Syntax constraints in parameter set signaling of subpictures

1. A method of processing video data, comprising:

determining, for a conversion between a video comprising a picture which is divided into one or more subpictures and a bitstream of the video, a position of a subpicture of the one or more subpictures based on a first syntax element and a second syntax element included in the bitstream; and

performing the conversion based on the determining,

wherein the first syntax element indicates a horizontal position of a top-left coding tree unit of the 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 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 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 luma samples.

2. The method of claim 1 , wherein a third syntax element is included in the bitstream to specify a width of the subpicture in unit of CtbSizeY and a length of the third syntax element is derived to be Ceil(Log 2(S1)) bits, and

wherein a fourth syntax element is included in the bitstream to specify a height of the subpicture in unit of CtbSizeY and a length of the fourth syntax element is derived to be Ceil(Log 2(S2)) bits.

3. The method of claim 1 , wherein S3 and S4 are an integer multiple of Max(8, MinCbSizeY), respectively.

4. The method of claim 1 , wherein the bitstream conforms to a format rule which specifies that different subpictures have different subpicture ID values.

5. The method of claim 4 , wherein when a value of one or more syntax elements is equal to 0, a default value is assigned to the subpicture ID value of the subpicture,

wherein, when the value of the one or more syntax elements is equal to 0, a subpicture ID mapping is not explicitly signaled, neither in the sequence parameter set nor in the picture parameter set.

6. The method of claim 5 , wherein when the value of the one or more syntax elements is equal to 0, a fifth syntax element specifying ID of the subpicture is not included in a picture parameter set and a sixth syntax element specifying ID of the subpicture is not included in a sequence parameter set.

7. The method of claim 5 , wherein the subpicture is an i-th subpicture in the picture and the default value is equal to i, and wherein i is an integer smaller than a number of subpictures in the picture.

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

9. The method of claim 1 , wherein the conversion includes decoding the picture from the bitstream.

10. 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 comprising a picture which is divided into one or more subpictures and a bitstream of the video, a position of a subpicture of the one or more subpictures based on a first syntax element and a second syntax element included in the bitstream; and

perform the conversion based on the determining,

wherein the first syntax element indicates a horizontal position of a top-left coding tree unit of the 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 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 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 luma samples.

11. The apparatus of claim 10 , wherein a third syntax element is included in the bitstream to specify a width of the subpicture in unit of CtbSizeY and a length of the third syntax element is derived to be Ceil(Log 2(S1)) bits, and

wherein a fourth syntax element is included in the bitstream to specify a height of the subpicture in unit of CtbSizeY and a length of the fourth syntax element is derived to be Ceil(Log 2(S2)) bits.

12. The apparatus of claim 10 , wherein S3 and S4 are an integer multiple of Max(8, MinCbSizeY), respectively.

13. The apparatus of claim 10 , wherein the bitstream conforms to a format rule which specifies that different subpictures have different subpicture ID values.

14. The apparatus of claim 13 , wherein when a value of one or more syntax elements is equal to 0, a default value is assigned to the subpicture ID value of the subpicture,

wherein when the value of the one or more syntax elements is equal to 0, a subpicture ID mapping is not explicitly signalled, neither in the sequence parameter set nor in the picture parameter set.

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

determine, for a conversion between a video comprising a picture which is divided into one or more subpictures and a bitstream of the video, a position of a subpicture of the one or more subpictures based on a first syntax element and a second syntax element included in the bitstream; and

perform the conversion based on the determining,

wherein the first syntax element indicates a horizontal position of a top-left coding tree unit of the 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 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 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 luma samples.

16. The non-transitory computer-readable storage medium of claim 15 , wherein a third syntax element is included in the bitstream to specify a width of the subpicture in unit of CtbSizeY and a length of the third syntax element is derived to be Ceil(Log 2(S1)) bits, and wherein a fourth syntax element is included in the bitstream to specify a height of the subpicture in unit of CtbSizeY and a length of the fourth syntax element is derived to be Ceil(Log 2(S2)) bits.

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 comprising a picture which is divided into one or more subpictures, a position of a subpicture of the one or more subpictures based on a first syntax element and a second syntax element included in the bitstream; and

generating the bitstream based on the determining,

wherein the first syntax element indicates a horizontal position of a top-left coding tree unit of the 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 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 luma samples, and wherein S4 is the 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 luma samples.

18. The non-transitory computer-readable recording medium of claim 17 , wherein a third syntax element is included in the bitstream to specify a width of the subpicture in unit of CtbSizeY and a length of the third syntax element is derived to be Ceil(Log 2(S1)) bits, and wherein a fourth syntax element is included in the bitstream to specify a height of the subpicture in unit of CtbSizeY and a length of the fourth syntax element is derived to be Ceil(Log 2(S2)) bits.