Delta motion vector in prediction refinement with optical flow process

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 current video block, an applicability of an optical flow operation with an affine mode according to a first rule that specifies a condition for applying the optical flow operation;
• generate initial prediction samples of a sub-block of the current video block coded with the affine mode;
• apply the optical flow operation to generate final prediction samples for the sub-block by deriving a prediction refinement based on motion vector differences dMvH and/or dMvV, wherein dMvH and dMvV indicate motion vector differences along a horizontal direction and a vertical direction;
• and
• perform a conversion between the current video block and a bitstream of a video based on the final prediction samples,
• wherein a clipping operation is performed on the dMvH and/or dMvV before the deriving of the prediction refinement according to a second rule,
• wherein the second rule specifies to perform the clipping operation before the prediction refinement being acquired and wherein the clipping operation is performed based on a precision of the dMvH and/or dMvV, and
• wherein the dMvH and/or dMvV is clipped as follows:
• dMvH[posX][posY]=Clip3(−2K−1,2K−1−1,dMvH[posX][posY]);
• dMvV[posX][posY]=Clip3(−2K−1,2K−1−1,dMvV[posX][posY]).

A non-transitory computer-readable storage medium storing instructions that cause a processor to:

• determine, for a current video block, an applicability of an optical flow operation with an affine mode according to a first rule that specifies a condition for applying the optical flow operation;
• generate initial prediction samples of a sub-block of the current video block coded with the affine mode;
• apply the optical flow operation to generate final prediction samples for the sub-block by deriving a prediction refinement based on motion vector differences dMvH and/or dMvV, wherein dMvH and dMvV indicate motion vector differences along a horizontal direction and a vertical direction;
• and
• perform a conversion between the current video block and a bitstream of a video based on the final prediction samples,
• wherein a clipping operation is performed on the dMvH and/or dMvV before the deriving of the prediction refinement according to a second rule,
• wherein the second rule specifies to perform the clipping operation before the prediction refinement being acquired and wherein the clipping operation is performed based on a precision of the dMvH and/or dMvV, and
• wherein the dMvH and/or dMvV is clipped as follows:
• dMvH[posX][posY]=Clip3(−2K−1,2K−1−1,dMvH[posX][posY]);
• dMvV[posX][posY]=Clip3(−2K−1,2K−1−1,dMvV[posX][posY]).

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 current video block, an applicability of an optical flow operation with an affine mode according to a first rule that specifies a condition for applying the optical flow operation;
• generating initial prediction samples of a sub-block of the current video block coded with the affine mode;
• applying the optical flow operation to generate final prediction samples for the sub-block by deriving a prediction refinement based on motion vector differences dMvH and/or dMvV, wherein dMvH and dMvV indicate motion vector differences along a horizontal direction and a vertical direction;
• and
• generating the bitstream of the video based on the final prediction samples,
• wherein a clipping operation is performed on the dMvH and/or dMvV before the deriving of the prediction refinement according to a second rule,
• wherein the second rule specifies to perform the clipping operation before the prediction refinement being acquired and wherein the clipping operation is performed based on a precision of the dMvH and/or dMvV, and
• wherein the dMvH and/or dMvV is clipped as follows:
• dMvH[posX][posY]=Clip3(−2K−1,2K−1−1,dMvH[posX][posY]);
• dMvV[posX][posY]=Clip3(−2K−1,2K−1−1,dMvV[posX][posY]).