Image coding method based on affine motion prediction, and device for same

1 . A picture decoding method performed by a decoding apparatus, the method comprising:

obtaining motion prediction information from a bitstream;

generating an affine motion vector predictor (MVP) candidate list for a current block, the affine MPV candidate list including at least one of a first affine MVP candidate block or a second affine MVP candidate block, the generating the affine MVP candidate list comprising: selecting, from a first block group, the first affine MVP candidate block that is coded according to an affine motion model, and selecting, from a second block group, the second affine MVP candidate block that is coded according to the affine motion model, wherein the first block group includes (i) a first neighboring block being positioned at a bottom-left corner of a current block and (ii) a second neighboring block being positioned at a left boundary of the current block, and wherein the second block group includes (i) a third neighboring block being positioned at a top-right corner of the current block, (ii) a fourth neighboring block being positioned at a top boundary of the current block, and (iii) a fifth neighboring block being positioned at a top-left corner of the current block;

selecting one of the first and second affine MVP candidate blocks included in the affine MVP candidate list by using an affine MVP candidate index in the motion prediction information;

deriving control point motion vector predictors (CPMVPs) for respective control points (CPs) of the current block based on the selected one of the first and second affine MVP candidate blocks;

deriving control point motion vector differences (CPMVDs) for the respective CPs of the current block based on information on the CPMVDs for the respective CPs included in the obtained motion prediction information;

deriving control point motion vectors (CPMVs) for the respective CPs of the current block based on the CPMVPs and the CPMVDs;

deriving sub-block motion vectors for the current block based on the CPMVs for the respective CPs of the current block;

deriving prediction samples for the current block based on the sub-block motion vectors for the current block;

deriving residual samples for the current block based on residual information; and

generating reconstructed samples for the current block based on the derived prediction samples and the derived residual samples, wherein the first block group excludes the third, fourth, and fifth neighboring blocks, and wherein the second block group excludes the first and second neighboring blocks.

2 . The picture decoding method of claim 1 , wherein the motion prediction information includes information regarding a context index for the affine MVP candidate index, wherein a value of the information regarding the context index is equal to 0 based on existence of at least one neighboring block for the current block coded according to the affine motion model, wherein the value of the information on the context index is equal to 1 when no neighboring block for the current block coded according to the affine motion model exists, and wherein the affine MVP candidate index is coded based on a context adaptive binary arithmetic coding by using a context model indicated by the context index.

3 . A non-transitory computer-readable storage medium storing thereon a computer program and a video bitstream, wherein when processed by one or more processors, the computer program causes the one or more processors to implement a picture decoding method to decode the bitstream to generate a video or a picture, the picture decoding method comprising:

obtaining motion prediction information from a bitstream;

generating an affine motion vector predictor (MVP) candidate list for a current block, the affine MPV candidate list including at least one of a first affine MVP candidate block or a second affine MVP candidate block, the generating the affine MVP candidate list comprising: selecting, from a first block group, the first affine MVP candidate block that is coded according to an affine motion model, and selecting, from a second block group, the second affine MVP candidate block that is coded according to the affine motion model, wherein the first block group includes (i) a first neighboring block being positioned at a bottom-left corner of a current block and (ii) a second neighboring block being positioned at a left boundary of the current block, and wherein the second block group includes (i) a third neighboring block being positioned at a top-right corner of the current block, (ii) a fourth neighboring block being positioned at a top boundary of the current block, and (iii) a fifth neighboring block being positioned at a top-left corner of the current block;

selecting one of the first and second affine MVP candidate blocks included in the affine MVP candidate list by using an affine MVP candidate index in the motion prediction information;

deriving control point motion vector predictors (CPMVPs) for respective control points (CPs) of the current block based on the selected one of the first and second affine MVP candidate blocks;

deriving control point motion vector differences (CPMVDs) for the respective CPs of the current block based on information on the CPMVDs for the respective CPs included in the obtained motion prediction information;

deriving control point motion vectors (CPMVs) for the respective CPs of the current block based on the CPMVPs and the CPMVDs;

deriving sub-block motion vectors for the current block based on the CPMVs for the respective CPs of the current block;

deriving prediction samples for the current block based on the sub-block motion vectors for the current block;

deriving residual samples for the current block based on residual information; and

generating reconstructed samples for the current block based on the derived prediction samples and the derived residual samples, wherein the first block group excludes the third, fourth, and fifth neighboring blocks, and wherein the second block group excludes the first and second neighboring blocks.

4 . The non-transitory computer-readable storage medium of claim 3 , wherein the motion prediction information includes information regarding a context index for the affine MVP candidate index, wherein a value of the information regarding the context index is equal to 0 based on existence of at least one neighboring block for the current block coded according to the affine motion model, wherein the value of the information on the context index is equal to 1 when no neighboring block for the current block coded according to the affine motion model exists, and wherein the affine MVP candidate index is coded based on a context adaptive binary arithmetic coding by using a context model indicated by the context index.