Construction of affine candidates in video processing

An apparatus of video processing 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 current video block of a video and a bitstream of the video, a mode of operation of an affine ultimate motion vector expression (UMVE) mode;
• and
• perform the conversion based on the determining;
• wherein the current video block is coded with an affine merge mode and motion vector differences in the affine UMVE mode that includes a starting point of motion information, a motion magnitude and a motion direction for the current video block;
• wherein one or more fields in the bitstream correspond to the mode of operation;
• and
• wherein the one or more fields include:
• a list size field that indicates a size of a base affine merge candidate list for an affine merge mode with prediction offsets that is used by the UMVE mode, wherein the one or more fields further include a sub-block merge list size field whose value indicates a size of a candidate list for a sub-block merge mode, and wherein the size of the base affine merge candidate list for the affine UMVE mode is less than or equal to the size of the candidate list for the sub-block merge mode, or
• wherein the one or more fields include a table field that signals a distance table or a direction table for the affine merge mode with prediction offsets, wherein the table field signals a size K1 of the distance table, and a size K2 of the direction table, and wherein first K1 elements of the distance table or first K2 elements of the direction table are valid.

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 conversion between a current video block of a video and a bitstream of the video, a mode of operation of an affine ultimate motion vector expression (UMVE) mode;
• and
• generating the bitstream based on the determining;
• wherein the current video block is coded with an affine merge mode and motion vector differences in the affine UMVE mode that includes a starting point of motion information, a motion magnitude and a motion direction for the current video block;
• wherein one or more fields in the bitstream correspond to the mode of operation;
• and
• wherein the one or more fields includes a list size field that indicates a size of a base affine merge candidate list for an affine merge mode with prediction offsets that is used by the UMVE mode, wherein the one or more fields further include a sub-block merge list size field whose value indicates a size of a candidate list for a sub-block merge mode, and wherein the size of the base affine merge candidate list for the affine UMVE mode is less than or equal to the size of the candidate list for the sub-block merge mode, or
• wherein the one or more fields include a table field that signals a distance table or a direction table for the affine merge mode with prediction offsets, wherein the table field signals a size K1 of the distance table, and a size K2 of the direction table, and wherein first K1 elements of the distance table or first K2 elements of the direction table are valid.