Global and locally adaptive warped motion compensation in video compression
Real motion is often complex, and cannot always be sufficiently described by a translational model for the purpose of video coding. Translational motion models can only map a rectangular to a rectangular of the same size in a reference frame, but real motion observed in videos often go beyond that. For example, motion due to camera shake, panning and zoom might require transformations that support shearing, scaling, rotation and changes in aspect ratio. In order to handle these instances adequately for video encoding, we introduce two coding modes — a global motion mode and a locally adaptive warped motion mode, that make use of affine and/or homographic projections to more accurately describe and predict complex motion. These warped modes respectively capture global motion computed at the frame level, or local motion computed at the block level based on local motion statistics. We pay special attention to efficient SIMD friendly decoder-side warping operations to make these tools practically useful. Together, these techniques prove to yield considerable coding gains on standard test sets.