An video decoding method performed by a decoding device, the method comprising: 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 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; 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 characterized in that: when the size of the current block is 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 intra-prediction mode is one of directional intra prediction modes with a prediction angle greater than 0, 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 when the size of the current block is not smaller than the first specific value, the interpolation filter for the target sample is derived as a Gaussian filter, wherein when the size of the current block is smaller than the first specific value and the prediction angle of the intra-prediction mode is smaller than the second specific value, the interpolation filter for the target sample is derived as a cubic filter, and wherein when the size of the current block is smaller than the first specific value and the prediction angle of the intra-prediction mode is not smaller than the second specific value, the interpolation filter for the target sample is derived as the Gaussian filter. 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 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; 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 characterized in that: when the size of the current block is 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 intra-prediction mode is one of directional intra prediction modes with a prediction angle greater than 0, 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 when the size of the current block is not smaller than the first specific value, the interpolation filter for the target sample is derived as a Gaussian filter, wherein when the size of the current block is smaller than the first specific value and the prediction angle of the intra-prediction mode is smaller than the second specific value, the interpolation filter for the target sample is derived as a cubic filter, and wherein when the size of the current block is smaller than the first specific value and the prediction angle of the intra-prediction mode is not smaller than the second specific value, the interpolation filter for the target sample is derived as the Gaussian filter. when the size of the current block is 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 intra-prediction mode is one of directional intra prediction modes with a prediction angle greater than 0, 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 when the size of the current block is not smaller than the first specific value, the interpolation filter for the target sample is derived as a Gaussian filter, wherein when the size of the current block is smaller than the first specific value and the prediction angle of the intra-prediction mode is smaller than the second specific value, the interpolation filter for the target sample is derived as a cubic filter, and wherein when the size of the current block is smaller than the first specific value and the prediction angle of the intra-prediction mode is not smaller than the second specific value, the interpolation filter for the target sample is derived as the Gaussian filter.