Complexity reduction of in-loop filtering for compressed image restoration in HEVC

This paper presents a method for reducing the complexity of adaptive filtering for image restoration adopted in High Efficiency Video Coding (HEVC), called Adaptive Loop Filter (ALF). Although ALF can greatly improve the PSNR of decoded frames, its impact is not uniform across an entire frame, but depends on the local texture. Based on this fact, the proposed method first predicts the effectiveness of ALF for each pixel by score calculation, and skips all ALF processes for the pixels whose scores are less than a threshold. This threshold is adaptively determined for each frame in the encoding process, and is transmitted to a decoder. Experimental evaluation clearly demonstrates that the proposed method achieves significant reduction of the computation time of ALF without noticeable encoding performance loss.