Perceptual color transformations for wide color gamut video coding

1. A computer-implemented method, comprising:

converting an additive color model signal into a uniform color space signal having chromaticity components and a luminance component;

transforming the chromaticity components by applying a chromaticity-specific transform to the chromaticity components, wherein the chromaticity-specific transform maps a main triangular representation of the chromaticity components in the uniform color space to a four-sided polygonal representation of the chromaticity components in the uniform color space;

transforming the luminance component by applying a luminance-specific transform to the luminance component;

quantizing the transformed chromaticity components and luminance component; and

encoding the quantized chromaticity components and luminance component.

2. The computer-implemented method of claim 1 , wherein the additive color model signal comprises a red-green-blue (RGB) color model signal.

3. The computer-implemented method of claim 1 , wherein the uniform color space signal comprises a YUV color space signal.

4. The computer-implemented method of claim 3 , wherein mapping a main triangular representation of the chromaticity components in the uniform color space comprises subdividing the main triangular representation of the chromaticity components in the YUV color space into a mesh comprising a plurality of secondary triangles.

5. The computer-implemented method of claim 4 , wherein each of the vertices of the main triangular representation comprises one of the primaries of the additive color model.

6. The computer-implemented method of claim 5 , further comprising defining a midway vertex between two of the primaries of the additive color model.

7. The computer-implemented method of claim 6 , further comprising mapping each of the vertices of the main triangular representation and the defined midway vertex to a rectangular representation of the chromaticity components in the YUV color space.

8. The computer-implemented method of claim 7 , wherein the rectangular representation comprises a unit square.

9. The computer-implemented method of claim 7 , wherein respective vertices of each of the plurality of secondary triangles is mapped to the rectangular representation of the chromaticity components in the YUV color space using mean value coordinates.

10. The computer-implemented method of claim 9 , wherein values of each of the chromaticity components in the YUV color space are mapped to the rectangular representation thereof using barycentric coordinates corresponding to the values of each of the chromaticity components.

11. The computer-implemented method of claim 6 , further comprising mapping each of the vertices of the main triangular representation and the defined midway vertex to a four-sided polygonal representation of the chromaticity components in the YUV color space.

12. The computer-implemented method of claim 11 , further comprising applying at least one of a plurality of constraints affecting the mapping of each of the vertices of the main triangular representation, each of the vertices of each of the plurality of secondary triangles, and the defined midway vertex to the four-sided polygonal representation.

13. The computer-implemented method of claim 12 , further comprising adjusting characteristics of at least one of the plurality of constraints to modify the chromaticity-specific transform.

14. The computer-implemented method of claim 1 , wherein the luminance-specific transform comprises an opto-electrical transfer function.

15. A non-transitory computer readable medium having computer executable program code embodied thereon, the computer executable program code configured to cause a processor to:

decode quantized color and brightness components of an input bitstream carrying an image signal represented by a first color space model having brightness and color components;

dequantize the quantized color and brightness components;

apply a first inverse transform to the dequantized color component, wherein the first inverse transform maps a four-sided polygonal representation of the color component in the first color space model to a triangular representation of the color components in the first color space model;

apply a second inverse transform to the dequantized brightness component separately from applying the first inverse transform to the color component; and

convert the image signal represented by a first color space model into that represented by a second color space model, the second color space model being an additive color space model.

16. The non-transitory computer readable medium of claim 15 , wherein the first inverse transform comprises one of an inverse direct transform or an inverse least squares transform, wherein the color and brightness components have been previously transformed to increase the number of code words available to a video codec for compressing the image signal.

17. The non-transitory computer readable medium of claim 16 , wherein the first color space model comprises a YUV color space model, the second color space model comprises a red-green-blue (RGB) color space model, and the video codec comprises a high efficiency video coding (HEVC) codec.

18. The non-transitory computer readable medium of claim 15 , wherein the image signal comprises one of a standard dynamic range (SDR) image signal, a high dynamic range (HDR) image signal, a standard color gamut (SCG) image signal, or a wide color gamut (WCG) image signal.

19. A system, comprising:

a processor comprising:

a converter,

a first transformer,

an opto-electrical transfer function, and

an encoder;

a non-transitory computer-readable medium coupled to the converter, the first transformer, the opto-electrical transfer function, and the encoder, the non-transitory computer-readable medium having computer executable program code embodied thereon, the computer executable program code configured to cause a processor to:

convert an image signal represented by a first color space model into that represented by a second color space model having brightness and color components using the converter;

apply a first transform to the color component using the first transformer, wherein the first transformer maps a triangular representation of the color components in the second color space model to a four-sided polygonal representation of the color components in the second color space model;

apply a second transform to the brightness component using the encoder separately from applying the first transform to the color component;

quantize the transformed color and brightness components using the quantizer; and

encode the quantized color and brightness components and output a bitstream using the encoder.

20. The system of claim 19 , wherein the first transform is invertible.