Encoder, decoder, encoding method, and decoding method

A decoder comprising:

• memory;
• and
• circuitry coupled to the memory and configured to:
• determine a merge mode to be applied to a current block, the merge mode including a sub-block merge mode and a first merge mode different from the sub-block merge mode, wherein
• in the merge mode, inter-prediction parameters are inferred from a neighboring block neighboring the current block, and
• in the sub-block merge mode, the current block includes a plurality of sub-blocks, and inter-prediction parameters are provided for each of the plurality of sub-blocks;
• in response to the merge mode being determined to be the first merge mode, generate a prediction image for the current block by performing a bi-directional optical flow prediction process, wherein the bi-directional optical flow prediction process uses a spatial gradient for the current block;
• and
• in response to the merge mode being determined to be the sub-block merge mode, generate a prediction image for the current block by not performing the bi-directional optical flow prediction process.

A non-transitory computer readable medium storing a bitstream and computer-executable instructions,

• wherein the bitstream includes mode information and motion information according to which the computer-executable instructions cause a decoder to perform:
• using the mode information to determine a merge mode to be applied to a current block, the merge mode including a sub-block merge mode and a first merge mode different from the sub-block merge mode, wherein
• in the merge mode, inter-prediction parameters are inferred from a neighboring block neighboring the current block, and
• in the sub-block merge mode, the current block includes a plurality of sub-blocks, and inter-prediction parameters are provided for each of the plurality of sub-blocks;
• and
• using the motion information to generate a prediction image for the current block, wherein, in generating the prediction image,
• a bi-directional optical flow prediction process is performed in response to the merge mode being determined to be the first merge mode, wherein the bi-directional optical flow prediction process uses a spatial gradient for the current block;
• and
• the bi-directional optical flow prediction process is not performed in response to the merge mode being determined to be the sub-block merge mode.