Method and apparatus for colour correction during HDR to SDR conversion

1. A method comprising:

deriving chroma components (C 2 12 , C 3 12 ) of a third image by correcting chroma components (C 2 1 ,C 3 1 ) of a second image according to a luma component (C 1 1,pre ) of said second image and a reconstructed component obtained by applying a mapping function to said luma component (C 1 1,pre ) of said second image, said chroma components (C 2 1 ,C 3 1 ) of the second image being obtained by applying a conversion matrix to components (C m 10 ) of a first image; and

adapting coefficients of the conversion matrix which are relative to a chroma component of said third image independently of coefficients of the conversion matrix which are relative to another chroma component of said third image to ensure that there is no clipping on chroma components of the third image.

2. The method of claim 1 , wherein the method further comprises a step of:

deriving a sixth image by applying an inverse of the conversion matrix on components of a fifth image whose chroma components (C 2 3 , C 3 3 ) are derived by inverse correcting chroma components (C 2 2 , C 3 2 ) of a fourth image according to a luma component (C 1 2 ) of said fourth image, said fourth image being obtained from components of said third image; and

compensating the adaptation of coefficients of the conversion matrix by adapting the coefficients of the inverse of the conversion matrix.

3. The method of claim 2 , wherein adapting coefficients of the conversion matrix comprises dividing said coefficients by at least one correcting factor and adapting the coefficients of the inverse of the conversion matrix comprises multiplying said coefficients by said at least one correcting factor.

4. The method of claim 2 , wherein the correction of the conversion matrix applies to all pixels of an image.

5. The method of claim 2 , wherein the correction of the conversion matrix applies only to pixels whose luminance level belongs to a specific luminance range.

6. The method of claim 1 , wherein the method further comprises a step of transmitting an information data representative of the conversion matrix with adapting coefficients, of the inverse of the conversion matrix with adapting coefficients, or at least one correcting factor.

7. The method of claim 1 , wherein the method further comprises a step of:

deriving a sixth image by applying an inverse of the conversion matrix on components of a fifth image whose chroma components (C 2 3 , C 3 3 ) are derived by inverse correcting chroma components (C 2 2 , C 3 2 ) of a fourth image according to a luma component (C 1 2 ) of said fourth image and a chroma correcting function, said fourth image being obtained from components of said third image; and

compensating the adaptation of coefficients of the conversion matrix by adapting the chroma correction function for at least one luminance range that does not need to be corrected.

8. The method of claim 7 , wherein a corrected chroma correcting function is derived by dividing a given chroma correcting function by a correcting factor.

9. The method of claim 8 , wherein the method further comprises a step of transmitting an information data representative of the corrected chroma correcting function or said correcting factor.

10. The method of claim 1 , wherein adapting coefficients of the conversion matrix is based on a correcting factor and two bounds of a luminance range for a chroma component of an image, and wherein the values of said correcting factor and said two bounds of a luminance range are collected over a given period of time, and a filtered version of each of these values is delivered.

11. A non-transitory computer readable medium storing instructions which, when executed by a computer, cause the computer to carry out the method of claim 1 .

12. A device comprising means for a processor configured to:

derive chroma components (C 2 12 , C 3 12 ) of a third image by correcting chroma components (C 2 1 ,C 3 1 ) of a second image according to a luma component (C 1 1,pre ) of said second image and a reconstructed component obtained by applying a mapping function to said luma component (C 1 1,pre ) of said second image, said chroma components (C 2 1 ,C 3 1 ) of the second image being obtained by applying a conversion matrix to components (C m 10 ) of a first image; and

adapt coefficients of the conversion matrix which are relative to a chroma component of said third image independently of coefficients of the conversion matrix which are relative to another chroma component of said third image to ensure that there is no clipping on chroma components of the third image.

13. The device of claim 12 , wherein the processor is further configured to:

derive a sixth image by applying an inverse of the conversion matrix on components of a fifth image whose chroma components (C 2 3 , C 3 3 ) are derived by inverse correcting chroma components (C 2 2 , C 3 2 ) of a fourth image according to a luma component (C 1 2 ) of said fourth image, said fourth image being obtained from components of said third image; and

compensate the adaptation of coefficients of the conversion matrix by adapting the coefficients of the inverse of the conversion matrix.

14. The device of claim 13 , wherein adapting coefficients of the conversion matrix comprises dividing said coefficients by at least one correcting factor and adapting the coefficients of the inverse of the conversion matrix comprises multiplying said coefficients by said at least one correcting factor.

15. The device of claim 13 , wherein the correction of the conversion matrix applies to all pixels of an image.

16. The device of claim 13 , wherein the correction of the conversion matrix applies only to pixels whose luminance level belongs to a specific luminance range.

17. The device of claim 12 , wherein the processor is further configured to transmit an information data representative of the conversion matrix with adapting coefficients, of the inverse of the conversion matrix with adapting coefficients, or at least one correcting factor.

18. The device of claim 12 , wherein the processor is further configured to:

derive a sixth image by applying an inverse of the conversion matrix on components of a fifth image whose chroma components (C 2 3 , C 3 3 ) are derived by inverse correcting chroma components (C 2 2 , C 3 2 ) of a fourth image according to a luma component (C 1 2 ) of said fourth image and a chroma correcting function, said fourth image being obtained from components of said third image; and

compensate the adaptation of coefficients of the conversion matrix by adapting the chroma correction function for at least one luminance range that does not need to be corrected.

19. The device of claim 18 , wherein a corrected chroma correcting function is derived by dividing a given chroma correcting function by a correcting factor.

20. The device of claim 19 , wherein the processor is further configured to transmit an information data representative of the corrected chroma correcting function or said correcting factor.

21. The device of claim 12 , wherein adapting coefficients of the conversion matrix is based on a correcting factor and two bounds of a luminance range for a chroma component of an image, and wherein the values of said correcting factor and said two bounds of a luminance range are collected over a given period of time, and a filtered version of each of these values is delivered.