A Distortion Propagation Oriented CU-tree Algorithm for x265

Rate-distortion optimization (RDO) is widely used in video coding to improve coding efficiency. Conventionally, RDO is applied to each block independently to avoid high computational complexity. However, various prediction techniques introduce spatio-temporal dependency between blocks, therefore the independent RDO is not optimal. Specifically, because of the motion compensation, the distortion of reference blocks will affect the quality of subsequent prediction blocks. And considering this temporal dependency in RDO can improve the global rate-distortion (R-D) performance. x265 leveraged on a lookahead module to analyze the temporal dependency between blocks, and weighted the quality of each block based on its reference strength. However, the original algorithm in x265 ignored the impacts of quantization, and this shortcoming degraded the R-D performance of x265. In this paper, we propose a new linear distortion propagation model to estimate the temporal dependency, which introduces the impacts of quantization. And from a perspective of global RDO, a corresponding adaptive quantization formula is presented. The proposed algorithm was conducted in x265 version 3.2. Experiments revealed that, the proposed algorithm achieved average 15.43% PSNR-based and 23.81% SSIM-based BD-rate reductions, which outperformed the original algorithm in x265 by 4.14% and 9.68%, respectively.