Subpicture signaling in video coding

1. A method of video bitstream processing, comprising:

parsing, for decoding a video picture comprising subpictures, a syntax structure to obtain type information about network abstraction layer (NAL) units making up the video picture and whether the network abstraction layer bit units have mixed NAL unit types in a range IDR_W_RADL to CRA_NUT,

wherein the syntax structure is constrained to a particular value in case that the video picture includes only one NAL unit; and

decoding the subpictures based on the parsing.

2. The method of claim 1 , wherein the syntax structure is included in a picture parameter set (PPS).

3. The method of claim 2 , wherein a value of the syntax structure indicates that a video picture referring to the PPS that includes the syntax structure includes multiple NAL units and the multiple NAL units do not have a same value of NAL unit type.

4. The method of claim 3 , wherein the value of the syntax structure further specifies that the video picture is not an intra random access picture, IRAP.

5. The method of claim 1 , wherein a value of the syntax structure is disallowed for use in case that the video picture corresponds to a single NAL unit.

6. The method of claim 1 , wherein, for a particular subpicture of the video picture having a particular NAL unit type, the parsing comprises parsing according to a rule that a slice having a NAL unit type different than the particular NAL unit type is included in the particular subpicture.

7. The method of claim 1 , wherein the parsing comprises parsing the video bitstream according to a rule that a particular subpicture of the video picture is allowed to include a slice only when the video bitstream indicates that the particular subpicture is to be treated as a picture.

8. The method of claim 1 , wherein the parsing comprises parsing the video bitstream according to a rule that disables, for a slice in a particular subpicture of the video picture, use of reference pictures preceding the video picture in a decoding order.

9. A video decoding apparatus comprising a processor, the processor being configured for implementing a video bitstream processing method, comprising:

parsing, for decoding a video picture comprising subpictures comprising one or more slices, a syntax structure in a picture parameter set, PPS, of the video bitstream to obtain type information about one or more network abstraction layer (NAL) units making up the video picture and whether the network abstraction layer bit units have mixed NAL unit types in a range IDR_W_RADL to CRA_NUT, wherein the syntax structure is constrained to a particular value in case that the video picture includes only one NAL unit; and

reconstructing the video picture based on the parsing.

10. The apparatus of claim 9 , wherein a value of the syntax structure indicates that every video picture referring to the PPS that includes the syntax structure includes multiple NAL units and the multiple NAL units do not have a same value of NAL unit type and the video picture is not IRAP.

11. The apparatus of claim 9 , wherein, for a particular subpicture of the video picture having a particular NAL unit type, the parsing comprises parsing according to a rule that a slice having a NAL unit type different than the particular NAL unit type is included in the particular subpicture.

12. The apparatus of claim 9 , wherein the parsing comprises parsing the video bitstream according to a rule that a particular subpicture of the video picture is allowed to include a slice only when the video bitstream indicates that the particular subpicture is to be treated as a picture.

13. The apparatus of claim 9 , wherein the parsing comprises parsing the video bitstream according to a rule that disables, for a slice in a particular subpicture of the video picture, use of reference pictures preceding the video picture in a decoding order.

14. A video bitstream encoding method, comprising:

constructing, for encoding a video picture comprising subpictures, a syntax structure that includes type information about network abstraction layer (NAL) units making up the video picture and whether the network abstraction layer units have mixed NAL unit types in a range IDR_W_RADL to CRA_NUT, wherein the constructing comprises:

determining that the video picture is to be coded using only one NAL unit, and based on the determination that the video picture is to be coded using only one NAL unit, constraining the syntax structure to a particular value; and

generating the video bitstream by including the syntax structure and by encoding the video picture.

15. The method of claim 14 , wherein the syntax structure is included in a picture parameter set (PPS).

16. The method of claim 15 , wherein a value of the syntax structure indicates that a video picture referring to the PPS that includes the syntax structure includes multiple NAL units and the multiple NAL units do not have a same value of NAL unit type.

17. The method of claim 16 , wherein the value of the syntax structure further specifies that the video picture is not an intra random access point, IRAP, and

wherein a value of the syntax structure is disallowed for use in case that the video picture corresponds to a single NAL unit.

18. A video encoding apparatus comprising a processor configured for:

constructing, for encoding a video picture comprising one or more subpictures, a syntax structure that includes type information about one or more network abstraction layer (NAL) units making up the video picture and whether the network abstraction layer units have mixed NAL unit types in a range IDR_W_RADL to CRA_NUT video picture is an intra random access picture (TRAP), wherein the constructing comprises:

determining that the video picture is to be coded using only one NAL unit, and based on the determination that the video picture is to be coded using only one NAL unit, constraining the syntax structure to a particular value; and

generating the video bitstream by including the syntax structure and by encoding the video picture.

19. The apparatus of claim 18 , wherein, for a particular subpicture of the video picture having a particular NAL unit type, the parsing comprises parsing according to a rule that a slice having a NAL unit type different than the particular NAL unit type is included in the particular subpicture.

20. The apparatus of claim 18 , wherein the constructing comprises constructing the video bitstream according to a rule that a particular subpicture of the video picture is allowed to include a slice only when the video bitstream indicates that the particular subpicture is to be treated as a picture.