A pre-filtering approach to exploit decoupled prediction and transform block structures in video coding

Recent video coding techniques allow for decoupling of the transform block partition from that employed for prediction. For example, HEVC allows a transform block to overlap multiple prediction blocks. This paper is premised on the observation that in order to truly realize the potential of such enhanced flexibility, it is necessary to account for and mitigate considerable side effects due to stitching together independently predicted blocks, including the emergence of spurious high frequency components from sharp transitions across boundaries, which undermine the transform efficacy. The proposed solution involves an appropriately designed pre-filtering approach to mitigate boundary transition effects whenever a transform spans data from multiple prediction blocks. Moreover, this filtering technique enables extending the flexibility in decoupling prediction and transform structures, as various restrictions may now be eliminated. In particular, it makes it possible and beneficial to allow a transform block to span residual data from both inter and intra predicted blocks, whereas HEVC necessarily forces a single type of prediction in each coding unit. The method is further extended to include motion refinement that accounts for the pre-filtering approach. Experiments provide evidence for consistent coding gains over HEVC and VP9.