HEVC encoder optimisations using adaptive coding unit visiting order

The flexible partitioning scheme and increased number of prediction modes in the High Efficiency Video Coding (HEVC) standard are largely responsible for both its high compression efficiency and computational complexity. Each frame in HEVC is partitioned in Coding Tree Units (CTUs) of fixed size, which are then recursively partitioned in Coding Units (CUs). In typical implementations, CUs in a CTU are visited from top to bottom at each level of recursion. In this paper, a different approach is used in which CUs in a CTU can be adaptively visited also in reverse order from bottom to top. Three novel algorithms to reduce complexity of HEVC depth selection, mode decision and inter-prediction are presented, based on this adaptive visiting order. Experimental results show that the proposed encoder achieves on average 38.2% speed-ups compared to fast reference HEVC implementations with pre-built speed-ups enabled, for very limited efficiency losses.