Constraints on collocated pictures in video coding

1. A method of video processing, comprising:

performing a conversion between a video comprising a current picture and a bitstream of the video,

wherein the bitstream conforms to a format rule, and

wherein the format rule specifies that the bitstream comprises a flag for disabling a use of a temporal motion vector prediction tool for the current picture based on whether a reference picture in a decoded picture buffer has a spatial resolution and scaling window offsets that are identical to that of the current picture.

2. The method of claim 1 , wherein the flag equals zero when no reference picture in the decoded picture buffer has a same spatial resolution and a same scaling window offsets as that of the current picture.

3. The method of claim 1 , wherein the flag equals zero when no reference picture in the decoded picture buffer is in an active entry of one or more reference picture lists of all slices in the current picture.

4. The method of claim 1 , wherein the flag is included in a picture header.

5. The method of claim 4 , wherein the flag is ph_temporal_mvp_enabled_flag.

6. The method of claim 1 , wherein the format rule specifies that a slice header associated with a slice of the current picture comprises one or more constraints on a collocated picture of the current picture, and

wherein the one or more constraints specifies that a variable indicating that a reference picture resampling process is disabled for the collocated picture.

7. The method of claim 6 , wherein the variable is RprConstraintsActiveFlag, and wherein the variable having a value of zero indicates that the reference picture resampling process is disabled for the collocated picture.

8. The method of claim 6 , wherein the one or more constraints further specifies that a number of subpictures and a coding tree unit (CTU) size of the collocated picture are identical to that of the current picture.

9. The method of claim 1 , wherein a collocated picture is used in the temporal motion vector prediction tool.

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 a current picture and a bitstream of the video,

wherein the bitstream conforms to a format rule, and

wherein the format rule specifies that the bitstream comprises a flag for disabling a use of a temporal motion vector prediction tool for the current picture based on whether a reference picture in a decoded picture buffer has a spatial resolution and scaling window offsets that are identical to that of the current picture.

13. The apparatus of claim 12 , wherein the flag equals zero when no reference picture in the decoded picture buffer has a same spatial resolution and a same scaling window offsets as that of the current picture, or

wherein the flag equals zero when no reference picture in the decoded picture buffer is in an active entry of one or more reference picture lists of all slices in the current picture.

14. The apparatus of claim 12 , wherein the flag is included in a picture header, and wherein the flag is ph_temporal_mvp_enabled_flag.

15. The apparatus of claim 12 , wherein the format rule specifies that a slice header associated with a slice of the current picture comprises one or more constraints on a collocated picture of the current picture,

wherein the one or more constraints specifies that a variable indicating that a reference picture resampling process is disabled for the collocated picture,

wherein the variable is RprConstraintsActiveFlag, and wherein the variable having a value of zero indicates that the reference picture resampling process is disabled for the collocated picture, and

wherein the one or more constraints further specifies that a number of subpictures and a coding tree unit (CTU) size of the collocated picture are identical to that of the current picture.

16. The apparatus of claim 12 , wherein a collocated picture is used in the temporal motion vector prediction tool.

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

perform a conversion between a video comprising a current picture and a bitstream of the video,

wherein the bitstream conforms to a format rule, and

wherein the format rule specifies that the bitstream comprises a flag for disabling a use of a temporal motion vector prediction tool for the current picture based on whether a reference picture in a decoded picture buffer has a spatial resolution and scaling window offsets that are identical to that of the current picture.

18. The non-transitory computer-readable storage medium of claim 17 , wherein the flag equals zero when no reference picture in the decoded picture buffer has a same spatial resolution and a same scaling window offsets as that of the current picture, or

wherein the flag equals zero when no reference picture in the decoded picture buffer is in an active entry of one or more reference picture lists of all slices in the current picture,

wherein the flag is included in a picture header,

wherein the flag is ph_temporal_mvp_enabled_flag,

wherein the format rule specifies that a slice header associated with a slice of the current picture comprises one or more constraints on a collocated picture of the current picture,

wherein the one or more constraints specifies that a variable indicating that a reference picture resampling process is disabled for the collocated picture,

wherein the variable is RprConstraintsActiveFlag, and wherein the variable having a value of zero indicates that the reference picture resampling process is disabled for the collocated picture, and

wherein the one or more constraints further specifies that a number of subpictures and a coding tree unit (CTU) size of the collocated picture are identical to that of the current picture,

wherein the collocated picture is used in the temporal motion vector prediction tool.

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 comprising a current picture,

wherein the bitstream conforms to a format rule, and

wherein the format rule specifies that the bitstream comprises a flag for disabling a use of a temporal motion vector prediction tool for the current picture based on whether a reference picture in a decoded picture buffer has a spatial resolution and scaling window offsets that are identical to that of the current picture.

20. The non-transitory computer-readable recording medium of claim 19 , wherein the flag equals zero when no reference picture in the decoded picture buffer has a same spatial resolution and a same scaling window offsets as that of the current picture, or

wherein the flag equals zero when no reference picture in the decoded picture buffer is in an active entry of one or more reference picture lists of all slices in the current picture,

wherein the flag is included in a picture header,

wherein the flag is ph_temporal_mvp_enabled_flag,

wherein the format rule specifies that a slice header associated with a slice of the current picture comprises one or more constraints on a collocated picture of the current picture,

wherein the one or more constraints specifies that a variable indicating that a reference picture resampling process is disabled for the collocated picture,

wherein the variable is RprConstraintsActiveFlag,

wherein the variable having a value of zero indicates that the reference picture resampling process is disabled for the collocated picture,

wherein the one or more constraints further specifies that a number of subpictures and a coding tree unit (CTU) size of the collocated picture are identical to that of the current picture, and

wherein the collocated picture is used in the temporal motion vector prediction tool.