Context-Based Fractional Sample Refinement for HEVC Compliant Encoding

The H.265/MPEG-H High Efficiency Video Coding compliant encoding process faces the challenge of high computational complexity. Particularly, in the case of inter-picture prediction, most of the computational resources are allocated for the motion estimation (ME) process. In turn, ME and motion compensation enable improving coding efficiency by addressing the blocks of video frames as corresponding displacements from one or more reference blocks. These displacements do not necessarily have to be limited to integer sample positions, but may have an accuracy of half sample or quarter sample positions, which are identified during fractional sample refinement. In this paper, a context-based scheme for fractional sample refinement is proposed. The scheme takes the advantage of already obtained information in prior ME steps and provides significant flexibility in terms of parameterization. In this way, it adaptively achieves a desired tradeoff between computational complexity and coding efficiency. According to the experimental results obtained for an example algorithm utilizing the proposed framework, a significant decrease in the number of search points can be achieved. For instance, considering only 6 instead of 16 fractional sample positions results in a tradeoff of only 0.4% Bjøntegaard Delta-rate loss for high-definition video sequences compared with the conventional interpolation-and-search method.