REFERENCE PICTURE RESAMPLING

1 . A method of processing video data, comprising:

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

wherein the rule includes a bitstream constraint which specifies that a number of entries in a reference picture list of the current picture is greater than 0 in response to (1) one or more slices in the current picture being allowed to have a slice type other than an Intra (I) slice type, and (2) reference picture list (RPL) information being present in a picture header.

2 . The method of claim 1 , wherein the reference picture list is a reference picture list 0 .

3 . The method of claim 1 , wherein a first syntax flag in the picture header indicates whether the one or more slices in the current picture are allowed to have the slice type other than the Intra (I) slice type.

4 . The method of claim 1 , wherein a second syntax flag in a picture parameter set indicates whether the RPL information is present in the picture header instead of slice headers.

5 . The method of claim 1 , wherein the number of entries in the reference picture list of the current picture is indicated by a third syntax element included in the bitstream.

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

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

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

wherein the rule includes a bitstream constraint which specifies that a number of entries in a reference picture list of the current picture is greater than 0 in response to (1) one or more slices in the current picture being allowed to have a slice type other than an Intra (I) slice type, and (2) reference picture list (RPL) information being present in a picture header.

9 . The apparatus of claim 8 , wherein the reference picture list is a reference picture list 0 .

10 . The apparatus of claim 8 , wherein a first syntax flag in the picture header indicates whether the one or more slices in the current picture are allowed to have the slice type other than the Intra (I) slice type.

11 . The apparatus of claim 8 , wherein a second syntax flag in a picture parameter set indicates whether the RPL information is present in the picture header instead of slice headers.

12 . The apparatus of claim 8 , wherein the number of entries in the reference picture list of the current picture is indicated by a third syntax element included in the bitstream.

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

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

wherein the rule includes a bitstream constraint which specifies that a number of entries in a reference picture list of the current picture is greater than 0 in response to (1) one or more slices in the current picture being allowed to have a slice type other than an Intra (I) slice type, and (2) reference picture list (RPL) information being present in a picture header.

14 . The non-transitory computer-readable storage medium of claim 13 , wherein the reference picture list is a reference picture list 0 .

15 . The non-transitory computer-readable storage medium of claim 13 , wherein a first syntax flag in the picture header indicates whether the one or more slices in the current picture are allowed to have the slice type other than the Intra (I) slice type, and a second syntax flag in a picture parameter set indicates whether the RPL information is present in the picture header instead of slice headers.

16 . The non-transitory computer-readable storage medium of claim 13 , wherein the number of entries in the reference picture list of the current picture is indicated by a third syntax element included in the bitstream.

17 . 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 from a current picture in the video according to a rule,

wherein the rule includes a bitstream constraint which specifies that a number of entries in a reference picture list of the current picture is greater than 0 in response to (1) one or more slices in the current picture being allowed to have a slice type other than an Intra (I) slice type, and (2) reference picture list (RPL) information being present in a picture header.

18 . The non-transitory computer-readable recording medium of claim 17 , wherein the reference picture list is a reference picture list 0 .

19 . The non-transitory computer-readable recording medium of claim 17 , wherein a first syntax flag in the picture header indicates whether the one or more slices in the current picture are allowed to have the slice type other than the Intra (I) slice type, and a second syntax flag in a picture parameter set indicates whether the RPL information is present in the picture header instead of slice headers.

20 . The non-transitory computer-readable recording medium of claim 17 , wherein the number of entries in the reference picture list of the current picture is indicated by a third syntax element included in the bitstream.