Computationally efficient adaptive loop filtering design with dd1ectional features for video coding

To improve coding efficiency and visual quality in video coding, adaptive loop filtering (ALF) methods using directional features to classify pixels for filter adaptation have been proposed in the literature. However, the associated complexity has been identified as an obstacle for realistic hardware implementation and deployment. This paper presents an ALF design, which efficiently classifies video blocks by computing the direction and strength of local gradients only at subset of pixels. As compared to the state-of-the-art ALF approach utilizing directional features for classification, our design achieves appealing complexity-efficiency trade-off, with more than 85% reductions in computation, 75% reduction in terms of the number of pixels to be accessed, and reduces the worst-case number of filters from 15 to 8; while being able to preserve more than 85% of ALF gams in different coding structures.