Enhanced Frame Context Initialization for Video Coding Beyond AV1

Entropy coding is an integral part of all modern hybrid block-based video codecs. The context adaptive binary arithmetic coding (CABAC) is a normative part of ITU-T/ISO/IEC video coding standards (H.264/AVC, HEVC and VVC), while the Alliance for Open Media (AOMedia) video standard (AV1) utilizes a context adaptive multi-symbol version of it for entropy coding. Recent explorations of new coding tools beyond AV1 capabilities have led to the improvement of the context adaptive multi-symbol arithmetic entropy coder. In this study, improvements to the context initialization process of the entropy coder are discussed in detail and results reported. The improvements include a) optimal selection of reference frame pairs and b) context modeling from selected reference frame pairs for tile context initialization. Two variants of the proposed method, Variant1 and Variant2 are implemented on top of the reference codebase (research-v8.0.0). Experimental results with CTCv7 show that for random access (RA) configuration, average BDRATE (PSNR-YUV) gains of -0.13% and -0.18% are achievable for Variant1 and Variant2 respectively. Meanwhile, for low delay (LD) configuration the reported gains are -0.47% and -0.50% respectively for Variant1 and Variant2.