Image encoding/decoding method and apparatus for performing bi-directional prediction, and method for transmitting bitstream

An image decoding apparatus comprising:

• a memory;
• and
• at least one processor,
• wherein the at least one processor is configured to:
• derive a constructed subblock merge candidate of a current block, based on an inter prediction mode of the current block being a subblock merge mode;
• construct a subblock merge candidate list including the constructed subblock merge candidate;
• select a subblock merge candidate of the current block based on the subblock merge candidate list;
• generate a prediction block of the current block based on motion information of the selected subblock merge candidate,
• wherein the deriving the constructed subblock merge candidate comprises deriving a weight index of bi-prediction for the constructed subblock merge candidate.

An image encoding method performed by an image encoding apparatus, the image encoding method comprising:

• generate a prediction block of a current block by performing inter prediction on the current block based on motion information of the current block;
• and
• encoding the motion information of the current block,
• wherein the encoding the motion information of the current block comprises:
• deriving a constructed subblock merge candidate of the current block, based on an inter prediction mode of the current block being a subblock merge mode;
• constructing a subblock merge candidate list including the constructed subblock merge candidate;
• and
• encoding the motion information of the current block based on the subblock merge candidate list,
• wherein the deriving the constructed subblock merge candidate comprises deriving a weight index of bi-prediction for the constructed subblock merge candidate.