A method for decoding an image by a decoding apparatus according to the present invention comprises the steps of: deriving first motion information of a current block in a first direction and second motion information of the current block in a second direction; deriving a first reference sample on the basis of the first motion information; deriving a second reference sample on the basis of the second motion information; deriving a first weight for the first reference sample and a second weight for the second reference sample; and generating a prediction sample of the current block by performing a weighted summation of the first and second reference samples on the basis of the first and second weights, wherein the first motion information comprises a first reference picture index and a first MV and the second motion information comprises a second reference picture index and a second MV.