Interplay between subpictures and in-loop filtering

1. A method of video processing, comprising:

determining, for a conversion between a chroma block of a current processing unit of a current subpicture of a current picture of a video and a bitstream of the video, that a cross component adaptive loop filtering operation is applied to the chroma block, wherein the current picture comprises one or more subpictures; and

performing the conversion based on the determining,

wherein in the cross component adaptive loop filtering operation, a chroma sample of the chroma block is filtered based on information of luma samples,

wherein one or more luma samples located outside the current processing unit are excluded from the filtering of the chroma sample,

wherein the bitstream conforms to a format rule which specifies that different subpictures have different subpicture ID values, wherein when a value of one or more syntax elements is equal to 0 indicating that subpicture ID mapping is not explicitly signalled in a sequence parameter set or in a picture parameter set, a default value is assigned to the subpicture ID value of the subpicture.

2. The method of claim 1 , wherein the current processing unit is defined by an adaptive loop filtering virtual boundary.

3. The method of claim 1 , wherein one or more luma samples located outside an adaptive loop filtering virtual boundary are excluded from the filtering of the chroma sample.

4. The method of claim 3 , wherein one or more luma samples inside the adaptive loop filtering virtual boundary is used to replace the one or more luma samples located outside an adaptive loop filtering virtual boundary and is included in the filtering of the chroma sample.

5. The method of claim 1 , wherein the current picture further comprises a video block, and

wherein when an edge of the video block coincide with a boundary of a subpicture in the current picture and an in-loop filtering operation across the boundary of the subpicture is disabled, a deblocking filter process is not applied to the edge.

6. The method of claim 5 , wherein a syntax element indicating whether the in-loop filtering operation process across the boundary of the subpicture is disabled is included in the bitstream.

7. The method of claim 1 , wherein a syntax element indicating a maximum block size used for a transform skip coding tool is conditionally included in a sequence parameter set in the bitstream based on a value of a transform skip enabled flag included in the sequence parameter set in the bitstream.

8. The method of claim 1 , wherein the current picture further comprises a video block,

wherein a history-based motion vector prediction table is maintained to be used for motion vector prediction,

wherein when the video block is coded in a first mode, the history-based motion vector prediction table is refrained from being updated based on the video block, and

wherein in the first mode, a partition scheme dividing the video block into two partitions is allowed, and at least one of the two partitions is non-square and non-rectangular.

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

10. The method of claim 1 , wherein the conversion includes decoding the current picture 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, for a conversion between a chroma block of a current processing unit of a current subpicture of a current picture of a video and a bitstream of the video, that a cross component adaptive loop filtering operation is applied to the chroma block, wherein the current picture comprises one or more subpictures; and

perform the conversion based on the determining,

wherein in the cross component adaptive loop filtering operation, a chroma sample of the chroma block is filtered based on information of luma samples,

wherein one or more luma samples located outside the current processing unit are excluded from the filtering of the chroma sample,

wherein the bitstream conforms to a format rule which specifies that different subpictures have different subpicture ID values, wherein when a value of one or more syntax elements is equal to 0 indicating that subpicture ID mapping is not explicitly signalled in a sequence parameter set or in a picture parameter set, a default value is assigned to the subpicture ID value of the subpicture.

12. The apparatus of claim 11 , wherein the current processing unit is defined by an adaptive loop filtering virtual boundary.

13. The apparatus of claim 11 , wherein one or more luma samples located outside an adaptive loop filtering virtual boundary are excluded from the filtering of the chroma sample.

14. The apparatus of claim 13 , wherein one or more luma samples inside the adaptive loop filtering virtual boundary is used to replace the one or more luma samples located outside an adaptive loop filtering virtual boundary and is included in the filtering of the chroma sample.

15. The apparatus of claim 11 , wherein the current picture further comprises a video block, and

wherein when an edge of the video block coincide with a boundary of a subpicture in the current picture and an in-loop filtering operation across the boundary of the subpicture is disabled, a deblocking filter process is not applied to the edge.

16. The apparatus of claim 15 , wherein a syntax element indicating whether the in-loop filtering operation process across the boundary of the subpicture is disabled is included in the bitstream.

17. The apparatus of claim 11 , wherein a syntax element indicating a maximum block size used for a transform skip coding tool is conditionally included in a sequence parameter set in the bitstream based on a value of a transform skip enabled flag included in the sequence parameter set in the bitstream.

18. The apparatus of claim 11 , wherein the current picture further comprises a video block,

wherein a history-based motion vector prediction table is maintained to be used for motion vector prediction,

wherein when the video block is coded in a first mode, the history-based motion vector prediction table is refrained from being updated based on the video block, and

wherein in the first mode, a partition scheme dividing the video block into two partitions is allowed, and at least one of the two partitions is non-square and non-rectangular.

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

determine, for a conversion between a chroma block of a current processing unit of a current subpicture of a current picture of a video and a bitstream of the video, that a cross component adaptive loop filtering operation is applied to the chroma block, wherein the current picture comprises one or more subpictures; and

perform the conversion based on the determining,

wherein in the cross component adaptive loop filtering operation, a chroma sample of the chroma block is filtered based on information of luma samples,

wherein one or more luma samples located outside the current processing unit are excluded from the filtering of the chroma sample,

wherein the bitstream conforms to a format rule which specifies that different subpictures have different subpicture ID values, wherein when a value of one or more syntax elements is equal to 0 indicating that subpicture ID mapping is not explicitly signalled in a sequence parameter set or in a picture parameter set, a default value is assigned to the subpicture ID value of the subpicture.

20. 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 chroma block of a current processing unit of a current subpicture of a current picture of a video, that a cross component adaptive loop filtering operation is applied to the chroma block, wherein the current picture comprises one or more subpictures; and

generating the bitstream based on the determining,

wherein in the cross component adaptive loop filtering operation, a chroma sample of the chroma block is filtered based on information of luma samples,

wherein one or more luma samples located outside the current processing unit are excluded from the filtering of the chroma sample,

wherein the bitstream conforms to a format rule which specifies that different subpictures have different subpicture ID values, wherein when a value of one or more syntax elements is equal to 0 indicating that subpicture ID mapping is not explicitly signalled in a sequence parameter set or in a picture parameter set, a default value is assigned to the subpicture ID value of the subpicture.