Efficient AV1 Video Coding Using a Multi-layer Framework

This paper proposes a multi-layer multi-reference prediction framework for effective video compression. Current AOM/AV1 baseline uses three reference frames for the inter prediction of each video frame. This paper first presents a new coding tool that extends the total number of reference frames in both forward and backward prediction directions. A multi-layer framework is then described, which suggests the encoder design and places different reference frames within one Golden Frame (GF) group to different layers. The multi-layer framework leverages the existing coding tools in the AV1 baseline, including the tool of "show_existing_frame" and the reference frame buffer update module of a wide flexibility. The use of extended ALTREF_FRAMEs is proposed, and multiple ALTREF_FRAME candidates are selected and widely spaced within one GF group. ALTREF_FRAME is a constructed, no-show reference obtained through temporal filtering of a look-ahead frame. In the multi-layer structure, one reference frame may serve different roles for the encoding of different frames through the virtual index manipulation. The experimental results have been collected over several video test sets of various resolutions and characteristics both texture- and motion-wise, which demonstrate that the proposed approach achieves a consistent coding gain compared to the AV1 baseline. For instance, using PSNR as the distortion metric, an average bitrate saving of 5.57+% in BDRate is obtained for the CIF-level resolution set, some of which has a gain of up to 13+%, and 4.47% on average for the VGA-level resolution set, some of which up to 18+%.