Symmetric motion vector difference coding
Symmetric motion vector difference coding is described. One example video processing method includes determining, for a conversion between a block of a current picture of video and a bitstream representation of the block, whether a symmetric motion vector difference (SMVD) mode is allowable for the block, wherein whether the SMVD mode is allowable depends on a derivation process of two target reference pictures of the current picture, which includes a first step for searching a forward target reference picture in the reference picture list 0 and searching a backward target reference picture in the reference picture list 1, and a second step for searching a backward target reference picture in the reference picture list 0 and searching a forward target reference picture in the reference picture list 1, outputs of the first step and the second step being independent of each other; and performing the conversion based on the determination.
1. A method of processing video data, comprising:
determining, for a conversion between a block of a current picture of a video and a bitstream of the video, that a first coding mode is applied to the block based on a derivation process of two target reference pictures of the current picture, wherein the derivation process includes:
performing a first step for searching a forward target reference picture in a reference picture list 0 and searching a backward target reference picture in a reference picture list 1, wherein target reference picture indices in both the reference picture list 0 and the reference picture list 1 are initialized to be unavailable;
keeping a target reference picture index in one of the reference picture list 0 and the reference picture list 1 unavailable due to the forward target reference picture or the backward target reference picture not being found in the one of the reference picture list 0 and the reference picture list 1 in the first step;
resetting a target reference picture index in another of the reference picture list 0 and the reference picture list 1 available due to the forward target reference picture or the backward target reference picture being found in the another of the reference picture list 0 and the reference picture list 1 in the first step;
resetting both the target reference picture indices to be unavailable before a second step is invoked due to the target reference picture index in the one of the reference picture list 0 and the reference picture list 1 being unavailable; and
performing the second step for searching a backward target reference picture in the reference picture list 0 and searching a forward target reference picture in the reference picture list 1; and
performing the conversion based on the determination,
wherein in the first coding mode, a syntax structure indicating a motion vector difference of a reference picture list X is not present in the bitstream, and the motion vector difference of the reference picture list X is derived based on a motion vector difference of a reference picture list (1-X), wherein X is equal to 0 or 1.
2. The method of claim 1 , wherein resetting both the target reference picture indices to be unavailable comprises resetting both RefIdxSymL0 and RefIdxSymL1 to −1, wherein RefIdxSymL0 indicates the target reference picture index in the reference picture list 0 and RefIdxSymL1 indicates the target reference picture index in the reference picture list 1.
3. The method of claim 1 , wherein whether the first coding mode is allowable depends on whether both the target reference pictures are found in one step of the first step and the second step.
4. The method of claim 3 , wherein when both the target reference pictures are found in the first step, the first coding mode is allowed.
5. The method of claim 3 , wherein when both the target reference pictures are found in the second step, the first coding mode is allowed.
6. The method of claim 3 , wherein when at least one of the two target reference pictures are not found in the first step and at least one of the two target reference pictures are not found in the second step, the first coding mode is disallowed.
7. The method of claim 1 , wherein a picture order of the forward target reference picture is smaller than a picture order of the current picture and a picture order of the backward target reference picture is larger than the picture order of the current picture.
8. The method of claim 1 , wherein the conversion includes decoding the block of video from the bitstream.
9. The method of claim 1 , wherein the conversion includes encoding the block of video into the bitstream.
10. 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:
determine, for a conversion between a block of a current picture of a video and a bitstream of the video, that a first coding mode is applied to the block based on a derivation process of two target reference pictures of the current picture, wherein the derivation process includes:
performing a first step for searching a forward target reference picture in a reference picture list 0 and searching a backward target reference picture in a reference picture list 1, wherein target reference picture indices in both the reference picture list 0 and the reference picture list 1 are initialized to be unavailable;
keeping a target reference picture index in one of the reference picture list 0 and the reference picture list 1 unavailable due to the forward target reference picture or the backward target reference picture not being found in the one of the reference picture list 0 and the reference picture list 1 in the first step;
resetting a target reference picture index in another of the reference picture list 0 and the reference picture list 1 available due to the forward target reference picture or the backward target reference picture being found in the another of the reference picture list 0 and the reference picture list 1 in the first step;
resetting both the target reference picture indices to be unavailable before a second step is invoked due to the target reference picture index in the one of the reference picture list 0 and the reference picture list 1 being unavailable; and
performing the second step for searching a backward target reference picture in the reference picture list 0 and searching a forward target reference picture in the reference picture list 1; and
perform the conversion based on the determination,
wherein in the first coding mode, a syntax structure indicating a motion vector difference of a reference picture list X is not present in the bitstream, and the motion vector difference of the reference picture list X is derived based on a motion vector difference of a reference picture list (1-X), wherein X is equal to 0 or 1.
11. The apparatus of claim 10 , wherein resetting both the target reference picture indices to be unavailable comprises resetting both RefIdxSymL0 and RefIdxSymL1 to −1, wherein RefIdxSymL0 indicates the target reference picture index in the reference picture list 0 and RefIdxSymL1 indicates the target reference picture index in the reference picture list 1.
12. The apparatus of claim 10 , wherein whether the first coding mode is allowable depends on whether both the target reference pictures are found in one step of the first step and the second step.
13. The apparatus of claim 12 , wherein when both the target reference pictures are found in the first step, the first coding mode is allowed.
14. The apparatus of claim 12 , wherein when both the target reference pictures are found in the second step, the first coding mode is allowed.
15. The apparatus of claim 12 , wherein when at least one of the two target reference pictures are not found in the first step and at least one of the two target reference pictures are not found in the second step, the first coding mode is disallowed.
16. The apparatus of claim 10 , wherein a picture order of the forward target reference picture is smaller than a picture order of the current picture and a picture order of the backward target reference picture is larger than the picture order of the current picture.
17. A non-transitory computer-readable storage medium storing instructions that cause a processor to:
determine, for a conversion between a block of a current picture of a video and a bitstream of the video, that a first coding mode is applied to the block based on a derivation process of two target reference pictures of the current picture, wherein the derivation process includes:
performing a first step for searching a forward target reference picture in a reference picture list 0 and searching a backward target reference picture in a reference picture list 1, wherein target reference picture indices in both the reference picture list 0 and the reference picture list 1 are initialized to be unavailable;
keeping a target reference picture index in one of the reference picture list 0 and the reference picture list 1 unavailable due to the forward target reference picture or the backward target reference picture not being found in the one of the reference picture list 0 and the reference picture list 1 in the first step;
resetting a target reference picture index in another of the reference picture list 0 and the reference picture list 1 available due to the forward target reference picture or the backward target reference picture being found in the another of the reference picture list 0 and the reference picture list 1 in the first step;
resetting both the target reference picture indices to be unavailable before a second step is invoked due to the target reference picture index in the one of the reference picture list 0 and the reference picture list 1 being unavailable; and
performing the second step for searching a backward target reference picture in the reference picture list 0 and searching a forward target reference picture in the reference picture list 1; and
perform the conversion based on the determination,
wherein in the first coding mode, a syntax structure indicating a motion vector difference of a reference picture list X is not present in the bitstream, and the motion vector difference of the reference picture list X is derived based on a motion vector difference of a reference picture list (1-X), wherein X is equal to 0 or 1.
18. The non-transitory computer-readable storage medium of claim 17 , wherein resetting both the target reference picture indices to be unavailable comprises resetting both RefIdxSymL0 and RefIdxSymL1 to −1, wherein RefIdxSymL0 indicates the target reference picture index in the reference picture list 0 and RefIdxSymL1 indicates the target reference picture index in the reference picture list 1.
19. The non-transitory computer-readable storage medium of claim 17 , wherein a picture order of the forward target reference picture is smaller than a picture order of the current picture and a picture order of the backward target reference picture is larger than the picture order of the current picture.
20. 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:
determining, for a block of a current picture of the video, that a first coding mode is applied to the block based on a derivation process of two target reference pictures of the current picture, wherein the derivation process includes:
performing a first step for searching a forward target reference picture in a reference picture list 0 and searching a backward target reference picture in a reference picture list 1, wherein target reference picture indices in both the reference picture list 0 and the reference picture list 1 are initialized to be unavailable;
keeping a target reference picture index in one of the reference picture list 0 and the reference picture list 1 unavailable due to the forward target reference picture or the backward target reference picture not being found in the one of the reference picture list 0 and the reference picture list 1 in the first step;
resetting a target reference picture index in another of the reference picture list 0 and the reference picture list 1 available due to the forward target reference picture or the backward target reference picture being found in the another of the reference picture list 0 and the reference picture list 1 in the first step;
resetting both the target reference picture indices to be unavailable before a second step is invoked due to the target reference picture index in the one of the reference picture list 0 and the reference picture list 1 being unavailable; and
performing the second step for searching a backward target reference picture in the reference picture list 0 and searching a forward target reference picture in the reference picture list 1; and
generating the bitstream based on the determination,
wherein in the first coding mode, a syntax structure indicating a motion vector difference of a reference picture list X is not present in the bitstream, and the motion vector difference of the reference picture list X is derived based on a motion vector difference of a reference picture list (1-X), wherein X is equal to 0 or 1.