a memory; and

at least one processor connected to the memory, the at least one processor configured to:

acquire image information including inter-prediction mode information through a bitstream;

generate a subblock merge candidate list of a current block based on the inter-prediction mode information;

select one candidate among candidates included in the subblock merge candidate list; and

generate prediction samples of the current block based on motion information for the selected candidate and a weight index for the selected candidate,

wherein the subblock merge candidate list comprises a constructed affine merge candidate and a subblock-based temporal merge candidate,

wherein the constructed affine merge candidate is generated based on at least two of a control point CP0 related to a top-left corner of the current block, a control point CP1 related to a top-right corner of the current block, a control point CP2 related to a bottom-left corner of the current block, and a control point CP3 related to a bottom-right corner of the current block,

wherein based on a case that the constructed affine merge candidate is generated based on {CP0, CP2, CP3}, a weight index for the constructed affine merge candidate is equal to a weight index for the CP0,

wherein based on a case that the constructed affine merge candidate is generated based on {CP1, CP2, CP3}, the weight index for the constructed affine merge candidate is equal to a weight index for the CP1,

wherein a value of a weight index for the subblock-based temporal merge candidate is derived as zero (0), and