Interplay between subpictures and in-loop filtering

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, and
• wherein one or more luma samples located outside the current processing unit are excluded from the filtering of the chroma sample;
• wherein a position of the current subpicture is determined based on a first syntax element and a second syntax element included in the bitstream;
• wherein the first syntax element indicates a horizontal position of a top-left coding tree unit of the current subpicture in unit of CtbSizeY and a length of the first syntax element is derived to be Ceil(Log 2(S1)) bits,
• wherein the second syntax element indicates a vertical position of the top-left coding tree unit of the current subpicture in unit of CtbSizeY and a length of the second syntax element is derived to be Ceil(Log 2(S2)) bits,
• wherein S1 and S2 are greater than 0, and CtbSizeY denotes a size of a coding tree block, and
• wherein S1 is equal to (S3+CtbSizeY−1)/CtbSizeY, and S2 is equal to (S4+CtbSizeY−1)/CtbSizeY, wherein S3 is a value of a syntax element included in a sequence parameter set and specifying a maximum width of a decoded picture referring to the sequence parameter set in units of the luma samples, and wherein S4 is a value of a syntax element included in the sequence parameter set and specifying a maximum height of a decoded picture referring to the sequence parameter set in units of the luma samples.

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, and
• wherein one or more luma samples located outside the current processing unit are excluded from the filtering of the chroma sample;
• wherein a position of the current subpicture is determined based on a first syntax element and a second syntax element included in the bitstream;
• wherein the first syntax element indicates a horizontal position of a top-left coding tree unit of the current subpicture in unit of CtbSizeY and a length of the first syntax element is derived to be Ceil(Log 2(S1)) bits,
• wherein the second syntax element indicates a vertical position of the top-left coding tree unit of the current subpicture in unit of CtbSizeY and a length of the second syntax element is derived to be Ceil(Log 2(S2)) bits,
• wherein S1 and S2 are greater than 0, and CtbSizeY denotes a size of a coding tree block, and
• wherein S1 is equal to (S3+CtbSizeY−1)/CtbSizeY, and S2 is equal to (S4+CtbSizeY−1)/CtbSizeY, wherein S3 is a value of a syntax element included in a sequence parameter set and specifying a maximum width of a decoded picture referring to the sequence parameter set in units of the luma samples, and wherein S4 is a value of a syntax element included in the sequence parameter set and specifying a maximum height of a decoded picture referring to the sequence parameter set in units of the luma samples.

A method for storing a bitstream of a video, comprising:

• determining, for a chroma block of a current processing unit of a current picture of a current subpicture 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;
• generating the bitstream based on the determining;
• and
• storing the bitstream in a non-transitory computer-readable recording medium,
• wherein in the cross component adaptive loop filtering operation, a chroma sample of the chroma block is filtered based on information of luma samples, and
• wherein one or more luma samples located outside the current processing unit are excluded from the filtering of the chroma sample;
• wherein a position of the current subpicture is determined based on a first syntax element and a second syntax element included in the bitstream;
• wherein the first syntax element indicates a horizontal position of a top-left coding tree unit of the current subpicture in unit of CtbSizeY and a length of the first syntax element is derived to be Ceil(Log 2(S1)) bits,
• wherein the second syntax element indicates a vertical position of the top-left coding tree unit of the current subpicture in unit of CtbSizeY and a length of the second syntax element is derived to be Ceil(Log 2(S2)) bits,
• wherein S1 and S2 are greater than 0, and CtbSizeY denotes a size of a coding tree block, and
• wherein S1 is equal to (S3+CtbSizeY−1)/CtbSizeY, and S2 is equal to (S4+CtbSizeY−1)/CtbSizeY, wherein S3 is a value of a syntax element included in a sequence parameter set and specifying a maximum width of a decoded picture referring to the sequence parameter set in units of the luma samples, and wherein S4 is a value of a syntax element included in the sequence parameter set and specifying a maximum height of a decoded picture referring to the sequence parameter set in units of the luma samples.