Method and apparatus for color difference transform

1. A method for encoding a video signal, the method comprising:

transforming, by a processor executing a color difference encoding transform, an input video signal encoded by tri-stimulus luminance and chromacity values of a color space and having video frames characterized by a dominant white point into a de-correlated video signal encoded by modified luminance and color difference values;

adapting the color difference encoding transform at least in part by adapting calculation of the color difference values based on the dominant white point and

outputting the de-correlated video signal, based on the adapting.

2. The method of claim 1 , wherein the adapting the color difference encoding transform is further based on a pair of coefficients multiplied by respective ones of the color difference values.

3. The method of claim 2 , further comprising determining the pair of coefficients using respective ratios of a normalized encoding function operating on image chromaticity inputs.

4. The method of claim 2 , further comprising determining the pair of coefficients based simultaneously on the dominant white point and at least one of average or median picture levels.

5. The method of claim 1 , wherein the adapting the color difference encoding transform causes the compressed video signal to be more highly compressed than without the adapting.

6. The method of claim 1 , further comprising determining the dominant white point based on the frames of the input video signal, using the processor.

7. The method of claim 6 , wherein the processor changes values of the dominant white point between sets of the frames, based on the determining.

8. The method of claim 6 , wherein determining the dominant white point comprises obtaining a value of the dominant white point from metadata associated with the frames.

9. The method of claim 6 , wherein determining the dominant white point comprises analyzing at least one image in the frames, using the processor.

10. The method of claim 1 , wherein the input signal is based on an International Commission of Illumination (CIE) XYZ color space encoded using a non-linear encoding function.

11. The method of claim 10 , wherein adapting the color difference encoding transform is further based on the non-linear encoding function.

12. An apparatus comprising a processor coupled to a memory, the memory holding instructions for:

transforming, by a color difference encoding transform, an input video signal encoded by tri-stimulus luminance and chromacity values of a color space and having video frames characterized by a dominant white point into a de-correlated video signal encoded by modified luminance and color difference values;

adapting the color difference encoding transform at least in part by adapting calculation of the color difference values based on the dominant white point; and

outputting the de-correlated video signal, based on the adapting.

13. A method for decoding a video signal, the method comprising:

transforming, by a processor executing a color difference decoding transform, an input de-correlated video signal encoded by modified luminance and color difference values of a color space and having video frames characterized by a dominant white point into a correlated video signal encoded by original tri-stimulus luminance and chromaticity values;

adapting the color difference decoding transform at least in part by adapting calculation of the chromaticity values based on the dominant white point; and

outputting the correlated video signal.

14. The method of claim 13 , wherein the adapting the color difference decoding transform is further based on a pair of coefficients multiplied by respective ones of the chromaticity values.

15. The method of claim 14 , further comprising determining the pair of coefficients using respective ratios of a normalized encoding function operating on image chromaticity inputs.

16. The method of claim 14 , further comprising determining the pair of coefficients based simultaneously on the dominant white point and at least one of average or median picture levels.

17. The method of claim 14 , further comprising determining the dominant white point based on the frames of the encoded input video signal, using the processor.

18. The method of claim 17 , wherein the processor changes values of the dominant white point between sets of the frames, based on the determining.

19. The method of claim 17 , wherein determining the dominant white point comprises obtaining a value of the dominant white point from metadata associated with the frames.

20. The method of claim 17 , wherein determining the dominant white point comprises analyzing at least one image in the frames, using the processor.

21. The method of claim 13 , wherein the output signal is based on an International Commission of Illumination (CIE) XYZ color space decoded using a non-linear decoding function.

22. The method of claim 13 , wherein adapting the color difference decoding transform is further based on the non-linear decoding function.

23. An apparatus comprising a processor coupled to a memory, the memory holding instructions for:

transforming, by a color difference decoding transform, an input de-correlated video signal encoded by modified luminance and color difference values of a color space and having video frames characterized by a dominant white point into a correlated video signal encoded by original tri-stimulus luminance and chromaticity values;

adapting the color difference decoding transform based on the dominant white point at least in part by adapting calculation of the chromaticity values; and

outputting the correlated video signal.