Image encoding/decoding method and device for deriving weight index of bidirectional 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:
• reconstruct an inter prediction mode of a current block;
• derive a subblock merge candidate based on neighboring blocks of the current block, based on the inter prediction mode of the current block being a subblock merge mode;
• derive motion information of the current block based on motion information of the subblock merge candidate;
• generate a prediction block of the current block by performing inter prediction on the current block based on the motion information of the current block;
• and
• reconstruct the current block based on the prediction block,
• wherein the motion information comprises a weight index for bi-prediction.

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

• generating a prediction block of a current block by performing inter prediction on the current block based on motion information of the current block;
• encoding the current block based on the prediction block;
• and
• encoding the motion information of the current block,
• wherein the encoding the motion information of the current block comprising:
• determining an inter prediction mode of the current block;
• deriving a subblock merge candidate based on neighboring blocks of the current block, based on the inter prediction mode of the current block being a subblock merge mode;
• and
• encoding the motion information of the current block based on motion information of the subblock merge candidate,
• wherein the motion information comprises a weight index for bi-prediction.