colors, and as such redistributing the image data values in the input image data 302 can increase the accuracy of color space conversion using the second color space conversion process 210.
In some cases, if a suitable LUT 1004 is not available, the method 100 may include generating the LUT 1004 based on the input color space and the output color space.
In some examples, not illustrated, the first color space conversion process 206 or the second color space conversion process 210 may include computing transformations between an input color space and an output color space on-the-fly rather than relying on the use of static mapping information, such as a CCM or a 3D LUT. In one such example, not illustrated, the second color space conversion process 210 includes determining the conversion operation 1104 for transforming image data values represented in the input color space to image data values represented in the output color space and applying the conversion operation 1104. The conversion function 1104 may be applied to the one or more image data values in the input video data 202 to generate the second processed image data 208 including the third image data values 216.
The processing circuitry 1202 may include any suitable combination of processing hardware. Examples of processing circuitry which may be employed include display processing units (DPU), which include fixed function hardware which is specifically configured to perform to the method 100, central processing units (CPU), graphical processing units (GPU), image signal processors (ISP) or other suitable type of processing units. In some examples, a combination of multiple types of processing units may be included the processing circuitry 1202. Additionally, or alternatively, the processing circuitry 1202 may include other application specific processing circuitry such as an application specific integrated circuit configured to execute a method as described above with respect to
In examples where the processing circuitry 1202 comprises one or more general purpose processing units such as CPUs, GPUs, and so forth, the computer system 1200 may comprise storage 1204. The storage 1204 may store computer executable instruction 1206 which, when executed by the one or more general purpose processing units, cause the computer system 1200 to perform the method 100 described above.
The computer system 1200 shown is an example of a subsystem of a computing device. For example, the computer system 1200 may be part of a personal computer, a server, a mobile computing device, such as a smart telephone or tablet computer, and so forth. In practice, there may be many more modules connected to, or part of the computer system 1200, including for example, communication modules for sending and receiving video data 202, and image data 212. The computer system 1200 may be communicable with one or more further computer systems 1200 using the communication modules through wired or wireless means. For example, the communications modules may include wireless communication modules such as WiFi, Bluetooth, or cellular communications modules arranged to communicate with further computing devices over cellular communications protocols. Additionally, or alternatively, the communication modules may be wired communication modules. In some examples, the computer system 1200 is in communication with a camera which generates the input video data 202. In this case, the computer system 1200 may be configured to convert the input video data 202 from an input color space, associated with the camera, to an output color space in which the video is to be viewed. The computer system 1200 may then transmit the output image data 212 generated for receipt by further computing devices.
In some examples, the computer system 1200 includes a display 1208 such as an LED, OLED, LCD, Plasma, or any other suitable display which is capable of reproducing an image based on image data 212. The output color space used to represent the image in the output image data 212, may be dependent on the type of display 1208 which is included in the computer system 1200. Different display types may generally be capable of displaying different color gamuts based on the arrangement, size, type, and number of color elements included in the display. Hence, some displays may be capable of displaying a larger color gamut than other displays. To make use of the full color gamut which a display is capable of reproducing, different displays may be associated with different color spaces, that is to say that displays may including processing circuitry which is configured to process image data formatted according to one or more specific standards to represent colors. In some examples, the output color space used to represent the image in the output image data 212 may be the same color space as a color space associated with the display 1208. The gamut which is representable with a given display may not directly correspond to a color space as defined in a standard, but may be specific to the display.
While the example of
In some examples, in particular those described above with respect to
It is to be understood that any features described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other examples, or any combination of any other examples. Furthermore, equivalents and modification not described above may also be employed without departing from the scope of the accompanying claims.
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