A method by which a decoding device performs image decoding, according to the present invention, comprises the steps of: deriving an intra prediction mode of a current block; deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block; deriving reference samples for prediction of a target sample among the neighboring samples on the basis of the position of the target sample of the current block and the prediction angle of the intra prediction mode; determining an interpolation filter for the target sample; and deriving the prediction sample of the target sample on the basis of the interpolation filter and the reference samples.