Method and apparatus for processing image data

1. A method of encoding image data including first, second and third color components defining color characteristics of image portions in a color space, the method comprising for at least a portion of an image:

scaling a first color difference component and a second color difference component obtained from the first, second and third color components of the image portion, by applying a gain factor to extend a respective dynamic range of the first color difference component and the second color difference component, the first color difference component being defined as a weighted difference between the third color component and the first color component and the second color difference component being defined as a weighted difference between the second color component and the first color component, wherein the gain factor applied to a color difference component is dependent upon the color gamut of the color space; and

encoding the scaled first and second color difference components.

2. A method according to claim 1 , wherein the gain factor applied to a respective color difference component is such that the color difference component is within the range −0.5 to 0.5.

3. A method according to claim 1 , wherein scaling of the first and second color difference components is applied prior to quantization of the first and second color difference components.

4. A method according to claim 1 , wherein scaling of the first and second color difference components is applied after quantization of the first and second color difference components.

5. A method according to claim 1 , wherein the image portion is a high dynamic range image portion having a bit depth greater than 8 bits.

6. A method according to claim 1 , further comprising transmitting data representative of the gain factor applied.

7. A method according to claim 6 , wherein the data representative of the gain factor applied is included in a field of Video Usability Information.

8. A method according to claim 7 , wherein data indicating the color gamut used is included in a matrix_coeff field of Video Usability Information.

9. A method for decoding image data including first, second and third color components defining color characteristics of image portions in a color space, the method comprising for at least a portion of an image:

decoding a received data bitstream;

obtaining, from the decoded bitstream, a first color difference component and a second color difference component;

rescaling the first color difference component and the second color difference component by applying a rescaling factor corresponding to a gain factor applied to the first and second color difference components prior to encoding, wherein the gain factor is dependent upon the color gamut of the color space, the first color difference component being defined as a weighted difference between the third color component and the first color component and the second color difference component being defined as a weighted difference between the second color component and the first color component; and

processing the rescaled first and second color difference components to obtain corresponding color components of the image portion.

10. A method according to claim 9 , further comprising receiving data representative of the gain factor applied prior to encoding, in a field of Video Usability Information.

11. An encoder for encoding image data including first, second and third color components defining color characteristics of image portions in a color space, the encoder comprising a processor configured for:

scaling a first color difference component and a second color difference component, obtained from the first, second and third color components of an image portion, by applying a gain factor to extend their a respective dynamic range of the first color difference component and the second color difference component, the first color difference component being defined as a weighted difference between the third color component and the first color component and the second color difference component being defined as a weighted difference between the second color component and the first color component, wherein the gain factor applied to a color difference component is dependent upon the color gamut of the color space; and

encoding the scaled first and second color difference components.

12. An encoder according to claim 11 , wherein the gain factor applied to a respective color difference component is such that the color difference component is within the range −0.5 to 0.5.

13. An encoder according to claim 11 , wherein scaling of the first and second color difference components is applied prior to quantization of the first and second color difference components.

14. An encoder according to claim 11 , wherein scaling of the first and second color difference components is applied after quantization of the first and second color difference components.

15. An encoder according to claim 11 , wherein the image portion is a high dynamic range image portion having a bit depth greater than 8 bits.

16. An encoder according to claim 11 , wherein the processor is further configured for transmitting data representative of the gain factor applied.

17. An encoder according to claim 16 , wherein the data representative of the gain factor applied is included in a field of Video Usability Information.

18. An encoder according to claim 17 , wherein data indicating the color gamut used is included in a matrix_coeff field of Video Usability Information.

19. A decoder for decoding image data including first, second and third color components defining color characteristics of image portions in a color space, the decoder comprising a processor configured for:

decoding a received bitstream;

obtaining, from the decoded bitstream, a first color difference component and a second color difference component;

rescaling the first color difference component and the second color difference component by applying a rescaling factor corresponding to a gain factor applied to the first and second color difference components prior to encoding, wherein the gain factor is dependent upon the color gamut of the color space, the first color difference component being defined as a weighted difference between the third color component and the first color component and the second color difference component being defined as a weighted difference between the second color component and the first color component; and

processing the rescaled first and second color difference components to obtain corresponding color components of an image portion.

20. A decoder according to claim 12 , wherein the processor is further configured for receiving data representative of the gain factor applied prior to encoding, in a field of Video Usability Information.

21. A non-transitory computer-readable medium including instructions for causing one or more processors to perform encoding image data including first, second and third color components defining color characteristics of image portions in a color space, the encoding comprising for at least a portion of an image:

scaling a first color difference component and a second color difference component obtained from the first, second and third color components of the image portion, by applying a gain factor to extend a respective dynamic range of the first color difference component and the second color difference component, the first color difference component being defined as a weighted difference between the third color component and the first color component and the second color difference component being defined as a weighted difference between the second color component and the first color component, wherein the gain factor applied to a color difference component is dependent upon the color gamut of the color space; and

encoding the scaled first and second color difference components.

22. A non-transitory computer-readable medium according to claim 21 , wherein the gain factor applied to a respective color difference component is such that the color difference component is within the range −0.5 to 0.5.

23. A non-transitory computer-readable according to claim 21 , wherein scaling of the first and second color difference components is applied prior to quantization of the first and second color difference components.

24. A non-transitory computer-readable medium according to claim 21 , wherein scaling of the first and second color difference components is applied after quantization of the first and second color difference components.

25. A non-transitory computer-readable medium according to claim 21 , wherein the image portion is a high dynamic range image portion having a bit depth greater than 8 bits.

26. A non-transitory computer-readable medium according to claim 21 , wherein the encoding further comprises transmitting data representative of the gain factor applied.

27. A non-transitory computer-readable medium according to claim 26 , wherein the data representative of the gain factor applied is included in a field of Video Usability Information.

28. A non-transitory computer-readable medium according to claim 26 , wherein data indicating the color gamut used is included in a matrix_coeff field of Video Usability Information.