Interpolation filtering for intra prediction in Versatile Video Coding

In this paper, we propose a novel interpolation of reference samples for intra prediction in Versatile Video Coding (VVC). To interpolate a predictor value between two reference samples, the method uses four nearest reference samples, as does the existing cubic filter in VVC, but with a simpler design that does not require to pre-compute the filter coefficients. We model the signal as a sum of two components, where the first component is the classical linear interpolation, and the second component is a corrective term that accounts for the change due to the two farther samples. To arrive at this model, we model the signal as a sum of two quadratic functions where each quadratic function models the signal with three adjacent samples. The corrective term is attributed to the quadratic term in the resulting model, which can be calculated on the fly. Since models based on four samples can lead to large errors at edges of objects, we propose to use a thresholding method to decide between the proposed model and the usual linear interpolation. Besides BD-rate performance gain, the advantages of the proposed method are lower complexity, no memory storage of filter coefficients, and a uniform method for both Luma and Chroma components. The proposed method applied in VTM 7.0 intra prediction results in BD-rate gains of 0.13% for Luma and 0.30% for Chroma with lower decoding complexity.

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