Two-Pass Bi-Directional Optical Flow Via Motion Vector Refinement
Hongbin Liu, Li Zhang, Kai Zhang, Hsiao-Chiang Chuang, Yue Wang, Jizheng Xu · 2019
Bi-directional optical flow (BDOF) is an efficient coding tool that has been recently adopted into Versatile Video Coding (VVC) standard. With BDOF, bi-predictive prediction samples of one coding block are enhanced via higher-precision motion vectors (MVs) derived from its two reference blocks. In this way, the energy of prediction error could be reduced, resulting in better coding performance. In VVC, the derived motion information is only used to enhance prediction samples. In this paper, it is proposed to use the derived motion information to also refine decoded MVs. The refined MVs may be used as spatial motion vector prediction (MVP) for the following coding units (CUs), as the temporal MVP for the subsequent pictures, and in the deblocking filtering process. Furthermore, the refined MVs can be used to perform motion compensation (MC) again to further improve the quality of the prediction samples. Simulation results show that the proposed methods can achieve -1.18% BD-rate saving in average under the random access configuration on top of the existing BDOF design in VVC.