Encoder, a decoder and corresponding methods for inter-prediction

An inter-prediction decoding method for a video block, comprising:

• parsing an index from a bitstream;
• selecting, using the index, an initial motion vector from a list of candidate motion vectors for a current block;
• determining a plurality of search space positions according to the initial motion vector, wherein the plurality of search space positions include a central search space position;
• calculating fractional pixels by using bilinear interpolation;
• checking a matching cost for each of the plurality of search space positions according to a checking order;
• selecting, from the plurality of search space positions, a target search space position having a minimal matching cost;
• and
• determining a refining motion vector of the current block based on the initial motion vector and the target search space position, wherein the central search space position is checked first and pointed to by the initial motion vector.

An encoder, comprising:

• one or more processors;
• and
• memory coupled to the one or more processors and storing program instructions that, when executed by the one or more processors, cause the encoder to:
• select, using an index, an initial motion vector from a list of candidate motion vectors for a current block;
• determine a plurality of search space positions according to the initial motion vector, wherein the plurality of search space positions include a central search space position;
• calculate fractional pixels by using bilinear interpolation;
• check a matching cost for each of the plurality of search space positions according to a checking order;
• select from the plurality of search space positions a target search space position having a minimal matching cost;
• determine a refining motion vector for the current block based on the initial motion vector and the target search space position, wherein the central search space position is checked first and the pointed to by the initial motion vector;
• and
• encode the index into a bitstream.

A decoder, comprising:

• one or more processors;
• and
• memory coupled to the one or more processors and storing program instructions that, when executed by the one or more processors, cause the decoder to:
• parse an index from a bitstream;
• select, using an index, an initial motion vector from a list of candidate motion vectors for a current block;
• determine a plurality of search space positions according to the initial motion vector, wherein the plurality of search space positions include a central search space position;
• check matching costs for each of the plurality of search space positions according to a checking order;
• select from the plurality of search space positions a target search space position having a minimal matching cost;
• and
• determine a refining motion vector for the current block based on the initial motion vector and the target search space position, wherein the central search space position is checked first and the pointed to by the initial motion vector.

A non-transitory storage medium storing a bitstream and one or more instructions executable by at least one processor to perform operations of encoding or decoding of the bitstream, the operations comprising:

• selecting, using an index, an initial motion vector from a list of candidate motion vectors for a current block;
• determining a plurality of search space positions according to the initial motion vector, wherein the plurality of search space positions include a central search space position;
• checking matching costs for each of the plurality of search space positions according to a checking order;
• selecting from the plurality of search space positions a target search space position having a minimal matching cost;
• and
• determining a refining motion vector of the current block based on the initial motion vector and the target search space position, wherein the central search space position is checked first and the pointed to by the initial motion vector.