Early termination for optical flow refinement

An encoder comprising:

• a memory configured to store instructions;
• and
• a processor coupled to the memory, wherein the instructions when executed cause the processor to be configured for:
• obtaining a pair of initial motion vectors for a current block, wherein the current block is a sub-block;
• obtaining two first prediction blocks for the current block based on the pair of initial motion vectors;
• calculating a first matching cost according to the two first prediction blocks, wherein the first matching cost is a sum of absolute differences (SAD) between the two first prediction blocks;
• obtaining a pair of refined motion vectors based on the pair of initial motion vectors;
• obtaining two second prediction blocks for the current block according to the pair of refined motion vectors;
• obtaining a second matching cost corresponding to the two second prediction blocks;
• determining whether an optical flow refinement process is to be performed or not, according to at least one preset condition, the at least one preset condition comprising a condition of whether the second matching cost is less than a threshold;
• performing an optical flow refinement process on the two second prediction blocks for obtaining a final prediction block for the current block, when it is determined that the optical flow refinement process is to be performed;
• obtaining the final prediction block by a weighted sum of the two second prediction blocks when it is determined that the optical flow refinement process is not to be performed;
• obtaining a residual block based on the current block and the final prediction block;
• and
• encoding the residual block into a bitstream.

A non-transitory computer readable storage medium storing instructions, wherein when the instructions are performed by one or more processors, the one or more processors are configured to perform:

• obtaining a pair of initial motion vectors for a current block, wherein the current block is a sub-block;
• obtaining two first prediction blocks for the current block based on the pair of initial motion vectors;
• calculating a first matching cost according to the two first prediction blocks, wherein the first matching cost is a sum of absolute differences (SAD) between the two first prediction blocks;
• obtaining a pair of refined motion vectors based on the pair of initial motion vectors;
• obtaining two second prediction blocks for the current block according to the pair of refined motion vectors;
• obtaining a second matching cost corresponding to the two second prediction blocks;
• determining whether an optical flow refinement process is to be performed or not, according to at least one preset condition, the at least one preset condition comprising a condition of whether the second matching cost is less than a threshold;
• performing an optical flow refinement process on the two second prediction blocks for obtaining a final prediction block for the current block, when it is determined that the optical flow refinement process is to be performed;
• obtaining the final prediction block by a weighted sum of the two second prediction blocks when it is determined that the optical flow refinement process is not to be performed;
• obtaining a residual block based on the current block and the final prediction block;
• and
• encoding the residual block into the bitstream.