Encoder, decoder, encoding method, and decoding method

A decoder that decodes a video by performing motion compensation, the decoder comprising:

• memory;
• and
• circuitry accessible to the memory,
• wherein the circuitry accessible to the memory:
• decodes first control information indicating one mode that derives a motion vector of a block of an image in the video;
• decodes second control information indicating whether it is possible to perform the motion compensation in a unit of a sub-block included in the block;
• derives the motion vector of the block in the one mode;
• determines whether to perform the motion compensation in the unit of the sub-block or the motion compensation in a unit of the block according to whether it is possible to perform the motion compensation in the unit of the sub-block;
• derives a motion vector of the sub-block and performs the motion compensation in the unit of the sub-block using the motion vector of the sub-block when determining to perform the motion compensation in the unit of the sub-block;
• and
• performs the motion compensation in the unit of the block using the motion vector of the block when determining to perform the motion compensation in the unit of the block.

An encoding method of encoding a video by performing motion compensation, the encoding method comprising:

• encoding first control information indicating one mode that derives a motion vector of a block of an image in the video;
• encoding second control information indicating whether it is possible to perform the motion compensation in a unit of a sub-block included in the block;
• deriving the motion vector of the block in the one mode;
• determining whether to perform the motion compensation in the unit of the sub-block or the motion compensation in a unit of the block according to whether it is possible to perform the motion compensation in the unit of the sub-block;
• when it is determined to perform the motion compensation in the unit of the sub-block, deriving a motion vector of the sub-block and performing the motion compensation in the unit of the sub-block using the motion vector of the sub-block;
• and
• when it is determined to perform the motion compensation in the unit of the block, performing the motion compensation in the unit of the block using the motion vector of the block.

A decoding method of decoding a video by performing motion compensation, the decoding method comprising:

• decoding first control information indicating one mode that derives a motion vector of a block of an image in the video;
• decoding second control information indicating whether it is possible to perform the motion compensation in a unit of a sub-block included in the block;
• deriving the motion vector of the block in the one mode;
• determining whether to perform the motion compensation in the unit of the sub-block or the motion compensation in a unit of the block according to whether it is possible to perform the motion compensation in the unit of the sub-block;
• when it is determined to perform the motion compensation in the unit of the sub-block, deriving a motion vector of the sub-block and performing the motion compensation in the unit of the sub-block using the motion vector of the sub-block;
• and
• when it is determined to perform the motion compensation in the unit of the block, performing the motion compensation in the unit of the block using the motion vector of the block.