The Daala Directional Deringing Filter

This paper presents the deringing filter used in the Daala royalty-free video codec. The filter is based on a non-linear conditional replacement filter and is designed for vectorization efficiency. It takes into account the direction of edges and patterns being filtered. The filter works by identifying the direction of each block and then adaptively filtering along the identified direction. In a second pass, the blocks are also filtered in a different direction, with more conservative thresholds to avoid blurring edges. The proposed deringing filter is shown to improve the quality of both Daala and the Alliance for Open Media (AOM) AV1 video codec.