Border partition

1. A method for coding video data, comprising:

determining, for a conversion between a picture of a video and a bitstream of the video, that a first block of a picture segment covers at least one region that is outside a border of the picture segment, wherein the picture segment represents one of a plurality of segments into which the picture is segmented, wherein a size of the first block is M×N pixels;

selecting a second block of size K×L pixels, wherein (K≤M and L<N) or (K<M and L≤N), wherein the second block falls entirely within the picture segment and wherein the second block is used as a largest coding unit, a leaf coding block, or a coding tree block;

selecting a third block of size K′×L′ pixels by padding one or both dimensions of the second block, wherein K′ is set to 2 a , wherein 2 a ≥K and 2 a-1 <K and/or wherein L′ is set to 2 b , wherein 2 b >L and 2 b-1 <L, and the third block is another largest coding unit, another leaf coding block or another coding tree block; and

splitting, using a partition tree selected from a subset of partition trees applied to the first block, the third block, wherein the subset is determined based on a comparison between a ratio based on K′ and L′ and a threshold,

wherein the padding comprises a mirrored repetition or a motion-compensation based samples padding.

2. The method of claim 1 , wherein the conversion comprises encoding the picture into the bitstream.

3. The method of claim 1 , wherein the conversion comprises decoding the picture from the bitstream.

4. The method of claim 1 , wherein the size of the second block is backward compatible with a video coding standard.

5. The method of claim 4 , wherein parameters corresponding to the partition tree are communicated using signaling that is backward compatible with the video coding standard.

6. The method of claim 4 , wherein the video coding standard includes H.264/AVC (Advanced Video Coding) standard, H.265/HEVC (High Efficiency Video Coding) standard, or VVC (Versatile Video Coding) standard.

7. The method of claim 1 , wherein the padding the one or both dimensions comprises repeating outermost samples of the second block.

8. The method of claim 1 , further comprising:

determining that the size of the second block is not backward compatible with a video coding standard.

9. The method of claim 1 , further comprising entropy coding the partition tree using context-adaptive binary adaptive coding (CABAC).

10. The method of claim 1 , wherein the first block is a coding tree unit (CTU), a coding unit (CU), a prediction unit (PU), or a transform unit (TU).

11. The method of claim 1 , wherein a partition tree based on the size of the second block is different from the partition tree selected based on the size of the third block.

12. The method of claim 1 , wherein the picture segment is a slice or a tile.

13. The method of claim 1 , wherein the partition tree comprises subblocks, obtained by splitting the third block, being directly coded with no further splitting.

14. An apparatus for coding video data, the apparatus comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:

determine, for a conversion between a picture of a video and a bitstream of the video, that a first block of a picture segment covers at least one region that is outside a border of the picture segment, wherein the picture segment represents one of a plurality of segments into which the picture is segmented, wherein a size of the first block is M×N pixels;

select a second block of size K×L pixels, wherein (K≤M and L<N) or (K<M and L≤N), wherein the second block falls entirely within the picture segment and wherein the second block is used as a largest coding unit, a leaf coding block, or a coding tree block;

select a third block of size K′×L′ pixels by padding one or both dimensions of the second block, wherein K′ is set to 2 a , wherein 2 a ≤K and 2 a-1 <K and/or wherein L′ is set to 2 b , wherein 2 b ≥L and 2 b-1 <L, and the third block is another largest coding unit, another leaf coding block or another coding tree block; and

split, using a partition tree selected from a subset of partition trees applied to the first block, the third block, wherein the subset is determined based on a comparison between a ratio based on K′ and L′ and a threshold,

wherein the padding comprises a mirrored repetition or a motion-compensation based samples padding.

15. The apparatus of claim 14 , wherein the size of the second block is backward compatible with a video coding standard.

16. The apparatus of claim 14 , wherein a partition tree based on the size of the second block is different from the partition tree selected based on the size of the third block.

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

determine, for a conversion between a picture of a video and a bitstream of the video, that a first block of a picture segment covers at least one region that is outside a border of the picture segment, wherein the picture segment represents one of a plurality of segments into which the picture is segmented, wherein a size of the first block is M×N pixels;

select a second block of size K×L pixels, wherein (K≤M and L<N) or (K<M and L≤N), wherein the second block falls entirely within the picture segment and wherein the second block is used as a largest coding unit, a leaf coding block, or a coding tree block;

select a third block of size K′×L′ pixels by padding one or both dimensions of the second block, wherein K′ is set to 2 a , wherein 2 a K and 2 a-1 <K and/or wherein L′ is set to 2 b , wherein 2 b ≤L and 2 2-1 <L, and the third block is another largest coding unit, another leaf coding block or another coding tree block; and

split, using a partition tree selected from a subset of partition trees applied to the first block, the third block, wherein the subset is determined based on a comparison between a ratio based on K′ and L′ and a threshold,

wherein the padding comprises a mirrored repetition or a motion-compensation based samples padding.

18. The non-transitory computer readable media of claim 17 , wherein a partition tree based on the size of the second block is different from the partition tree selected based on the size of the third block.

19. A method for storing a bitstream of a video, comprises:

determining, for a picture of the video, that a first block of a picture segment covers at least one region that is outside a border of the picture segment, wherein the picture segment represents one of a plurality of segments into which a picture is segmented, wherein a size of the first block is M×N pixels;

selecting a second block of size K×L pixels, wherein (K≤M and L<N) or (K<M and L≤N), wherein the second block falls entirely within the picture segment and wherein the second block is used as a largest coding unit, a leaf coding block, or a coding tree block;

selecting a third block of size K′×L′ pixels by padding one or both dimensions of the second block, wherein K′ is set to 2 a , wherein 2 a ≥K and 2 a-1 <K and/or wherein L′ is set to 2 b , wherein 2 b ≥L and 2 b-1 <L, and the third block is another largest coding unit, another leaf coding block or another coding tree block;

splitting, using a partition tree selected from a subset of partition trees applied to the first block, the third block, wherein the subset is determined based on a comparison between a ratio based on K′ and L′ and a threshold;

generating the bitstream from a current block based on the splitting, and

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

wherein the padding comprises a mirrored repetition or a motion-compensation based samples padding.

20. The method of claim 19 , wherein a partition tree based on the size of the second block is different from the partition tree selected based on the size of the third block.