Encoder, decoder, encoding method, and decoding method

A decoder comprising:

• circuitry;
• and
• memory coupled to the circuitry, wherein
• a prediction mode for a current block to be decoded is an affine mode, and
• in operation, the circuitry:
• derives a base motion vector which is a motion vector to be used in a prediction process for the current block, and is a motion vector at a control point in the current block, the control point being an affine-mode control point;
• derives a first motion vector different from the base motion vector;
• derives a motion vector difference based on a difference between the base motion vector and the first motion vector;
• determines whether the motion vector difference is greater than a threshold;
• modifies a second motion vector when the motion vector difference is determined to be greater than the threshold, and does not modify the second motion vector when the motion vector difference is determined not to be greater than the threshold, the second motion vector being different from the base motion vector and the first motion vector;
• and
• decodes the current block using the second motion vector modified or the second motion vector not modified.

An encoding method, wherein

• a prediction mode for a current block to be encoded is an affine mode,
• the encoding method comprising:
• deriving a base motion vector which is a motion vector to be used in a prediction process for the current block, and is a motion vector at a control point in the current block, the control point being an affine-mode control point;
• deriving a first motion vector different from the base motion vector;
• deriving a motion vector difference based on a difference between the base motion vector and the first motion vector;
• determining whether the motion vector difference is greater than a threshold;
• modifying a second motion vector when the motion vector difference is determined to be greater than the threshold, and not modifying the second motion vector when the motion vector difference is determined not to be greater than the threshold, the second motion vector being different from the base motion vector and the first motion vector;
• and
• encoding the current block using the second motion vector modified or the second motion vector not modified.

A decoding method, wherein

• a prediction mode for a current block to be decoded is an affine mode,
• the decoding method comprising:
• deriving a base motion vector which is a motion vector to be used in a prediction process for the current block, and is a motion vector at a control point in the current block, the control point being an affine-mode control point;
• deriving a first motion vector different from the base motion vector;
• deriving a motion vector difference based on a difference between the base motion vector and the first motion vector;
• determining whether the motion vector difference is greater than a threshold;
• modifying a second motion vector when the motion vector difference is determined to be greater than the threshold, and not modifying the second motion vector when the motion vector difference is determined not to be greater than the threshold, the second motion vector being different from the base motion vector and the first motion vector;
• and
• decoding the current block using the second motion vector modified or the second motion vector not modified.