Affine motion prediction-based image decoding method and device using affne merge candidate list in image coding system

An image decoding method performed by a decoding apparatus, the method comprising:

• constructing a merge candidate list for deriving motion information of subblock units of a current block;
• deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block;
• deriving prediction samples for the current block based on the CPMVs for the CPs;
• and
• generating a reconstructed picture for the current block based on the derived prediction samples,
• wherein the merge candidate list includes (i) inherited affine candidates including a first inherited affine candidate and a second inherited affine candidate, and (ii) constructed affine candidates;
• wherein the first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block of the current block,
• wherein the second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block of the current block,
• wherein the CPs include a CP0, a CP1, and a CP2,
• wherein the CP0 is a point at a top left position of the current block, the CP1 is a point at a top right position of the current block, and the CP2 is a point at a bottom left position of the current block, and
• wherein each of the constructed affine candidates is derived based on a combination of a candidate motion vector for the CP0, a candidate motion vector for the CP1, and a candidate motion vector for the CP2.

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

• constructing a merge candidate list for deriving motion information of subblock units of a current block;
• deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block;
• and
• encoding image information including prediction information for the current block,
• wherein the merge candidate list includes (i) inherited affine candidates including a first inherited affine candidate and a second inherited affine candidate, and (ii) constructed affine candidates;
• wherein the first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block of the current block,
• wherein the second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block of the current block,
• wherein the CPs include a CP0, a CP1, and a CP2,
• wherein the CP0 is a point at a top left position of the current block, the CP1 is a point at a top right position of the current block, and the CP2 is a point at a bottom left position of the current block, and
• wherein each of the constructed affine candidates is derived based on a combination of a candidate motion vector for the CP0, a candidate motion vector for the CP1, and a candidate motion vector for the CP2.

A transmission method of data for an image, the method comprising:

• obtaining a bitstream for the image, wherein the bitstream is generated based on constructing a merge candidate list for deriving motion information of subblock units of a current block, deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block, and encoding image information including prediction information for the current block;
• and
• transmitting the data comprising the bitstream,
• wherein the merge candidate list includes (i) inherited affine candidates including a first inherited affine candidate and a second inherited affine candidate, and (ii) constructed affine candidates;
• wherein the first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block of the current block,
• wherein the second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block of the current block,
• wherein the CPs include a CP0, a CP1, and a CP2,
• wherein the CP0 is a point at a top left position of the current block, the CP1 is a point at a top right position of the current block, and the CP2 is a point at a bottom left position of the current block, and
• wherein each of the constructed affine candidates is derived based on a combination of a candidate motion vector for the CP0, a candidate motion vector for the CP1, and a candidate motion vector for the CP2.