Method for signaling subpicture partitioning in coded video stream

1. A method of decoding an encoded video bitstream, the method comprising: when subpicture information is signaled in a data structure of the encoded video bitstream, obtaining, from the encoded video bitstream, a first syntax element indicating a number of subpictures of a current picture and a second syntax element indicating whether a subpicture identifier mapping is signaled for the current picture; when the second syntax element indicates that the subpicture identifier mapping is signaled for the current picture: obtaining, from a picture parameter set (PPS) of the encoded video bitstream, respective third syntax elements for the subpictures of the current picture, the third syntax elements indicating subpicture identifiers for the subpictures of the current picture; deriving respective subpicture identifier values for the subpictures of the current picture using the respective third syntax elements, wherein, for any two different subpictures in the subpictures of the current picture, the respective subpicture identifier values are prohibited from being equal: and decoding the current picture based on the respective subpicture identifier values; and when the second syntax element indicates that the subpicture identifier mapping is not signaled for the current picture decoding the current picture based on the first syntax element and the second syntax element, without obtaining the third syntax elements.

2. The method of claim 1 , wherein the first syntax element is signaled in a sequence parameter set (SPS).

3. The method of claim 1 , wherein based on a picture size of the current picture being smaller than a size of a coding tree unit of a luma component of the current picture, the number of the subpictures is equal to one.

4. The method of claim 3 , wherein based on the number of the subpictures being equal to one, subpicture partitioning information for the current picture is not explicitly signaled.

5. The method of claim 1 , further comprising:

when the first syntax element indicates that the number of subpictures of the current picture is greater than one, obtaining, from the encoded video bitstream, a size of at least one subpicture of the subpictures.

6. The method of claim 5 , wherein the size of the at least one subpicture of the subpictures comprises a height and a width of the at least one subpicture of the subpictures.

7. The method of claim 5 , wherein the size of the at least one subpicture of the subpictures is indicated in the encoded video bitstream in units of a size of a coding tree unit of a luma component of the current picture.

8. The method of claim 5 , wherein a size of another subpicture is determined based on the size of the at least one subpicture of the subpictures.

9. The method of claim 1 , wherein deriving the respective subpicture identifier values for the subpictures of the current picture using the respective third syntax elements comprises determining, for each subpicture of the subpictures, whether to use an identifier from the third syntax elements or an identifier signaled in a sequence parameter set.

10. The method of claim 1 , wherein a length of each of the third syntax elements is determined using a fourth syntax element from the PPS of the encoded video bitstream.

11. A method of video encoding, the method comprising:

obtaining video data comprising a plurality of pictures, including a current picture;

when the video data comprises subpicture information, signaling, via an encoded video bitstream, a first syntax element indicating a number of subpictures of the current picture and a second syntax element indicating whether a subpicture identifier mapping is signaled for the current picture;

when a subpicture identifier mapping is signaled for the current picture, signaling, via a picture parameter set (PPS) of the encoded video bitstream, respective third syntax elements for the subpictures of the current picture, the third syntax elements indicating subpicture identifiers for the subpictures of the current picture; and

when the subpicture identifier mapping is not signaled for the current picture, forgoing signaling the third syntax elements.

12. The method of claim 11 , wherein the first syntax element is signaled via a sequence parameter set of the encoded video bitstream.

13. The method of claim 11 , further comprising signaling, via a sequence parameter set of the encoded video bitstream, a fourth syntax element indicating a subpicture identifier.

14. The method of claim 11 , further comprising signaling, via the PPS of the encoded video bitstream, a fifth syntax element indicating a length of the third syntax elements.

15. A method of processing visual media data, the method comprising:

obtaining video data comprising a plurality of pictures, including a current picture; and

performing a conversion between the video data and a bitstream of encoded video data according to a format rule, wherein the format rule specifies that:

a first syntax element indicating a number of subpictures of the current picture and a second syntax element indicating whether a subpicture identifier mapping are signaled for the current picture are signaled in the bitstream when the video data comprises subpicture information;

when a subpicture identifier mapping is signaled for the current picture,

respective third syntax elements indicating subpicture identifiers for the subpictures of the current picture are signaled via a picture parameter set (PPS) of the bitstream; and

when the subpicture identifier mapping is not signaled for the current picture, the respective third syntax elements are foregone from signaling via the PPS of the bitstream.

16. The method of claim 15 , wherein the first syntax element is signaled via a sequence parameter set of the bitstream.

17. The method of claim 15 , wherein the bitstream further comprises a fourth syntax element signaled via a sequence parameter set, the fourth syntax element indicating a subpicture identifier.

18. The method of claim 15 , wherein the bitstream further comprises a fifth syntax element signaled via the picture parameter set, the fifth syntax element indicating a length of the third syntax elements.