Fast HEVC coding using reverse CU visiting

The High Efficiency Video Coding (HEVC) standard makes use of flexible partitioning to achieve high compression ratios. Each frame is divided in Coding Tree Units (CTUs) of fixed size, which are further partitioned into Coding Units (CUs) following a recursive quadtree structure. Typically, CUs at each level of recursion are tested to select the optimal coding configuration, hence this process is extremely demanding in terms of computational complexity. In this paper, a method to reduce complexity of HEVC quadtree configuration selection and mode decision is presented, based on a reverse bottom-to-top visiting order of CUs in the quadtree. By visiting smallest CUs first, information can be extracted to make decisions on larger CUs. The encoder adaptively selects whether a CTU is encoded using the reverse CU visiting, allowing for considerably faster encoding under all conditions. Experimental results show that the algorithm achieves on average 21% speedups over previous state-of-the-art fast HEVC algorithms, and up to 36% for some sequences, at very limited efficiency losses.