A video processing method for a video encoder or decoder comprises receiving input data of a current block, determining an intra prediction mode and reference samples according to the intra prediction mode, determining an intra reference sample filter from a Gaussian interpolation filter and an alternative interpolation filter for the current block, applying the intra reference sample filter to the reference samples to generate an intra predictor for the current block, and encoding or decoding the current block based on the intra predictor. A determination between the Gaussian and alternative interpolation filters is depending on a comparison of a mode difference value calculated by the intra prediction mode with a size-dependent threshold. The size-dependent threshold is set to be equal to 24 for blocks with block size smaller than or equal to 32 samples according to an embodiment. The alternative interpolation filter may be a DCT-IF interpolation filter.