Methods of coding images/videos with alpha channels

1. A method of video processing, comprising:

performing a conversion between a video comprising a plurality of pictures and a bitstream of the video, wherein each picture comprises an alpha layer and a base layer;

wherein the bitstream comprises a current block in the alpha layer of a current picture that is predicted from one or more reference blocks in the base layer,

further comprising: determining whether motion information of one of one or more reference blocks is present within the bitstream or whether the motion information is inferred,

wherein determining whether the motion information is present or whether the motion is inferred comprises:

determining whether the motion information of one of one or more reference blocks is present within the bitstream based on at least one of: header information, a position of a coding unit, coded mode information for blocks containing a sample at an edge, block dimension information, block shape information, a color format indication, a coding tree structure, color component information, a temporal layer identifier, or coding standard information;

in accordance with a determination that the motion information of one of one or more reference blocks is present within the bitstream, obtaining the motion information of one of one or more reference blocks; and

in accordance with a determination that the motion information of one of one or more reference blocks is not present within the bitstream, inferring the motion information of one of one or more reference blocks.

2. The method of claim 1 , wherein at least one of the one or more reference blocks is located in the current picture; or

at least one of the one or more reference blocks is located in at least one picture different from the current picture.

3. The method of claim 2 , wherein the at least one picture comprises a predefined number (k) of previously-coded pictures.

4. The method of claim 1 , wherein an indication of prediction direction for the current block is excluded from the bitstream.

5. The method of claim 1 , wherein the current block is predicted by performing a function based on a sample of the one or more reference blocks.

6. The method of claim 5 , wherein the function includes at least one of a weighted average, a maximum function, or a filtering function.

7. The method of claim 1 , wherein motion information of at least one of the one or more reference blocks is included in the bitstream.

8. The method of claim 7 , wherein the motion information includes an index of a picture including one of the one or more reference blocks.

9. The method of claim 1 , wherein a motion vector of one of the one or more reference blocks is excluded from the bitstream and inferred to a default value.

10. The method of claim 1 , further comprising determining a predicted motion vector of the current block, and wherein the current block is determined using a motion vector difference based on the predicted motion vector and an actual motion vector.

11. The method of claim 10 , wherein the bitstream includes the motion vector difference.

12. The method of claim 10 , wherein the motion vector difference is excluded from the bitstream and inferred to a default value.

13. The method of claim 10 , wherein the predicted motion vector of the current block is determined using a plurality of motion vectors corresponding to a plurality of different pictures.

14. The method of claim 10 , wherein the current block is predicted using the one or more reference blocks, and the one or more reference blocks have a same predicted motion vector.

15. The method of claim 10 , wherein the current block is predicted using the one or more reference blocks, and the one or more reference blocks have different motion vector predictions.

16. The method of claim 10 , wherein the bitstream includes an index of a picture including one of one or more reference blocks prior to the motion vector difference.

17. The method of claim 1 , wherein the bitstream includes a motion vector or a motion vector difference associated with one of one or more reference blocks based on an index of a picture including the one of one or more reference blocks.

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

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

20. 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 plurality of pictures and a bitstream of the video, wherein each picture comprises an alpha layer and a base layer;

wherein the bitstream comprises a current block in the alpha layer of a current picture that is predicted from one or more reference blocks in the base layer,

and further determine whether motion information of one of one or more reference blocks is present within the bitstream or whether the motion information is inferred,

wherein the processor is caused to determine whether the motion information is present or whether the motion is inferred by:

determining whether the motion information of one of one or more reference blocks is present within the bitstream based on at least one of: header information, a position of a coding unit, coded mode information for blocks containing a sample at an edge, block dimension information, block shape information, a color format indication, a coding tree structure, color component information, a temporal layer identifier, or coding standard information;

in accordance with a determination that the motion information of one of one or more reference blocks is present within the bitstream, obtaining the motion information of one of one or more reference blocks; and

in accordance with a determination that the motion information of one of one or more reference blocks is not present within the bitstream, inferring the motion information of one of one or more reference blocks.

21. 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:

performing a conversion between a video comprising a plurality of pictures and the bitstream of the video, wherein each picture comprises an alpha layer and a base layer;

wherein the bitstream comprises a current block in the alpha layer of a current picture that is predicted from one or more reference blocks in the base layer,

further comprising: determining whether motion information of one of one or more reference blocks is present within the bitstream or whether the motion information is inferred

wherein determining whether the motion information is present or whether the motion is inferred comprises:

determining whether the motion information of one of one or more reference blocks is present within the bitstream based on at least one of: header information, a position of a coding unit, coded mode information for blocks containing a sample at an edge, block dimension information, block shape information, a color format indication, a coding tree structure, color component information, a temporal layer identifier, or coding standard information;

in accordance with a determination that the motion information of one of one or more reference blocks is present within the bitstream, obtaining the motion information of one of one or more reference blocks; and

in accordance with a determination that the motion information of one of one or more reference blocks is not present within the bitstream, inferring the motion information of one of one or more reference blocks.

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

perform a conversion between a video comprising a plurality of pictures and a bitstream of the video, wherein each picture comprises an alpha layer and a base layer;

wherein the bitstream comprises a current block in the alpha layer of a current picture that is predicted from one or more reference blocks in the base layer,

and further determine whether motion information of one of one or more reference blocks is present within the bitstream or whether the motion information is inferred

wherein the processor is caused to determine whether the motion information is present or whether the motion is inferred by:

determining whether the motion information of one of one or more reference blocks is present within the bitstream based on at least one of: header information, a position of a coding unit, coded mode information for blocks containing a sample at an edge, block dimension information, block shape information, a color format indication, a coding tree structure, color component information, a temporal layer identifier, or coding standard information;

in accordance with a determination that the motion information of one of one or more reference blocks is present within the bitstream, obtaining the motion information of one of one or more reference blocks; and

in accordance with a determination that the motion information of one of one or more reference blocks is not present within the bitstream, inferring the motion information of one of one or more reference blocks.