High efficiency video coding (HEVC) standard is within the block-based hybrid coding framework, which essentially adopts prediction unit (PU) as the basic motion compensation unit. However, in the case of tiny motion, the actual motion vectors (MVs) for each sample may differ from the PU's MV, thus resulting in more residual energy. In this paper, a novel pixel-wise motion refinement method (PMR) is presented by extending the traditional bidirectional optical flow (BIO) to PUs with two unidirectional reference blocks. To get the robust MV for each sample, a median filtering process is introduced to MV shifting values of the neighboring pixels. Furthermore, a PU level mode-dependent pixel-wise motion refinement (MPMR) scheme is also presented to improve the coding performance. Simulation results demonstrate that the proposed method achieves 0.83% bitrate reduction on average for HEVC test sequences under HM12.0 low delay B (LDB) configuration, and achieves up to 4.92% bitrate reduction for sequences with complex motion, e.g. affine motion.
['Lei Zhao', 'Jian Zhang', 'Siwei Ma', 'Wen Gao']
Mode-dependent pixel-wise motion refinement for HEVC