Method and device for optimizing encoding/decoding of compensation offsets for a set of reconstructed samples of an image

1. A method of encoding at least a part of an image comprising a plurality of samples, each sample comprising at least two components, the encoded image comprising at least one filtering parameter, the method comprising:

encoding a first chroma component and a second chroma component of at least one sample of the image to provide at least one encoded sample;

decoding the at least one encoded sample; and

computing the at least one filtering parameter as a function of the difference between at least one chroma component of the at least one sample of the image and the at least one corresponding chroma component of the at least one corresponding decoded sample, the at least one filtering parameter being computed so as to be used in a loop filter as a common filtering parameter for sample adaptive offset loop filtering both the first and second chroma components of a decoded sample;

wherein when the sample adaptive offset loop filtering is of a first type at least one filtering parameter is computed to be used as a common filtering parameter for the filtering of the first type, the first type of sample adaptive offset loop filtering being one of edge-type filtering and band-type filtering, and when the sample adaptive offset loop filtering is of a second type, different from the first type, different filtering parameters are computed to be used as dedicated filtering parameters for filtering the first and second chroma components respectively, the second type of sample adaptive offset loop filtering being the other of edge-type filtering and band-type filtering.

2. A method according to claim 1 , wherein the first and second chroma components are U and V chroma components respectively.

3. A method according to claim 1 , wherein at least one filtering parameter is computed to be used as a dedicated filtering parameter for filtering an individual one of the first and second chroma components of the at least one decoded sample.

4. A method according to claim 3 , wherein offsets are dedicated filtering parameters.

5. A method according to claim 1 , wherein the or one said filtering parameter is a flag indicating, for a given sample, that one or more filtering parameters used for filtering another sample are to be used to provide corresponding filtering parameters for filtering the given sample.

6. A method according to claim 1 , wherein one said filtering parameter is a flag indicating, for a given sample in a first said part of the image, that one or more filtering parameters used for filtering another sample in a second said part of the image, different from said first part, are to be used to provide corresponding filtering parameters for filtering the given sample.

7. A method according to claim 1 , wherein one said filtering parameter is a left flag indicating, for a given sample in a first said part of the image, that one or more filtering parameters used for filtering another sample in a second said part of the image, to the immediate left of said first part, are to be used to provide corresponding filtering parameters for filtering the given sample, and another said filtering parameter is an upper flag indicating, for said given sample, that one or more filtering parameters used for filtering another sample in a third said part of the image, immediately above said first part, are to be used to provide corresponding filtering parameters for filtering the given sample.

8. A non-transitory computer-readable storage medium storing instructions of a computer program for implementing each step of a method according to claim 1 .

9. A method of decoding at least a part of an image comprising a plurality of samples, each sample comprising at least two components, the encoded image comprising at least one filtering parameter, the method comprising:

decoding a first chroma component and a second chroma component of at least one encoded sample to provide a first chroma component and a second chroma component of at least one reconstructed sample;

decoding the at least one filtering parameter; and

performing sample adaptive offset loop filtering on the at least one reconstructed sample, the filtering comprising using at least one decoded filtering parameter as a common filtering parameter for filtering both the first chroma component and the second chroma component of the at least one reconstructed sample;

wherein when the sample adaptive offset loop filtering is of a first type at least one decoded filtering parameter is used as a common filtering parameter for the filtering of the first type, the first type of sample adaptive offset loop filtering being one of edge-type filtering and band-type filtering, and when the sample adaptive offset loop filtering is of a second type, different from the first type, different decoded filtering parameters are used as dedicated filtering parameters for filtering the first and second chroma components respectively, the second type of sample adaptive offset loop filtering being the other of edge-type filtering and band-type filtering.

10. A method according to claim 9 , wherein the first and second chroma components are U and V chroma components respectively.

11. A method according to claim 9 , wherein at least one decoded filtering parameter is dedicated for filtering an individual one of the first and second chroma components of the at least one reconstructed sample.

12. A method according to claim 11 , wherein offsets are dedicated filtering parameters.

13. A method according to claim 9 , wherein the or one said filtering parameter is a flag indicating, for a given reconstructed sample, that one or more filtering parameters used for filtering another reconstructed sample are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample.

14. A method according to claim 9 , wherein one said filtering parameter is a flag indicating, for a given reconstructed sample in a first said part of the image, that one or more filtering parameters used for filtering another reconstructed sample in a second said part of the image, different from the first said part, are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample.

15. A method according to claim 9 , wherein one said filtering parameter is a left flag indicating, for a given reconstructed sample in a first said part of the image, that one or more filtering parameters used for filtering another reconstructed sample in a second said part of the image, to the immediate left of the first said part, are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample, and another said filtering parameter is an upper flag indicating, for said given reconstructed sample, that one or more filtering parameters used for filtering another reconstructed sample in a third said part of the image, immediately above said first part, are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample.

16. A non-transitory computer-readable storage medium storing instructions of a computer program for implementing each step of a method according to claim 9 .

17. An encoding device for encoding at least a part of an image comprising a plurality of samples, each sample comprising at least two components, the encoded image comprising at least one filtering parameter, the encoding device comprising:

an encoding unit which encodes a first chroma component and a second chroma component of at least one sample of the image to provide at least one encoded sample;

a decoding unit which decodes the at least one encoded sample; and

a computing unit which computes the at least one filtering parameter as a function of the difference between at least one chroma component of the at least one sample of the image and the at least one corresponding chroma component of the at least one corresponding decoded sample, the at least one filtering parameter being computed so as to be used in a loop filter as a common filtering parameter for sample adaptive offset loop filtering both the first and second chroma components of a decoded sample;

wherein the computing unit is operable, when the sample adaptive offset loop filtering is of a first type, to compute at least one filtering parameter to be used as a common filtering parameter for the filtering of the first type, the first type of sample adaptive offset loop filtering being one of edge-type filtering and band-type filtering, and is further operable, when the sample adaptive offset loop filtering is of a second type, different from the first type, to compute different filtering parameters to be used as dedicated filtering parameters for filtering the first and second chroma components respectively, the second type of sample adaptive offset loop filtering being the other of edge-type filtering and band-type filtering.

18. A decoding device for decoding at least a part of an image comprising a plurality of samples, each sample comprising at least two components, the encoded image comprising at least one filtering parameter, the decoding device comprising:

a sample decoding unit which decodes a first chroma component and a second chroma component of at least one encoded sample to provide a first chroma component and a second chroma component of at least one reconstructed sample;

a parameter decoding unit which decodes the at least one filtering parameter; and

a filtering unit which performs sample adaptive offset loop filtering on the at least one reconstructed sample, the filtering comprising using at least one decoded filtering parameter as a common filtering parameter for filtering both the first chroma component and the second chroma component of the at least one reconstructed sample;

wherein the filtering unit is operable, when the sample adaptive offset loop filtering is of a first type, to use at least one decoded filtering parameter as a common filtering parameter for the filtering of the first type, the first type of sample adaptive offset loop filtering being one of edge-type filtering and band-type filtering, and is further operable, when the sample adaptive offset loop filtering is of a second type, different from the first type, to use different decoded filtering parameters as dedicated filtering parameters for filtering the first and second chroma components respectively, the second type of sample adaptive offset loop filtering being the other of edge-type filtering and band-type filtering.

19. A device according to claim 18 , wherein the first and second chroma components are U and V chroma components respectively.

20. A device according to claim 18 , wherein at least one decoded filtering parameter is dedicated for filtering an individual one of the first and second chroma components of the at least one reconstructed sample.

21. A device according to claim 20 , wherein offsets are dedicated filtering parameters.

22. A device according to claim 18 , wherein the or one said filtering parameter is a flag indicating, for a given reconstructed sample, that one or more filtering parameters used for filtering another reconstructed sample are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample.

23. A device according to claim 18 , wherein one said filtering parameter is a flag indicating, for a given reconstructed sample in a first said part of the image, that one or more filtering parameters used for filtering another reconstructed sample in a second said part of the image, different from the first said part, are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample.

24. A device according to claim 18 , wherein one said filtering parameter is a left flag indicating, for a given reconstructed sample in a first said part of the image, that one or more filtering parameters used for filtering another reconstructed sample in a second said part of the image, to the immediate left of the first said part, are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample, and another said filtering parameter is an upper flag indicating, for said given reconstructed sample, that one or more filtering parameters used for filtering another reconstructed sample in a third said part of the image, immediately above said first part, are to be used to provide corresponding filtering parameters for filtering the given reconstructed sample.