Cross-component adaptive loop filter using luma differences

1. A method of processing video data, comprising:

determining, for a conversion between a video region of a video and a bitstream of the video, to apply a cross-component filter process on a chroma sample of the video region, wherein in the cross-component filter process, the chroma sample is refined based on applying filtering coefficients on luma sample differences; and

performing the conversion based on the determining,

wherein each luma sample difference corresponds to a difference between a value of a luma sample in a filter support region and a value of a corresponding luma sample of the chroma sample, and wherein the filter support region used for the cross-component filter process is a diamond shaped luma region including the corresponding luma sample.

2. The method of claim 1 , wherein the corresponding luma sample is located at (x, y) for the video region that is in 4:4:4 format, and wherein (x, y) indicates a coordinate of the chroma sample.

3. The method of claim 1 , wherein the corresponding luma sample is located at (2x, 2y) for the video region that is in 4:2:0 format, and wherein (x, y) indicates a coordinate of the chroma sample.

4. The method of claim 1 , wherein the corresponding luma sample is located at (2x, 2y) for the video region that is in 4:2:2 format, and wherein (x, y) indicates a coordinate of the chroma sample.

5. The method of claim 1 , wherein an indication of usage of the cross-component filter process is conditionally included in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled.

6. The method of claim 5 , wherein the indication is conditionally included in the bitstream further based on a chroma color format.

7. The method of claim 1 , wherein the conversion includes encoding the video into the bitstream.

8. The method of claim 1 , wherein the conversion includes decoding the video from the bitstream.

9. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:

determine, for a conversion between a video region of a video and a bitstream of the video, to apply a cross-component filter process on a chroma sample of the video region, wherein in the cross-component filter process, the chroma sample is refined based on applying filtering coefficients on luma sample differences; and

perform the conversion based on the determination,

wherein each luma sample difference corresponds to a difference between a value of a luma sample in a filter support region and a value of a corresponding luma sample of the chroma sample, and wherein the filter support region used for the cross-component filter process is a diamond shaped luma region including the corresponding luma sample.

10. The apparatus of claim 9 , wherein the corresponding luma sample is located at (x, y) for the video region that is in 4:4:4 format, and wherein (x, y) indicates a coordinate of the chroma sample; or

wherein the corresponding luma sample is located at (2x, 2y) for the video region that is in 4:2:0 format, and wherein (x, y) indicates a coordinate of the chroma sample.

11. The apparatus of claim 9 , wherein an indication of usage of the cross-component filter process is conditionally included in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled.

12. The apparatus of claim 11 , wherein the indication is conditionally included in the bitstream further based on a chroma color format.

13. A non-transitory computer-readable storage medium storing instructions that cause a processor to:

determine, for a conversion between a video region of a video and a bitstream of the video, to apply a cross-component filter process on a chroma sample of the video region, wherein in the cross-component filter process, the chroma sample is refined based on applying filtering coefficients on luma sample differences; and

perform the conversion based on the determination,

wherein each luma sample difference corresponds to a difference between a value of a luma sample in a filter support region and a value of a corresponding luma sample of the chroma sample, and wherein the filter support region used for the cross-component filter process is a diamond shaped luma region including the corresponding luma sample.

14. The non-transitory computer-readable storage medium of claim 13 , wherein the corresponding luma sample is located at (x, y) for the video region that is in 4:4:4 format, and wherein (x, y) indicates a coordinate of the chroma sample; or

wherein the corresponding luma sample is located at (2x, 2y) for the video region that is in 4:2:0 format, and wherein (x, y) indicates a coordinate of the chroma sample.

15. The non-transitory computer-readable storage medium of claim 13 , wherein an indication of usage of the cross-component filter process is conditionally included in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled.

16. The non-transitory computer-readable storage medium of claim 15 , wherein the indication is conditionally included in the bitstream further based on a chroma color format.

17. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises:

determining, for a video region of the video, to apply a cross-component filter process on a chroma sample of the video region, wherein in the cross-component filter process, the chroma sample is refined based on applying filtering coefficients on luma sample differences; and

generating the bitstream based on the determining,

wherein each luma sample difference corresponds to a difference between a value of a luma sample in a filter support region and a value of a corresponding luma sample of the chroma sample, and wherein the filter support region used for the cross-component filter process is a diamond shaped luma region including the corresponding luma sample.

18. The non-transitory computer-readable recording medium of claim 17 , wherein the corresponding luma sample is located at (x, y) for the video region that is in 4:4:4 format, and wherein (x, y) indicates a coordinate of the chroma sample; or

wherein the corresponding luma sample is located at (2x, 2y) for the video region that is in 4:2:0 format, and wherein (x, y) indicates a coordinate of the chroma sample.

19. The non-transitory computer-readable recording medium of claim 17 , wherein an indication of usage of the cross-component filter process is conditionally included in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled.

20. The non-transitory computer-readable recording medium of claim 19 , wherein the indication is conditionally included in the bitstream further based on a chroma color format.