Decoder side motion vector derivation

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 block of a video, an initial prediction sample;
• refine the initial prediction sample, based on an optical flow refinement technology, with a prediction sample offset to acquire a final prediction sample;
• and
• perform a conversion between the current block and a bitstream of the video based on the final prediction sample,
• wherein the prediction sample offset is determined based on at least one spatial gradient of the initial prediction sample, wherein the spatial gradient is calculated based on at least a difference between two first prediction samples from a same reference picture list, and
• wherein before calculating the difference between the two first prediction samples, values of the two first prediction sample are right shifted with a first value.

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

• determine, for a current block of a video, an initial prediction sample;
• refine the initial prediction sample, based on an optical flow refinement technology, with a prediction sample offset to acquire a final prediction sample;
• and
• perform a conversion between the current block and a bitstream of the video based on the final prediction sample,
• wherein the prediction sample offset is determined based on at least one spatial gradient of the initial prediction sample, wherein the spatial gradient is calculated based on at least a difference between two first prediction samples from a same reference picture list, and
• wherein before calculating the difference between the two first prediction samples, values of the two first prediction sample are right shifted with a first value.

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 block of a video, an initial prediction sample;
• refining the initial prediction sample, based on an optical flow refinement technology, with a prediction sample offset to acquire a final prediction sample;
• and
• generating the bitstream based on the final prediction sample,
• wherein the prediction sample offset is determined based on at least one spatial gradient of the initial prediction sample, wherein the spatial gradient is calculated based on at least a difference between two first prediction samples from a same reference picture list, and
• wherein before calculating the difference between the two first prediction samples, values of the two first prediction sample are right shifted with a first value.