Decoded picture buffer memory allocation and picture output in scalable video coding

1. A method of video processing, comprising:

performing a conversion between a video having one or more video layers and a bitstream of the video according to a format rule,

wherein the format rule specifies that a value of a flag that indicates whether to remove pictures previously decoded and stored in a decoded picture buffer from the decoded picture buffer when decoding an access unit of a certain type is included in the bitstream,

wherein the format rule specifies that a value of a variable that indicates whether pictures in the decoded picture buffer prior to a current picture in decoding order in the bitstream are output before the pictures are removed from the decoded picture buffer is set to a predetermined value when a first condition related with a greatest allowed value of a chroma format indicator or a greatest allowed value of a bit depth used to represent pixels of the video is satisfied,

wherein the value of the variable is set to the value of the flag when the first condition is not satisfied, and

wherein the first condition is not satisfied when a color format or a bit depth of pictures changes but the greatest allowed value of the chroma format indicator or the greatest allowed value of the bit depth does not change.

2. The method of claim 1 , wherein the format rule specifies that the value of the flag for the access unit is same for all pictures in the access unit.

3. The method of claim 1 , wherein the format rule specifies that the value of the flag for the access unit is same for all slices in the access unit.

4. The method of claim 1 , wherein the format rule specifies that the value of the flag is considered to be a value of a flag for pictures or slices of the access unit.

5. The method of claim 1 , wherein the flag is indicated in a picture header.

6. The method of claim 1 , wherein the flag is indicated in a slice header.

7. The method of claim 1 , wherein the conversion includes encoding the video into the bitstream.

8. The method of claim 1 , wherein the conversion includes decoding the video from the bitstream.

9. The method of claim 1 , wherein the greatest allowed value of the chroma format indicator and/or the greatest allowed value of a bit depth used to represent pixels of the video are included in a video parameter set.

10. The method of claim 1 , wherein the greatest allowed value of the chroma format indicator for a layer is applicable to all sequence parameter sets that are referred to by one or more layer video sequences in the layer.

11. 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 having one or more video layers and a bitstream of the video according to a format rule,

wherein the format rule specifies that a value of a flag that indicates whether to remove pictures previously decoded and stored in a decoded picture buffer from the decoded picture buffer when decoding an access unit of a certain type is included in the bitstream,

wherein the format rule specifies that a value of a variable that indicates whether pictures in the decoded picture buffer prior to a current picture in decoding order in the bitstream are output before the pictures are removed from the decoded picture buffer is set to a predetermined value when a first condition related with a greatest allowed value of a chroma format indicator or a greatest allowed value of a bit depth used to represent pixels of the video is satisfied,

wherein the value of the variable is set to the value of the flag when the first condition is not satisfied, and

wherein the first condition is not satisfied when a color format or a bit depth of pictures changes but the greatest allowed value of the chroma format indicator or the greatest allowed value of the bit depth does not change.

12. The apparatus of claim 11 , wherein the format rule specifies that the value of the flag for the access unit is same for all pictures in the access unit.

13. The apparatus of claim 11 , wherein the format rule specifies that the value of the flag for the access unit is same for all slices in the access unit.

14. The apparatus of claim 11 , wherein the format rule specifies that the value of the flag is considered to be a value of a flag for pictures or slices of the access unit.

15. The apparatus of claim 11 , wherein the flag is indicated in a picture header.

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

perform a conversion between a video having one or more video layers and a bitstream of the video according to a format rule,

wherein the format rule specifies that a value of a flag that indicates whether to remove pictures previously decoded and stored in a decoded picture buffer from the decoded picture buffer when decoding an access unit of a certain type is included in the bitstream,

wherein the format rule specifies that a value of a variable that indicates whether pictures in the decoded picture buffer prior to a current picture in decoding order in the bitstream are output before the pictures are removed from the decoded picture buffer is set to a predetermined value when a first condition related with a greatest allowed value of a chroma format indicator or a greatest allowed value of a bit depth used to represent pixels of the video is satisfied,

wherein the value of the variable is set to the value of the flag when the first condition is not satisfied, and

wherein the first condition is not satisfied when a color format or a bit depth of pictures changes but the greatest allowed value of the chroma format indicator or the greatest allowed value of the bit depth does not change.

17. The non-transitory computer-readable storage medium of claim 16 , wherein the format rule specifies that the value of the flag for the access unit is same for all slices in the access unit.

18. The non-transitory computer-readable storage medium of claim 16 , wherein the format rule specifies that the value of the flag is considered to be a value of a flag for pictures or slices of the access unit.

19. 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 the bitstream of the video having one or more video layers according to a format rule,

wherein the format rule specifies that a value of a flag that indicates whether to remove pictures previously decoded and stored in a decoded picture buffer from the decoded picture buffer when decoding an access unit of a certain type is included in the bitstream,

wherein the format rule specifies that a value of a variable that indicates whether pictures in the decoded picture buffer prior to a current picture in decoding order in the bitstream are output before the pictures are removed from the decoded picture buffer is set to a predetermined value when a first condition related with a greatest allowed value of a chroma format indicator or a greatest allowed value of a bit depth used to represent pixels of the video is satisfied,

wherein the value of the variable is set to the value of the flag when the first condition is not satisfied, and

wherein the first condition is not satisfied when a color format or a bit depth of pictures changes but the greatest allowed value of the chroma format indicator or the greatest allowed value of the bit depth does not change.