Method and device for performing image decoding on basis of intra prediction in image coding system

Disclosed is a video decoding method performed by a decoding device, the method comprising: obtaining prediction mode information for a current block from a bitstream; constructing a Most Probable Mode (MPM) list for the current block based on an intra prediction mode of a neighboring block of the current block; deriving an intra-prediction mode of the current block based on the prediction mode information and the MPM list; deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block; deriving reference samples for prediction of a target sample of the current block among the neighboring samples based on a position of the target sample and a prediction angle of the intra-prediction mode; determining an interpolation filter for the target sample based on the intra-prediction mode and a size of the current block, wherein the interpolation filter is determined as one of a Gaussian filter and a cubic filter; and deriving a prediction sample of the target sample based on the interpolation filter and the reference samples, wherein determining the interpolation filter for the target sample based on the intra-prediction mode and the size of the current block comprises: determining whether the size of the current block is smaller than a first specific value; and based on the size of the current block being smaller than the first specific value, determining whether the prediction angle of the intra-prediction mode is smaller than a second specific value, wherein the prediction angle of the intra prediction mode is determined as an angle between a prediction direction of the intra prediction mode and the closer one of a horizontal prediction direction and a vertical prediction direction, wherein based on the size of the current block being greater than or equal to a first specific value, the interpolation filter for the target sample is derived as the Gaussian filter, wherein based on the size of the current block being smaller than the first specific value and the prediction angle of the intra-prediction mode being smaller than the second specific value, the interpolation filter for the target sample is derived as the cubic filter, and wherein based on the size of the current block being smaller than the first specific value and the prediction angle of the intra-prediction mode being not smaller than the second specific value, the interpolation filter for the target sample is derived as the Gaussian filter.