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

1. An image decoding method performed by a decoding 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 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;

generating a reconstructed picture based on the prediction samples; and

filtering the reconstructed picture based on at least one of a deblocking filter, a sample adaptive offset, or adaptive loop filter,

wherein based on a size of the current block being W×H, and an x component of a top-left sample position of the current block being a and a v component of the top-left sample position being b, the left neighboring block is a block including a sample at (a−1, b+H−1) coordinates.

2. The image decoding method of claim 1 , wherein a position of the reference block is derived based on a motion vector of the left neighboring block.

3. The image decoding method of claim 2 , wherein the motion vector for deriving the position of the reference block is fixed to the motion vector of the left neighboring block.

4. The image decoding method of claim 1 ; wherein the deriving the motion information of the sub-blocks of the current block comprises:

selecting the subblock-based temporal merging candidate from the subblock merge candidate list; and

deriving the motion information of the sub-blocks of the current block based on the subblock-based temporal merging candidate.

5. The image decoding method of claim 4 , wherein motion information of a target sub-block among the sub-blocks is derived based on motion information of a collocated sub-block for the target sub-block included in the subblock-based temporal merging candidate.

6. 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;

determining to apply at least one of a deblocking filter, a sample adaptive offset, or adaptive loop filter; and

encoding image information including prediction information for the current block,

wherein based on a size of the current block being W×H, and an x component of a top-left sample position of the current block being a and a y component of the top-left sample position being b, the left neighboring block is a block including a sample at (a−1, b+H−1) coordinates.

7. The image encoding method of claim 6 , wherein a position of the reference block is derived based on a motion vector of the left neighboring block.

8. The image encoding method of claim 7 , wherein the motion vector for deriving the position of the reference block is fixed to the motion vector of the left neighboring block.

9. The image encoding method of claim 6 , wherein the deriving the motion information of the sub-blocks of the current block comprises:

selecting the subblock-based temporal merging candidate from the subblock merge candidate list; and

deriving the motion information of the sub-blocks of the current block based on the subblock-based temporal merging candidate.

10. The image encoding method of claim 9 , wherein motion information of a target sub-block among the sub-blocks is derived based on motion information of a collocated sub-block for the target sub-block included in the subblock-based temporal merging candidate.

11. A non-transitory computer-readable storage medium storing bitstream generated by a method, 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;

determining to apply at least one of a deblocking filter, a sample adaptive offset, or adaptive loop filter; and

encoding image information including prediction information for the current block,

wherein based on a size of the current block being W×H, and an x component of a top-left sample position of the current block being a and a y component of the top-left sample position being b, the left neighboring block is a block including a sample at (a−1, b+H−1) coordinates.