Image encoding/decoding method and device for performing prediction on basis of HMVP candidate, and method for transmitting bitstream

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

• determining a prediction mode of a current block;
• constructing a merge candidate list for the current block, based on the prediction mode of the current block being a triangle partition mode (TPM);
• determining a merge candidate for two partitions of the current block based on the merge candidate list;
• deriving motion information of the two partitions of the current block based on the determined merge candidate;
• generating a prediction block of the current block based on the motion information;
• storing motion information of the current block;
• updating a history-based motion vector predictor (HMVP) buffer based on the motion information of the current block;
• and
• encoding a merge index specifying the determined merge candidate in a bitstream,
• wherein the updating the HMVP buffer is performed based on motion information of a 4×4 sample unit at a predetermined position in the current block, and
• wherein, based on a width and height of the current block being respectively cbWidth and cbHeight, the predetermined position is derived as (cbWidth>>1, cbHeight>>1) based on a partition direction is diagonal partition and is derived as (cbWidth>>1−1, cbHeight>>1) based on a partition direction is inverse diagonal partition.