Encoder, decoder, encoding method, and decoding method

An encoder comprising:

• a memory;
• and
• circuitry accessible to the memory and configured to:
• for a current block,
• derive motion vectors of the current block by a merge mode;
• determine whether a first process is to be applied and whether a second process is to be applied so that at least one of the first process and the second process is determined not to be applied, the first process being a bi-directional optical flow process in which the circuitry refers to a spatial gradient of luminance;
• in a first condition in which the first process is determined to be applied and the second process is determined not to be applied,
• generate prediction images of the current block based on reference pictures and the motion vectors, and
• generate a final prediction image based on the prediction images and the spatial gradient of luminance;
• and
• in a second condition in which the first process is determined not to be applied and the second process is determined to be applied,
• generate prediction images of the current block based on reference pictures and the motion vectors, and
• generate a final prediction image by weighting the prediction images.

A method of transmitting a bitstream comprising:

• for a current block,
• deriving motion vectors of the current block by a merge mode;
• determining whether a first process is to be applied and whether a second process is to be applied so that at least one of the first process and the second process is determined not to be applied, the first process being a bi-directional optical flow process in which a spatial gradient of luminance is referred to;
• in a first condition in which the first process is determined to be applied and the second process is determined not to be applied,
• generating prediction images of the current block based on reference pictures and the motion vectors, and
• generating a final prediction image based on the prediction images and the spatial gradient of luminance;
• and
• in a second condition in which the first process is determined not to be applied and the second process is determined to be applied,
• generating prediction images of the current block based on reference pictures and the motion vectors, and
• generating a final prediction image by weighting the prediction images,
• generating the bitstream;
• and
• transmitting the bitstream.