Random access point access unit in scalable video coding

1. A method of video processing, comprising:

performing a conversion between a video comprising one or more pictures in one or more video layers and a bitstream of the video,

wherein the bitstream comprises a coded video sequence that includes an access unit,

wherein the bitstream further comprises a first syntax element indicating whether the access unit includes a first type of picture for each video layer making up the coded video sequence,

wherein the first syntax element corresponds to an access unit delimiter network abstraction layer unit, and

wherein the access unit delimiter network abstraction layer unit is an only access unit delimiter network abstraction layer unit present in each intra random access point access unit or gradual decoding refresh access unit when a second syntax element, which is indicative of a maximum allowed number of layers in each coded video sequence that refers to a video parameter set, is greater than one.

2. The method of claim 1 , wherein the access unit includes the first type of picture for each video layer; and the access unit is configured to start a new coded video sequence, or each picture of the access unit is the first type of picture.

3. The method of claim 1 , wherein the first type of picture for each video layer is an intra random access point picture or a gradual decoding refresh picture.

4. The method of claim 1 , wherein the first syntax element is a flag indicating whether the access unit containing an access unit delimiter is an intra random access point access unit or a gradual decoding refresh access unit.

5. The method of claim 1 , wherein the second syntax element indicates the maximum allowed number of layers by indicating the maximum allowed number minus 1.

6. The method of claim 1 , wherein the first syntax element equaling one is indicative of all slices in the access unit comprising an identical network abstraction layer unit type in a range of IDR_W_RADL to GDR_NUT, inclusive.

7. The method of claim 6 , wherein the first syntax element equaling one and the network abstraction layer unit type being IDR_W_RADL or IDR_N_LP are indicative of the access unit being a coded video sequence start access unit.

8. The method of claim 6 , wherein a variable for each of the pictures in the access unit equaling one and the network abstraction layer unit type being CRA_NUT or GDR_NUT are indicative of the access unit being a coded video sequence start access unit.

9. The method of claim 8 , wherein the variable indicates whether pictures in a decoded picture buffer prior to a current picture in decoding order are output before the pictures are recovered.

10. The method of claim 1 , wherein the conversion comprises decoding the video from the bitstream.

11. The method of claim 1 , wherein the conversion comprises encoding the video into the bitstream.

12. 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 perform a conversion between a video comprising one or more pictures in one or more video layers and a bitstream of the video,

wherein the bitstream comprises a coded video sequence that includes an access unit,

wherein the bitstream further comprises a first syntax element indicating whether the access unit includes a first type of picture for each video layer making up the coded video sequence,

wherein the first syntax element corresponds to an access unit delimiter network abstraction layer unit, and

wherein the access unit delimiter network abstraction layer unit is an only access unit delimiter network abstraction layer unit present in each intra random access point access unit or gradual decoding refresh access unit when a second syntax element, which is indicative of a maximum allowed number of layers in each coded video sequence that refers to a video parameter set, is greater than one.

13. The apparatus of claim 12 , wherein the first type of picture for each video layer is an intra random access point picture or a gradual decoding refresh picture.

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

perform a conversion between a video comprising one or more pictures in one or more video layers and a bitstream of the video,

wherein the bitstream comprises a coded video sequence that includes an access unit, and

wherein the bitstream further comprises a first syntax element indicating whether the access unit includes a first type of picture for each video layer making up the coded video sequence,

wherein the first syntax element corresponds to an access unit delimiter network abstraction layer unit, and

wherein the access unit delimiter network abstraction layer unit is an only access unit delimiter network abstraction layer unit present in each intra random access point access unit or gradual decoding refresh access unit when a second syntax element, which is indicative of a maximum allowed number of layers in each coded video sequence that refers to a video parameter set, is greater than one.

15. The non-transitory computer-readable storage medium of claim 14 , wherein the first type of picture for each video layer is an intra random access point picture or a gradual decoding refresh picture.

16. 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:

generating a bitstream of a video comprising one or more pictures in one or more video layers,

wherein the bitstream comprises a coded video sequence that includes an access unit,

wherein the bitstream further comprises a first syntax element indicating whether the access unit includes a first type of picture for each video layer making up the coded video sequence,

wherein the first syntax element corresponds to an access unit delimiter network abstraction layer unit, and

wherein the access unit delimiter network abstraction layer unit is an only access unit delimiter network abstraction layer unit present in each intra random access point access unit or gradual decoding refresh access unit when a second syntax element, which is indicative of a maximum allowed number of layers in each coded video sequence that refers to a video parameter set, is greater than one.

17. The non-transitory computer-readable recording medium of claim 16 , wherein the first type of picture for each video layer is an intra random access point picture or a gradual decoding refresh picture.