Inter-prediction method and device based on dmvr and bdof

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

• deriving a refined L0 motion vector and a refined L1 motion vector by applying decoder-side motion vector refinement (DMVR) based on an L0 motion vector and an L1 motion vector of a current block;
• deriving prediction samples for the current block based on L0 prediction samples derived based on the refined L0 motion vector and L1 prediction samples derived based on the refined L1 motion vector;
• deriving residual samples based on the prediction samples;
• and
• encoding image information including information on the residual samples,
• wherein the L0 prediction samples are derived based on an L0 reference picture and the refined L0 motion vector, and the L1 prediction samples are derived based on an L1 reference picture and the refined L1 motion vector,
• wherein the deriving of the prediction samples comprises deriving BDOF flag information related to indicating whether to apply bi-directional optical flow (BDOF), based on whether a first picture order count (POC) difference between a current picture and the L0 reference picture and a second POC difference between the current picture and the L1 reference picture are the same,
• wherein based on a case where the first POC difference and the second POC difference are the same, a value of the BDOF flag information is set to a value related to applying the BDOF to the current block.

A non-transitory computer-readable digital storage medium storing a bitstream generated by a method, the method comprising:

• deriving a refined L0 motion vector and a refined L1 motion vector by applying decoder-side motion vector refinement (DMVR) based on an L0 motion vector and an L1 motion vector of a current block;
• deriving prediction samples for the current block based on L0 prediction samples derived based on the refined L0 motion vector and L1 prediction samples derived based on the refined L1 motion vector;
• and
• deriving residual samples based on the prediction samples;
• and
• encoding image information including information on the residual samples to generate the bitstream,
• wherein the L0 prediction samples are derived based on an L0 reference picture and the refined L0 motion vector, and the L1 prediction samples are derived based on an L1 reference picture and the refined L1 motion vector,
• wherein the deriving of the prediction samples comprises deriving BDOF flag information related to indicating whether to apply bi-directional optical flow (BDOF), based on whether a first picture order count (POC) difference between a current picture and the L0 reference picture and a second POC difference between the current picture and the L1 reference picture are the same, and
• wherein based on a case where the first POC difference and the second POC difference are the same, a value of the BDOF flag information is set to a value related to applying the BDOF to the current block.