Method and apparatus for inter prediction in video coding system

A video encoding method performed by an encoding apparatus, the method comprising:

• generating a motion information candidate list of a current block;
• selecting one of candidates comprised in the motion information candidate list;
• deriving control point motion vectors (CPMVs) of the current block based on the selected candidate;
• deriving sub-block unit motion vectors of the current block based on the CPMVs;
• deriving a predicted block based on the sub-block unit motion vectors;
• generating a residual block for the current block based on the predicted block;
• and
• outputting a bitstream by encoding image information comprising information about the residual block,
• wherein the motion information candidate list comprises inherited affine candidates,
• wherein the inherited affine candidates are derived based on candidate blocks coded by an affine prediction among spatial neighboring blocks of the current block,
• wherein the inherited affine candidates includes a first inherited affine candidate and a second inherited affine candidate,
• wherein the first inherited affine candidate is derived from a first block group comprising a bottom-left corner neighboring block and a left neighboring block of the current block, and
• wherein the second inherited affine candidate is derived from a second block group comprising, a top-right corner neighboring block, and a top neighboring block of the current block.

A non-transitory computer readable storage medium storing encoded information causing a decoding apparatus to perform a video decoding method, the method comprising:

• generating a motion information candidate list of a current block;
• selecting one of candidates comprised in the motion information candidate list;
• deriving control point motion vectors (CPMVs) of the current block based on the selected candidate;
• deriving sub-block unit motion vectors of the current block based on the CPMVs;
• deriving a predicted block based on the sub-block unit motion vectors;
• and
• reconstructing a current picture based on the predicted block,
• wherein the motion information candidate list comprises inherited affine candidates,
• wherein the inherited affine candidates are derived based on candidate blocks coded by an affine prediction among spatial neighboring blocks of the current block, and
• wherein the inherited affine candidates includes a first inherited affine candidate and a second inherited affine candidate,
• wherein the first inherited affine candidate is derived from a first block group comprising a bottom-left corner neighboring block and a left neighboring block of the current block, and
• wherein the second inherited affine candidate is derived from a second block group comprising, a top-right corner neighboring block, and a top neighboring block of the current block.