Adaptive hierarchical motion estimation optimization for scalable HEVC

The scalable extension of the HEVC Video Coding Standard (H.265) offers elaborate mechanisms for motion vector prediction and estimation. S-HEVC builds on the standard by extending predictor lists for Coding Unit blocks, utilizing base-layer information in the inference of enhancement-layer Coding Units. The complex, exhaustive search schemes in use can be aided by hierarchical optimizations in subpixel motion estimation, which we propose for slow-moving CUs per frame. In this paper we implement and test an adaptive optimization of motion estimation in the standard (SHM 6.1 software release), based on a statistical analysis of the behavior of subpixel motion vector differentials in each spatial mode per Coding Unit. We propose that the least granular mode (64×64 PEL macro-block in current release) contains sufficient information at subpixel levels to decide best-mode selection, i.e., whether a complete recursion through the inner partitions (higher granularity) is required in the estimation of a CU motion vector. We further propose that subpixel motion estimation overheads can be avoided below a set threshold, given conditions set in base and enhancement layer motion estimation for priorly computed modes in the same CU. Both optimization methods are tested across a diverse set of video sequences, producing negligible quality penalties at for a sizable reduction in encoding time.