An Improved Framework of Affine Motion Compensation in Video Coding

Affine motion compensation (AMC) is a promising coding tool in Joint Exploration Model (JEM) developed by the Joint Video Exploration Team. AMC in JEM employs a 4-parameter affine model between the current block and its reference block. With this model, the motion vectors (MV) of each sub-block can be derived from the MVs at two control points. In this paper, we present a practical framework to further improve the AMC in JEM. First, we introduce a multi-model AMC approach, which allows the encoder to select either the 4-parameter affine model or the 6-parameter affine model adaptively. Second, we improve the affine inter-mode in two aspects. For the normative part, we present an efficient affine motion coding method, which replaces the affine MV Prediction (MVP) candidates in JEM with more accurate but simpler ones, and employs a second-order MVP. For the non-normative part, we enhance the motion estimation process for AMC, by regulating the optimization algorithm. Finally, we propose to unify the affine merge-mode and the normal merge-mode into a unified merge-mode, which combine affine merge candidates and normal merge candidates in a single merge candidate list. Partial of these methods have been adopted into the next generation video coding standard named Versatile Video Coding. Simulation results show that the proposed methods can achieve 1.67% BD rate savings in average for the random access configurations.