Encoder, decoder, encoding method, and decoding method

An encoder, comprising:

• circuitry;
• and
• memory, wherein
• using the memory, the circuitry:
• selects a mode from among a plurality of modes each for deriving a motion vector, and derives a motion vector for a current block via the mode selected;
• and
• performs inter prediction encoding on the current block, using the motion vector derived, via one of a skip mode and a non-skip mode that is different from the skip mode,
• the plurality of modes include a plurality of first modes each for predicting the motion vector for the current block based on an encoded block neighboring the current block without encoding information indicating a motion vector difference into a stream,
• in the skip mode, out of information indicating whether a second mode included in the plurality of first modes is used and information indicating whether a third mode that is included in the plurality of first modes and different from the second mode is used, only the information indicating whether the third mode is used is encoded into the stream,
• in the non-skip mode, the information indicating whether the second mode is used and the information indicating whether the third mode is used are encoded into the stream, and
• when the second mode is used, the current block is encoded via the non-skip mode regardless of presence or absence of a residual coefficient.

A decoder, comprising:

• circuitry;
• and
• memory, wherein
• using the memory, the circuitry:
• selects a mode from among a plurality of modes each for deriving a motion vector, and derives a motion vector for a current block via the mode selected;
• and
• performs inter prediction decoding on the current block, using the motion vector derived, via one of a skip mode and a non-skip mode that is different from the skip mode,
• the plurality of modes include a plurality of first modes each for predicting the motion vector for the current block based on a decoded block neighboring the current block without decoding information indicating a motion vector difference from a stream,
• in the skip mode, out of information indicating whether a second mode included in the plurality of first modes is used and information indicating whether a third mode that is included in the plurality of first modes and different from the second mode is used, only the information indicating whether the third mode is used is decoded from the stream,
• in the non-skip mode, the information indicating whether the second mode is used and the information indicating whether the third mode is used are decoded from the stream, and
• when the second mode is used, the current block is decoded via the non-skip mode regardless of presence or absence of a residual coefficient.

An encoding method, comprising:

• selecting a mode from among a plurality of modes each for deriving a motion vector, and deriving a motion vector for a current block via the mode selected;
• and
• performing inter prediction encoding on the current block, using the motion vector derived, via one of a skip mode and a non-skip mode that is different from the skip mode, wherein
• the plurality of modes include a plurality of first modes each for predicting the motion vector for the current block based on an encoded block neighboring the current block without encoding information indicating a motion vector difference into a stream,
• in the skip mode, out of information indicating whether a second mode included in the plurality of first modes is used and information indicating whether a third mode that is included in the plurality of first modes and different from the second mode is used, only the information indicating whether the third mode is used is encoded into the stream,
• in the non-skip mode, the information indicating whether the second mode is used and the information indicating whether the third mode is used are encoded into the stream, and
• when the second mode is used, the current block is encoded via the non-skip mode regardless of presence or absence of a residual coefficient.

A decoding method, comprising:

• selecting a mode from among a plurality of modes each for deriving a motion vector, and deriving a motion vector for a current block via the mode selected;
• and
• performing inter prediction decoding on the current block, using the motion vector derived, via one of a skip mode and a non-skip mode that is different from the skip mode, wherein
• the plurality of modes include a plurality of first modes each for predicting the motion vector for the current block based on a decoded block neighboring the current block without decoding information indicating a motion vector difference from a stream,
• in the skip mode, out of information indicating whether a second mode included in the plurality of first modes is used and information indicating whether a third mode that is included in the plurality of first modes and different from the second mode is used, only the information indicating whether the third mode is used is decoded from the stream,
• in the non-skip mode, the information indicating whether the second mode is used and the information indicating whether the third mode is used are decoded from the stream, and
• when the second mode is used, the current block is decoded via the non-skip mode regardless of presence or absence of a residual coefficient.