Affine motion prediction-based video decoding method and device using subblock-based temporal merge candidate in video coding system

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

• deriving a reference block in a reference picture based on a left neighboring block of a current block;
• deriving a subblock-based temporal merging candidate for the current block based on motion information of the reference block;
• deriving an inherited affine candidate for the current block;
• deriving a constructed affine candidate for the current block;
• constructing a subblock merge candidate list for the current block including the subblock-based temporal merging candidate, the inherited affine candidate and the constructed affine candidate;
• deriving motion information of sub-blocks of the current block based on the subblock merge candidate list;
• deriving prediction samples for the current block based on the motion information of the sub-blocks;
• and
• encoding image information including prediction information for the current block.

A non-transitory computer-readable storage medium storing image information on instructions causing a decoding apparatus to perform the following steps:

• deriving a reference block in a reference picture based on a left neighboring block of a current block;
• deriving a subblock-based temporal merging candidate for the current block based on the reference block;
• deriving an inherited affine candidate for the current block;
• deriving a constructed affine candidate for the current block;
• constructing a subblock merge candidate list for the current block including the subblock-based temporal merging candidate, the inherited affine candidate and the constructed affine candidate;
• deriving motion information of sub-blocks of the current block based on the subblock merge candidate list;
• deriving prediction samples for the current block based on motion information of the sub-blocks;
• and
• generating a reconstructed picture based on the prediction samples.