A Hybrid Weighted Compound Motion Compensated Prediction for Video Compression

Abstract

Compound motion compensated prediction that combines reconstructed reference blocks to exploit the temporal correlation is a major component in the hierarchical coding scheme. A uniform combination that applies equal weights to reference blocks regardless of distances towards the current frame is widely employed in mainstream codecs. Linear distance weighted combination, while reflecting the temporal correlation, is likely to ignore the quantization noise factor and hence degrade the prediction quality. This work builds on the premise that the compound prediction mode effectively embeds two functionalities - exploiting temporal correlation in the video signal and canceling the quantization noise from reference blocks. A modified distance weighting scheme is introduced to optimize the trade-off between these two factors. It quantizes the weights to limit the minimum contribution from both reference blocks for noise cancellation. We further introduces a hybrid scheme allowing the codec to switch between the proposed distance weighted compound mode and the averaging mode to provide more flexibility for the trade-off between temporal correlation and noise cancellation. The scheme is implemented in the AV1 codec as part of the syntax definition. It is experimentally demonstrated to provide on average 1.5% compression gains across a wide range of test sets.