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

1. An video decoding method performed by a decoding device, the method comprising:

deriving an intra-prediction mode of a current block, wherein the intra-prediction mode is one of directional intra prediction modes with a prediction angle greater than 0;

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 when the size of the current block is greater than or equal to a first specific value, the interpolation filter for the target sample is derived as the Gaussian filter, and

wherein when the size of the current block is smaller than the first specific value, the interpolation filter for the target sample is derived based on the intra-prediction mode and a second specific value.

2. The video decoding method of claim 1 , wherein

the deriving of a prediction sample of the target sample based on the interpolation filter and the reference samples comprises:

deriving filter coefficients of the interpolation filter based on the position of the target sample and the prediction angle; and

deriving the prediction sample of the target sample based on the filter coefficients and the reference samples.

3. The video decoding method of claim 1 , wherein

when a width and a height of the current block are equal, the size of the current block is set equal to the width of the current block.

4. The video decoding method of claim 1 , wherein

when a width and a height of the current block are different and the intra-prediction mode of the current block is a directional intra-prediction mode having vertical directionality, the size of the current block is set equal to the width of the current block.

5. The video decoding method of claim 1 , wherein

when a width and a height of the current block are different and the intra-prediction mode of the current block is a directional intra-prediction mode having horizontal directionality, the size of the current block is set equal to the height of the current block.

6. The video decoding method of claim 1 , wherein

when the size of the current block is smaller than the first specific value, it is determined whether the prediction angle of the intra-prediction mode of the current block is smaller than the second specific value, and

when 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

when 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 a Gaussian filter.

7. An video encoding method performed by an encoding device, the method comprising:

determining an intra-prediction mode of a current block, wherein the intra-prediction mode is one of directional intra prediction modes with a prediction angle greater than 0;

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;

deriving a prediction sample of the target sample based on the interpolation filter and the reference samples; and

encoding video information comprising prediction information for the current block,

wherein when the size of the current block is greater than or equal to a first specific value, the interpolation filter for the target sample is derived as the Gaussian filter, and

wherein when the size of the current block is smaller than the first specific value, the interpolation filter for the target sample is derived based on the intra-prediction mode and a second specific value.

8. A non-transitory computer-readable storage medium storing video information, the video information, when executed, causing a decoding apparatus to perform the following steps:

deriving an intra-prediction mode of a current block, wherein the intra-prediction mode is one of directional intra prediction modes with a prediction angle greater than 0;

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 when the size of the current block is greater than or equal to a first specific value, the interpolation filter for the target sample is derived as the Gaussian filter, and

wherein when the size of the current block is smaller than the first specific value, the interpolation filter for the target sample is derived based on the intra-prediction mode and a second specific value.