generating affine prediction samples for respective subblocks in a target block in a current picture to be decoded, by performing affine motion prediction on a subblock-by-subblock basis using a plurality of control point motion vectors of the target block;

determining whether to execute a first coding tool based on at least one of values of the control point motion vectors or an angle between the control point motion vectors;

in response to determining that the first coding tool is executed, modifying sample values of the affine prediction samples by compensating for motion according to sample positions in each of the subblocks using the control point motion vectors;

obtaining sample values of residual samples; and

decoding the target block based on the sample values of the residual samples and the modified sample values of the affine prediction samples,

wherein, when the values of the control point motion vectors are equal to each other, executing the first coding tool for modifying the sample values of the affine prediction samples generated by performing the affine motion prediction is skipped.