Encoder, decoder, encoding method, and decoding method

An encoder comprising:

• a processor;
• and
• memory, wherein
• using the memory, the processor:
• derives a representative motion vector indicating a representative position;
• determines, in a first reference picture for a current block, a first motion estimation region that includes the representative position indicated by the representative motion vector;
• calculates first evaluation values of a plurality of candidate regions included in the first motion estimation region, each of the first evaluation values being a difference between (i) a corresponding candidate region from among the plurality of candidate regions and (ii) a region in a second reference picture;
• determines a first adjacent region included in the first motion estimation region, the first adjacent region including a first candidate region and a vicinity of the first candidate region, the first candidate region being one of the plurality of the candidate regions having a smallest first evaluation value among the plurality of candidate regions included in the first motion estimation region;
• and
• determines the motion vector of the current block using a smallest second evaluation value among a plurality of regions included in the first adjacent region,
• wherein a first picture order count between a current picture including the current block and the first reference picture and a second picture order count between the current picture and the second reference picture is the same.

A non-transitory computer readable medium for use in a computer, the non- transitory computer readable medium storing a program for causing the computer to perform decoding processing, wherein the decoding processing comprises:

• deriving a representative motion vector indicating a representative position from a bitstream;
• determining, in a first reference picture for a current block, a first motion estimation region that includes the representative position indicated by the representative motion vector;
• calculating first evaluation values of a plurality of candidate regions included in the first motion estimation region, each of the first evaluation values being a difference between (i) a corresponding candidate region from among the plurality of candidate regions and (ii) a region in a second reference picture;
• determining a first adjacent region included in the first motion estimation region, the first adjacent region including a first candidate region and a vicinity of the first candidate region, the first candidate region being one of the plurality of the candidate regions having a smallest first evaluation value among the plurality of candidate regions included in the first motion estimation region;
• and
• determining a motion vector of the current block using a smallest second evaluation value among a plurality of regions included in the first adjacent region,
• wherein a first picture order count between a current picture including the current block and the first reference picture and a second picture order count between the current picture and the second reference picture is the same.