Inter layer prediction with different coding block size

1. A method of processing video data, comprising:

performing a conversion between a video unit of a current picture of a video and a bitstream of the video according to a rule,

wherein the rule specifies that, for a given picture to be available as a collocated reference picture whose motion information is used for the conversion, the given picture has a same coding tree unit size or a same coding tree block size as the current picture, and

wherein the rule specifies that a coding tool that uses the motion information is disabled in response to the given picture having a different coding tree unit size or a different coding tree block size, and wherein the coding tool is a temporal motion vector prediction coding tool or a subblock-based temporal motion vector prediction coding tool.

2. The method of claim 1 , wherein the video unit is a slice.

3. The method of claim 1 , wherein the collocated reference picture is identified by a list of collocated reference pictures and an index that specifies a reference index of the collocated reference picture.

4. The method of claim 3 , wherein the list of collocated reference pictures is determined by a first syntax flag specifying whether the collocated reference picture is derived from a reference picture list 0 or a reference picture list 1.

5. The method of claim 4 , wherein the first syntax flag is represented as sh_collocated_from_l0_flag, and wherein the reference index is represented as sh_collocated_ref_idx.

6. The method of claim 5 , wherein a value of a second syntax element that specifies a size of each coding tree unit for the collocated reference picture referred to by the reference index is equal to a value of a third syntax element that specifies a size of each coding tree unit for the current picture.

7. The method of claim 6 , wherein the second syntax element and the third syntax element are included in a sequence parameter set and are represented as sps_log2_ctu_size_minus5.

8. The method of claim 1 , wherein when a fourth syntax flag in a picture header specifies that usage of the motion information is disabled, a first syntax flag specifying whether the collocated reference picture is derived from a reference picture list 0 or a reference picture list 1 and a reference index of the collocated reference picture are omitted from the bitstream.

9. The method of claim 8 , wherein the fourth syntax flag is represented as ph_temporal_mvp_enabled_flag.

10. The method of claim 8 , wherein the first syntax flag is represented as sh_collocated_from_l0_flag or ph_collocated_from_l0_flag, and wherein the reference index is represented as sh_collocated_ref_idx or ph_collocated_ref_idx.

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

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

13. 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 unit of a current picture of a video and a bitstream of the video according to a rule,

wherein the rule specifies that, for a given picture to be available as a collocated reference picture whose motion information is used for the conversion, the given picture has a same coding tree unit size or a same coding tree block size as the current picture, and

wherein the rule specifies that a coding tool that uses the motion information is disabled in response to the given picture having a different coding tree unit size or a different coding tree block size, and wherein the coding tool is a temporal motion vector prediction coding tool or a subblock-based temporal motion vector prediction coding tool.

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

perform a conversion between a video unit of a current picture of a video and a bitstream of the video according to a rule,

wherein the rule specifies that, for a given picture to be available as a collocated reference picture whose motion information is used for the conversion, the given picture has a same coding tree unit size or a same coding tree block size as the current picture, and

wherein the rule specifies that a coding tool that uses the motion information is disabled in response to the given picture having a different coding tree unit size or a different coding tree block size, and wherein the coding tool is a temporal motion vector prediction coding tool or a subblock-based temporal motion vector prediction coding tool.

15. A method performed by a video processing apparatus, wherein the method comprises:

generating a bitstream of the video from a video unit of a current picture of the video according to a rule; and

storing the bitstream in a non-transitory computer-readable recording medium,

wherein the rule specifies that, for a given picture to be available as a collocated reference picture whose motion information is used for the generation, the given picture has a same coding tree unit size or a same coding tree block size as the current picture, and

wherein the rule specifies that a coding tool that uses the motion information is disabled in response to the given picture having a different coding tree unit size or a different coding tree block size, and wherein the coding tool is a temporal motion vector prediction coding tool or a subblock-based temporal motion vector prediction coding tool.

16. The apparatus of claim 13 , wherein the video unit is a slice.

17. The apparatus of claim 13 , wherein the collocated reference picture is identified by a list of collocated reference pictures and an index that specifies a reference index of the collocated reference picture.

18. The non-transitory computer-readable storage medium of claim 14 , wherein the video unit is a slice.

19. The non-transitory computer-readable storage medium of claim 14 , wherein the collocated reference picture is identified by a list of collocated reference pictures and an index that specifies a reference index of the collocated reference picture.

20. The method of claim 15 , wherein the video unit is a slice.