Motion vector derivation between color components

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.

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.

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.