Encoder, decoder, encoding method, and decoding method

1 . A decoder, comprising:

circuitry; and

memory, wherein

using the memory, the circuitry, in inter prediction processing:

derives a first motion vector of a first current block to be processed, using a motion vector of a previous block which has been previously processed;

derives a second motion vector of the first current block by performing a motion estimation in vicinity of a position specified by the first motion vector, the motion estimation including searching a position with an evaluation value lowest in vicinity of the position specified by the first motion vector;

generates a prediction image of the first current block by performing motion compensation using the second motion vector;

derives a third motion vector of a second current block to be processed after the first current block, using the first motion vector of the first current block, when the second current block is included in a first picture including the first current block;

derives a third motion vector of the second current block, using the second motion vector of the first current block, when the second current block is included in a second picture different from the first picture;

derives a fourth motion vector of the second current block by performing motion estimation in vicinity of a position specified by the third motion vector; and

generates a prediction image of the second current block by performing motion compensation using the fourth motion vector.

2 . The decoder according to claim 1 ,

wherein the evaluation value is a difference between a first region in vicinity of the position in a first reference picture and a second region in a second reference picture.

3 . An encoder, comprising:

circuitry; and

memory, wherein

using the memory, the circuitry, in inter prediction processing:

derives a first motion vector of a first current block to be processed, using a motion vector of a previous block which has been previously processed;

derives a second motion vector of the first current block by performing motion estimation in vicinity of a position specified by the first motion vector, the motion estimation including searching a position with an evaluation value lowest in vicinity of the position specified by the first motion vector;

generates a prediction image of the first current block by performing motion compensation using the second motion vector;

derives a third motion vector of a second current block to be processed after the first current block, using the first motion vector of the first current block, when the second current block is included in a first picture including the first current block;

derives a third motion vector of the second current block, using the second motion vector of the first current block, when the second current block is included in a second picture different from the first picture;

derives a fourth motion vector of the second current block by performing motion estimation in vicinity of a position specified by the third motion vector; and

generates a prediction image of the second current block by performing motion compensation using the fourth motion vector.

4 . A decoding method, comprising:

deriving a first motion vector of a first current block to be processed, using a motion vector of a previous block which has been previously processed;

deriving a second motion vector of the first current block by performing motion estimation in vicinity of a position specified by the first motion vector, the motion estimation including searching a position with an evaluation value lowest in vicinity of the position specified by the first motion vector;

generating a prediction image of the first current block by performing motion compensation using the second motion vector;

deriving a third motion vector of a second current block to be processed after the first current block, using the first motion vector of the first current block, when the second current block is included in a first picture including the first current block;

deriving a third motion vector of the second current block, using the second motion vector of the first current block, when the second current block is included in a second picture different from the first picture;

deriving a fourth motion vector of the second current block by performing motion estimation in vicinity of a position specified by the third motion vector; and

generating a prediction image of the second current block by performing motion compensation using the fourth motion vector.

5 . An encoding method, comprising:

deriving a first motion vector of a first current block to be processed, using a motion vector of a previous block which has been previously processed;

deriving a second motion vector of the first current block by performing motion estimation in vicinity of a position specified by the first motion vector, the motion estimation including searching a position with an evaluation value lowest in vicinity of the position specified by the first motion vector;

generating a prediction image of the first current block by performing motion compensation using the second motion vector;

deriving a third motion vector of a second current block to be processed after the first current block, using the first motion vector of the first current block, when the second current block is included in a first picture including the first current block;

deriving a third motion vector of the second current block, using the second motion vector of the first current block, when the second current block is included in a second picture different from the first picture;

deriving a fourth motion vector of the second current block by performing motion estimation in vicinity of a position specified by the third motion vector; and

generating a prediction image of the second current block by performing motion compensation using the fourth motion vector.