Efficient SAO coding algorithm for x265 encoder

x265 is an open-source HEVC encoder which utilizes many optimization techniques and thus has fast coding speed as well as excellent coding performance. As a key technique in HEVC, sample adaptive offset (SAO) is also adopted by x265 to reduce the distortion between reconstructed frames and original ones. Although x265 is implemented in parallel with many speed up techniques, less efforts have been focused on the reduction of SAO related computation, which makes SAO become the speed bottleneck due to its great computational complexity. In this paper, we first thoroughly investigate and analyze the implementation and complexity of SAO in x265. Then, we propose a fast algorithm to early terminate SAO process based on inter prediction mode, CTU spatial-domain correlations, and relations between luma and chroma. Experimental results show that the proposed algorithm can save 72.2% SAO processing time with only 0.52% BDBR increase or 0.014 dB BDPSNR loss which is negligible.