Motion vector derivation between color components
A method for video processing is provided to comprise: dividing a video block of a first color component to obtain a first set sub-blocks of the first color component; dividing a corresponding video block of a second color component to obtain a second set of sub-blocks of a second color component; deriving one or more motion vectors of the first set of sub-blocks based on one or more motion vectors of the second set of sub-blocks; and performing, based on the one or more motion vectors of the first set and second set of sub-blocks, a conversion between the video block and a coded representation of the video.
1. A method of processing video data, comprising:
determining, during a conversion between a current video unit of a video and a bitstream of the video, that motion vectors of control points for a luma block of the current video unit based on an affine mode, and wherein the current video unit comprises the luma block and a chroma block;
dividing the luma block into luma sub-blocks;
dividing the chroma block into chroma sub-blocks;
determining a luma motion vector for each luma sub-block based on the motion vectors of the control points, and determining a chroma motion vector for each chroma sub-block; and
reconstructing the luma block based on the luma motion vector of each luma sub-block;
wherein a color format of the current video unit is 4:2:0, the chroma block comprises at least one chroma group, and four chroma sub-blocks included in a same chroma group correspond to four luma sub-blocks; and
the four chroma sub-blocks included in the same chroma group share a same chroma motion vector which is derived based two luma motion vectors of two corresponding luma sub-blocks at diagonal or anti-diagonal positions.
2. The method of claim 1 , wherein a top-left one of the four luma sub-blocks has motion vector MV 0 , and a bottom-right one of the four luma sub-blocks has motion vector MV 1 , and wherein the same chroma motion vector is derived based on applying a scaling factor to an intermediate motion vector MV*, and the MV* is derived based on the MV 0 and the MV 1 .
3. The method of claim 2 , wherein the intermediate motion vector MV* is derived based on applying an offset-based averaging operation on the MV 0 and the MV 1 .
4. The method of claim 3 , wherein the intermediate motion vector MV*=Shift(MV 0 +MV 1 ,1), wherein Shift(x,1)=(x+offset)>>1, offset is equal to 0 or 1, and wherein >>represents a right shift operation.
5. The method of claim 1 , wherein a number of the control points is 2.
6. The method of claim 1 , wherein the control points include a top-left control point and a top-right control point.
7. The method of claim 1 , wherein a number of the control points is 3.
8. The method of claim 1 , wherein the control points include a top-left control point, a top-right control point and a bottom-left corner control point.
9. The method of claim 1 , further comprising:
reconstructing the chroma block based on the motion vectors of the chroma sub-blocks.
10. The method of claim 1 , wherein the conversion includes encoding the current video unit into the bitstream.
11. The method of claim 1 , wherein the conversion includes decoding the current video unit from the bitstream.
12. 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, during a conversion between a current video unit of a video and a bitstream of the video, that motion vectors of control points for a luma block of the current video unit based on an affine mode, and wherein the current video unit comprises the luma block and a chroma block;
divide the luma block into luma sub-blocks;
divide the chroma block into chroma sub-blocks;
determine a luma motion vector for each luma sub-block based on the motion vectors of the control points, and determining a chroma motion vector for each chroma sub-block; and
reconstruct the luma block based on the luma motion vector of each luma sub-block;
wherein a color format of the current video unit is 4:2:0, the chroma block comprises at least one chroma group, and four chroma sub-blocks included in a same chroma group correspond to four luma sub-blocks; and
the four chroma sub-blocks included in the same chroma group share a same chroma motion vector which is derived based two luma motion vectors of two corresponding luma sub-blocks at diagonal or anti-diagonal positions.
13. The apparatus of claim 12 , wherein a top-left one of the four luma sub-blocks has motion vector MV 0 , and a bottom-right one of the four luma sub-blocks has motion vector MV 1 , and wherein the same chroma motion vector is derived based on applying a scaling factor to an intermediate motion vector MV*, and the MV* is derived based on the MV 0 and the MV 1 .
14. The apparatus of claim 13 , wherein the intermediate motion vector MV* is derived based on applying an offset-based averaging operation on the MV 0 and the MV 1 .
15. The apparatus of claim 14 , wherein the intermediate motion vector MV*=Shift (MV 0 +MV 1 ,1), wherein Shift(x,1)=(x+offset)>>1, offset is equal to 0 or 1, and wherein >>represents a right shift operation.
16. The apparatus of claim 12 , wherein a number of the control points is 2.
17. The apparatus of claim 12 , wherein the control points include a top-left control point and a top-right control point.
18. The apparatus of claim 12 , wherein a number of the control points is 3.
19. A non-transitory computer-readable storage medium storing instructions that cause a processor to:
determine, during a conversion between a current video unit of a video and a bitstream of the video, that motion vectors of control points for a luma block of the current video unit based on an affine mode, and wherein the current video unit comprises the luma block and a chroma block;
divide the luma block into luma sub-blocks;
divide the chroma block into chroma sub-blocks;
determine a luma motion vector for each luma sub-block based on the motion vectors of the control points, and determining a chroma motion vector for each chroma sub-block; and
reconstruct the luma block based on the luma motion vector of each luma sub-block;
wherein a color format of the current video unit is 4:2:0, the chroma block comprises at least one chroma group, and four chroma sub-blocks included in a same chroma group correspond to four luma sub-blocks; and
the four chroma sub-blocks included in the same chroma group share a same chroma motion vector which is derived based two luma motion vectors of two corresponding luma sub-blocks at diagonal or anti-diagonal positions.
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, that motion vectors of control points for a luma block of a current video unit of the video based on an affine mode, and wherein the current video unit comprises the luma block and a chroma block;
dividing the luma block into luma sub-blocks;
dividing the chroma block into chroma sub-blocks;
determining a luma motion vector for each luma sub-block based on the motion vectors of the control points, and determining a chroma motion vector for each chroma sub-block;
reconstructing the luma block based on the luma motion vector of each luma sub-block to obtain reconstructed luma block; and
generating the bitstream at least based on the reconstructed luma block;
wherein a color format of the current video unit is 4:2:0, the chroma block comprises at least one chroma group, and four chroma sub-blocks included in a same chroma group correspond to four luma sub-blocks; and
the four chroma sub-blocks included in the same chroma group share a same chroma motion vector which is derived based two luma motion vectors of two corresponding luma sub-blocks at diagonal or anti-diagonal positions.