A switchable loop-restoration with side-information framework for the emerging AV1 video codec

Image restoration schemes have traditionally been targeted only for use in a blind scenario, where the aim is to improve the quality of an image or video after it has suffered degradations in capture, processing, storage, coding or transmission, at a time when the undegraded source is no longer available. However, these schemes can also be adopted for compression, because any information that can be restored can be saved, thereby aiding compressibility. In this use case, the source is known at the time of compression, and the encoder can send additional side-information to the decoder with the compressed bit-stream to specify how the decoder is supposed to restore. In this paper, we describe three novel schemes that are applied in-loop to reconstructed frames after a conventional deblocking loop filter has been applied. These schemes are switchable within a frame per suitably sized tile. The specific schemes described are based on separable symmetric Wiener filters, dual self-guided filters with subspace projection, and domain transform recursive filters. Results with the new coding tool are presented on the emerging AV1 video codec on standard test sets, showing upwards of 2% bit-rate savings.