Encoding method, decoding method, and processing method

A decoding method comprising:

• determining 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
• wherein 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, generating 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, generating a prediction image for the current block by not performing the bi-directional optical flow prediction process.

A method of processing a bitstream that includes mode information and motion information, the method comprising:

• 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
• wherein 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,
• wherein 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
• wherein 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.