Image decoding method for deriving predicted sample by using merge candidate and device therefor

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

• determining an inter-prediction mode of a current block and generating inter-prediction mode information indicating the inter-prediction mode;
• generating a subblock merge candidate list of the current block based on the inter-prediction mode;
• generating selection information which indicates one candidate among candidates included in the subblock merge candidate list;
• and
• encoding image information which includes the inter-prediction mode information and the selection information,
• wherein the subblock merge candidate list comprises a constructed affine 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 the subblock merge candidate list comprises a subblock-based temporal merge candidate, and
• wherein a value of a weight index for the subblock-based temporal merge candidate is derived as zero (0).

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

• determining an inter-prediction mode of a current block and generating inter-prediction mode information indicating the inter-prediction mode;
• generating a subblock merge candidate list of the current block based on the inter-prediction mode;
• generating selection information which indicates one candidate among candidates included in the subblock merge candidate list;
• and
• encoding image information to generate the bitstream, wherein the image information includes the inter-prediction mode information and the selection information,
• wherein the subblock merge candidate list comprises a constructed affine 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 the subblock merge candidate list comprises a subblock-based temporal merge candidate, and
• wherein a value of a weight index for the subblock-based temporal merge candidate is derived as zero (0).