A video decoding method comprising: generating a merge candidate list for a current block; specifying one merge candidate among a plurality of merge candidates included in the merge candidate list; deriving a first affine seed vector and a second affine seed vector of the current block based on a first affine seed vector and a second affine seed vector of the one merge candidate; deriving an affine vector for a subblock in the current block by using the first affine seed vector and the second affine seed vector of the current block, wherein the subblock is a region of a size smaller than that of the current block; and performing a motion compensation prediction for the subblock based on the affine vector, wherein the first affine seed vector and the second affine seed vector of the one merge candidate are derived based on motion information of a neighboring block adjacent to the current block, wherein the neighboring block is included in a coding tree unit different from a coding tree unit of the current block, the first affine seed vector and the second affine seed vector of the one merge candidate are derived based on motion vectors of a bottom-left subblock and a bottom-right subblock of the neighboring block, characterized in that the bottom-left subblock includes a bottom-left affine reference sample control point (xn4, yn4) positioned at a bottom-left corner of the neighboring block, and the bottom-right subblock adjoins a bottom-right affine reference sample control point (xn5, yn5) located at a right side of a bottom-right sample of the bottom-right subblock; wherein the first affine seed vector and the second affine seed vector of the one merge candidate are derived from a value obtained by applying a shifting operation to a width between the bottom-left subblock and the bottom-right subblock, wherein the shifting operation shifts the width by a scaling factor. generating a merge candidate list for a current block; specifying one merge candidate among a plurality of merge candidates included in the merge candidate list; deriving a first affine seed vector and a second affine seed vector of the current block based on a first affine seed vector and a second affine seed vector of the one merge candidate; deriving an affine vector for a subblock in the current block by using the first affine seed vector and the second affine seed vector of the current block, wherein the subblock is a region of a size smaller than that of the current block; and performing a motion compensation prediction for the subblock based on the affine vector, wherein the first affine seed vector and the second affine seed vector of the one merge candidate are derived based on motion information of a neighboring block adjacent to the current block, wherein the neighboring block is included in a coding tree unit different from a coding tree unit of the current block, the first affine seed vector and the second affine seed vector of the one merge candidate are derived based on motion vectors of a bottom-left subblock and a bottom-right subblock of the neighboring block, characterized in that the bottom-left subblock includes a bottom-left affine reference sample control point (xn4, yn4) positioned at a bottom-left corner of the neighboring block, and the bottom-right subblock adjoins a bottom-right affine reference sample control point (xn5, yn5) located at a right side of a bottom-right sample of the bottom-right subblock; wherein the first affine seed vector and the second affine seed vector of the one merge candidate are derived from a value obtained by applying a shifting operation to a width between the bottom-left subblock and the bottom-right subblock, wherein the shifting operation shifts the width by a scaling factor.