Signaling of cross-component adaptive loop filter

1. A method of processing video data, comprising:

determining, during a conversion between a video unit of a video and a bitstream of the video, to conditionally include an indication of usage of a cross-component filtering tool in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled, wherein in the cross-component filtering tool, values of chroma samples are refined by using values of luma samples; and

performing the conversion based on the determining,

wherein a first offset used in the cross-component filtering tool for a chroma sample of the chroma samples is derived based on values of luma samples in a filter support region and filter coefficients of the video unit, wherein the filter support region is used for the cross-component filtering tool and is a diamond shaped luma region;

wherein a second offset is derived using a clipping operation to the first offset, wherein the second offset is clipped to a given range; and

the conversion is performed by applying the cross-component filtering tool to refine the chroma sample based on the second offset.

2. The method of claim 1 , wherein the indication of usage of the cross-component filtering tool is conditionally included in the bitstream at the sequence parameter set (SPS) level further based on a chroma color format.

3. The method of claim 1 , wherein a first syntax element indicating an applicability of the cross-component filtering tool is included in the bitstream in a picture header or a slice header in case that the indication indicates that the cross-component filtering tool is enabled at the SPS level.

4. The method of claim 1 , wherein the indication is not explicitly included in case that the adaptive loop filter tool is disabled.

5. The method of claim 1 , wherein information related to the cross-component filtering tool is excluded from the bitstream in case that the adaptive loop filter tool is disabled.

6. The method of claim 5 , wherein the information related to the cross-component filtering tool includes at least one of a second syntax element which specifies that the cross-component filtering tool is enabled or not for a Cb color component, or a third syntax element which specifies that the cross-component filtering tool is enabled or not for a Cr color component.

7. The method of claim 6 , wherein the information related to the cross-component filtering tool further includes a fourth syntax element, which specifies a first identifier of a first adaptive loop filter that the Cb color component of a current slice refers to, when the second syntax element indicates that the cross-component filtering tool is enabled for the Cb color component; and wherein the information related to the cross-component filtering tool further includes a fifth syntax element, which specifies a second identifier of a second adaptive loop filter that the Cr color component of the current slice refers to, when the third syntax element indicates that the cross-component filtering tool is enabled for the Cr color component.

8. The method of claim 1 , wherein a conformance bitstream satisfies that an identifier of an adaptive loop filter for the cross-component filtering tool included in the bitstream shall refer to one available adaptive loop filter.

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

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

11. 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, during a conversion between a video unit of a video and a bitstream of the video, to conditionally include an indication of usage of a cross-component filtering tool in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled, wherein in the cross-component filtering tool, sample values of chroma samples are refined by using values of luma samples; and

perform the conversion based on the determining,

wherein a first offset used in the cross-component filtering tool for a chroma sample of the chroma samples is derived based on values of luma samples in a filter support region and filter coefficients of the video unit, wherein the filter support region is used for the cross-component filtering tool and is a diamond shaped luma region;

wherein a second offset is derived based on using a clipping operation to the first offset, wherein the second offset is clipped to a given range; and

the conversion is performed by applying the cross-component filtering tool to refine the chroma sample based on the second offset.

12. The apparatus of claim 11 , wherein a first syntax element indicating an applicability of the cross-component filtering tool is included in the bitstream in a picture header or a slice header in case that the indication indicates that the cross-component filtering tool is enabled at the SPS level.

13. The apparatus of claim 11 , wherein the indication of usage of the cross-component filtering tool is conditionally included in the bitstream further based on a chroma color format; and

wherein the indication is not explicitly included in case that the adaptive loop filter tool is disabled.

14. The apparatus of claim 11 , wherein information related to the cross-component filtering tool is excluded from the bitstream in case that the adaptive loop filter tool is disabled;

wherein the information related to the cross-component filtering tool includes at least one of a second syntax element which specifies that the cross-component filtering tool is enabled or not for a Cb color component, or a third syntax element which specifies that the cross-component filtering tool is enabled or not for a Cr color component;

wherein the information related to the cross-component filtering tool further includes a fourth syntax element, which specifies a first identifier of a first adaptive loop filter that the Cb color component of a current slice refers to, when the second syntax element indicates that the cross-component filtering tool is enabled for the Cb color component; and

wherein the information related to the cross-component filtering tool further includes a fifth syntax element, which specifies a second identifier of a second adaptive loop filter that the Cr color component of the current slice refers to, when the third syntax element indicates that the cross-component filtering tool is enabled for the Cr color component; and

wherein a conformance bitstream satisfies that the identifier of an adaptive loop filter for the cross-component filtering tool included in the bitstream shall refer to one available adaptive loop filter.

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

determine, during a conversion between a video unit of a video and a bitstream of the video, to conditionally include an indication of usage of a cross-component filtering tool in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled, wherein in the cross-component filtering tool, values of chroma samples are refined by using sample values of luma samples; and

perform the conversion based on the determining,

wherein a first offset used in the cross-component filtering tool for a chroma sample of the chroma samples is derived based on values of luma samples in a filter support region and filter coefficients of the video unit, wherein the filter support region is used for the cross-component filtering tool and is a diamond shaped luma region;

wherein a second offset is derived based on using a clipping operation to the first offset, wherein the second offset is clipped to a given range; and

the conversion is performed by applying the cross-component filtering tool to refine the chroma sample based on the second offset.

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

wherein a first syntax element indicating an applicability of the cross-component filtering tool is included in the bitstream in a picture header or a slice header in case that the indication indicates that the cross-component filtering tool is enabled at the SPS level; and

wherein the indication is not explicitly included in case that the adaptive loop filter tool is disabled.

17. The non-transitory computer-readable storage medium of claim 15 , wherein information related to the cross-component filtering tool is excluded from the bitstream in case that the adaptive loop filter tool is disabled;

wherein the information related to the cross-component filtering tool includes at least one of a second syntax element which specifies that the cross-component filtering tool is enabled or not for a Cb color component, or a third syntax element which specifies that the cross-component filtering tool is enabled or not for a Cr color component;

wherein the information related to the cross-component filtering tool further includes a fourth syntax element, which specifies a first identifier of a first adaptive loop filter that the Cb color component of a current slice refers to, when the second syntax element indicates that the cross-component filtering tool is enabled for the Cb color component; and

wherein the information related to the cross-component filtering tool further includes a fifth syntax element, which specifies a second identifier of a second adaptive loop filter that the Cr color component of the current slice refers to, when the third syntax element indicates that the cross-component filtering tool is enabled for the Cr color component; and

wherein a conformance bitstream satisfies that the identifier of an adaptive loop filter for the cross-component filtering tool included in the bitstream shall refer to one available adaptive loop filter.

18. 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 unit of a video, to conditionally include an indication of usage of a cross-component filtering tool in the bitstream at a sequence parameter set (SPS) level based on whether an adaptive loop filter tool is enabled, wherein in the cross-component filtering tool, values of chroma samples are refined by using values of luma samples; and

generating the bitstream based on the determining,

wherein a first offset used in the cross-component filtering tool for a chroma sample of the chroma samples is derived based on values of luma samples in a filter support region and filter coefficients of the video unit, wherein the filter support region is used for the cross-component filtering tool and is a diamond shaped luma region;

wherein a second offset is derived based on using a clipping operation to the first offset, wherein the second offset is clipped to a given range; and

the bitstream is generated by applying the cross-component filtering tool to refine the chroma sample based on the second offset.

19. The non-transitory computer-readable recording medium of claim 18 , wherein the indication of usage of the cross-component filtering tool is conditionally included in the bitstream further based on a chroma color format;

wherein a first syntax element indicating an applicability of the cross-component filtering tool is included in the bitstream in a picture header or a slice header in case that the indication indicates that the cross-component filtering tool is enabled at the SPS level; and

wherein the indication is not explicitly included in case that the adaptive loop filter tool is disabled.

20. The non-transitory computer-readable recording medium of claim 18 , wherein information related to the cross-component filtering tool is excluded from the bitstream in case that the adaptive loop filter tool is disabled;

wherein the information related to the cross-component filtering tool includes at least one of a second syntax element which specifies that the cross-component filtering tool is enabled or not for a Cb color component, or a third syntax element which specifies that the cross-component filtering tool is enabled or not for a Cr color component;

wherein the information related to the cross-component filtering tool further includes a fourth syntax element, which specifies a first identifier of a first adaptive loop filter that the Cb color component of a current slice refers to, when the second syntax element indicates that the cross-component filtering tool is enabled for the Cb color component; and

wherein the information related to the cross-component filtering tool further includes a fifth syntax element, which specifies a second identifier of a second adaptive loop filter that the Cr color component of the current slice refers to, when the third syntax element indicates that the cross-component filtering tool is enabled for the Cr color component; and

wherein a conformance bitstream satisfies that the identifier of an adaptive loop filter for the cross-component filtering tool included in the bitstream shall refer to one available adaptive loop filter.

21. The method of claim 1 , wherein in the cross-component filter tool, each value of the chroma samples is refined based on applying the filtering coefficients on luma sample differences, wherein each luma sample difference corresponds to a difference between a value of a luma sample in the filter support region and a value of a corresponding luma sample of the chroma sample.