Encoder, decoder, encoding method, and decoding method

A decoder, comprising:

• circuitry;
• and
• memory,
• wherein, using the memory, the circuitry:
• generates a prediction image to be used to decode a current block to be processed;
• derives a luminance correction parameter by referring to an image region which is included in an image including the current block and is outside the current block;
• and
• corrects the prediction image to be used to decode the current block, using the luminance correction parameter, and
• the luminance correction parameter is used in common between a plurality of blocks included in the image.

An encoding method, comprising:

• generating a prediction image to be used to encode a current block to be processed;
• deriving a luminance correction parameter by referring to an image region which is included in an image including the current block and is outside the current block;
• and
• correcting the prediction image to be used to encode the current block, using the luminance correction parameter,
• wherein the luminance correction parameter is used in common between a plurality of blocks included in the image.

A decoding method, comprising:

• generating a prediction image to be used to decode a current block to be processed;
• deriving a luminance correction parameter by referring to an image region which is included in an image including the current block and is outside the current block;
• and
• correcting the prediction image to be used to decode the current block, using the luminance correction parameter,
• wherein the luminance correction parameter is used in common between a plurality of blocks included in the image.