Improved Intra Mode Coding Beyond Av1

In AOMedia Video 1 (AV1), directional intra prediction modes are applied to model local texture patterns that present certain directionality. Each intra prediction direction is represented with a nominal mode index and a delta angle. The delta angle is entropy coded using shared context between luma and chroma, and the context is derived using the associated nominal mode. In this paper, two methods are proposed to further reduce the signaling cost of delta angles: cross-component delta angle coding, and context-adaptive delta angle coding, whereby the cross-component and spatial correlation of the delta angles are explored, respectively. The proposed methods were implemented on top of a recent version of libaom. Experimental results show that the proposed cross-component delta angle coding achieved average 0.4% BD-rate reduction with 4% encoding time saving over all intra configurations. By combining both methods, an average 1.2% BD-rate reduction is achieved.