Affine model-based image encoding/decoding method and device

1. A method of decoding an image, comprising:

generating a candidate list including merge candidates for motion information prediction of a current block in the image, wherein the merge candidates comprise at least one of a plurality of affine candidates or a subblock-based temporal candidate;

deriving, in units of subblocks of the current block, a motion vector of the current block based on the candidate list and a candidate index, the candidate index specifying one of the merge candidates in the candidate list;

generating a prediction block of the current block by performing inter prediction for the current block using the motion vector; and

reconstructing the current block based on the prediction block,

wherein, in response to a case where a merge candidate specified by the candidate index is one of the plurality of the affine candidates, deriving the motion vector comprises:

deriving a control point vector of the current block based on the specified merge candidate; and

deriving the motion vector based on the control point vector of the current block,

wherein, in response to a case where the specified merge candidate is the subblock-based temporal candidate, a motion vector of each of subblocks belonging to the current block is derived using a motion vector of a subblock in a collocated block corresponding to the each subblock in the current block, and

wherein the subblocks belonging to the current block share one reference picture.

2. The method of claim 1 , wherein the affine candidate includes at least one of a spatial candidate or a constructed candidate, and

wherein the spatial candidate is derived from a block encoded with an affine model among spatial neighboring blocks of the current block.

3. The method of claim 2 , wherein the control point vector of the current block includes at least one of a first control point vector corresponding to a top left sample of the current block, a second control point vector corresponding to a top right sample of the current block, or a third control point vector corresponding to a bottom left sample of the current block.

4. The method of claim 3 , wherein the control point vector of the spatial candidate is derived by considering whether a boundary of the current block is located on a boundary of coding tree block (CTU).

5. The method of claim 2 , wherein the constructed candidate is determined based on a combination of motion vectors of neighboring blocks adjacent to the current block.

6. The method of claim 3 , wherein, in response to the case where the specified merge candidate is the one of the plurality of the affine candidates, the motion vector of the current block is derived using at least one of the first control point vector, the second control point vector, a position of the subblock, or the size of the current block.

7. The method of claim 1 , wherein generating the candidate list is selectively performed by considering at least one of a prediction mode of a neighboring block of the current block or a size of the current block.

8. The method of claim 1 , wherein the collocated block belongs to a picture different from the current block, and

wherein the collocated block is representative of a block at a position shifted by a temporal vector from a position of the current block.

9. The method of claim 8 , wherein the temporal vector is determined based on only a left neighboring block among spatial neighboring blocks adjacent to the current block.

10. The method of claim 2 , wherein the candidate list is generated by adding the merge candidates in an order of the subblock-based temporal candidate, the spatial candidate, and the constructed candidate.

11. The method of claim 4 , wherein, in response to a case where the boundary of the current block is not located on the boundary of the CTU, the control point vector of the spatial candidate is derived based on a control point vector of a spatial neighboring block, and

wherein, in response to a case where the boundary of the current block is located on the boundary of the CTU, the control point vector of the spatial candidate is derived based on a motion vector of the spatial neighboring block.

12. A method of encoding an image, comprising:

generating a candidate list including merge candidates for motion information prediction of a current block in the image, wherein the merge candidates comprise at least one of a plurality of affine candidates or a subblock-based temporal candidate;

deriving, in units of subblocks of the current block, a motion vector of the current block based on the candidate list;

generating a prediction block of the current block by performing inter prediction for the current block using the motion vector; and

generating a residual block of the current block based on the prediction block,

wherein a candidate index specifying one of the merge candidates in the candidate list is encoded into a bitstream,

wherein, in response to a case where the motion vector of the current block is derived based on one of the plurality of the affine candidates, deriving the motion vector comprises:

deriving a control point vector of the current block based on the one of the plurality of the affine candidates; and

deriving the motion vector based on the control point vector of the current block,

wherein, in response to a case where the motion vector of the current block is derived based on the subblock-based temporal candidate, a motion vector of each of subblocks belonging to the current block is derived using a motion vector of a subblock in a collocated block corresponding to the each subblock in the current block, and

wherein the subblocks belonging to the current block share one reference picture.

13. A non-transitory computer-readable medium for storing a bitstream generated by an image encoding method, the image encoding method comprising:

generating a candidate list including merge candidates for motion information prediction of a current block in the image, wherein the merge candidates comprise at least one of a plurality of affine candidates or a subblock-based temporal candidate;

deriving, in units of subblocks of the current block, a motion vector of the current block based on the candidate list;

generating a prediction block of the current block by performing inter prediction for the current block using the motion vector; and

generating a residual block of the current block based on the prediction block,

wherein a candidate index specifying one of the merge candidates in the candidate list is encoded into a bitstream,

wherein, in response to a case where the motion vector of the current block is derived based on one of the plurality of the affine candidates, deriving the motion vector comprises:

deriving a control point vector of the current block based on the one of the plurality of the affine candidates; and

deriving the motion vector based on the control point vector of the current block,

wherein, in response to a case where the motion vector of the current block is derived based on the subblock-based temporal candidate, a motion vector of each of subblocks belonging to the current block is derived using a motion vector of a subblock in a collocated block corresponding to the each subblock in the current block, and

wherein the subblocks belonging to the current block share one reference picture.