Improved Intra Coding Beyond AV1 Using Adaptive Prediction Angles and Reference Lines

A fixed set of intra prediction angles by using the reconstructed samples in adjacent reference line are employed in AV1 to remove the spatial redundancy of video signals. Two methods are proposed in this paper to further improve the intra coding performance of AV1. Firsly, to better signal the intra prediction modes, only a subset of the intra prediction modes (IPMs) are allowed and signaled for each block, which is adaptively selected according to the IPMs of neighboring blocks. Secondly, to reduce the prediction errors when there is a strong discontinuity between the samples in current block and its adjacent reference samples, an adaptive reference line selection method is proposed by enabling farther reference lines for intra prediction. Experimental results show that, the proposed methods achieve 2.2% luma BD-rate savings with around 150% encoding time for intra coding on top of the libaom implementation of AV1.