Motion vector refinement for multi-reference prediction

An apparatus for storing a bitstream, wherein the apparatus comprises at least one processor, at least one storage medium storing computer executable instructions and at least one communication interface;

• wherein the at least one communication interface is configured to receive or transmit the bitstream and the at least one storage medium is configured to store the bitstream;
• and
• wherein the bitstream is generated by encoding a picture block by inter-prediction using a first motion vector associated with a first reference picture of a video and a second motion vector associated with a second reference picture of the video, and wherein when the instructions are executed by the processor the apparatus is caused to carry out a method comprises:
• obtaining an estimate of the first motion vector;
• determining, by performing a search within a search space specified based on the estimate of the first motion vector, the first motion vector;
• obtaining an estimate of the second motion vector;
• calculating, after the search to determine the first motion vector has been performed, the second motion vector, without performing a second search, based on:
• the estimate of the second motion vector, and
• a difference between the first motion vector and the estimate of the first motion vector;
• applying the first motion vector and the second motion vector for inter-prediction of the picture block;
• and
• generating the bitstream comprising the encoded picture block.