Adaptive Transforms for Inter-Predicted Residuals in Post-HEVC Video Coding

In October 2015, MPEG and ITU jointly launched the Joint Video Exploration Team (JVET) to prepare the next generation of video coding standard beyond HEVC. The tools added over HEVC, grouped into the Joint Exploration Model 3.0 (JEM-3.0), achieve over 25% of coding gains compared to HEVC for the Random-Access mode. In inter-coding mode, the transform stage consists in the Adaptive Multiple Transforms (AMT) which puts classical DCT-2 in competition with alternative transforms. In this paper, it is proposed to improve the AMT stage in inter-coding. The proposed method enables the encoder either to turn-off the AMT or to adjust its number of transforms. The introduced transforms are split into 3 nested sets: n°1 includes DST-7 and DCT-8; n°2 contains n°1 plus DST-7/DCT-8 combinations; n°3 contains n°2 plus 4 transforms learnt according to a Rate-Distortion Optimized Transform criterion (RDOT) [1]. The encoder performs a multiple-passes Coding-Tree-Unit (CTU) encoding to select the best set and signals its index into the bitstream, in a differential way. Acceleration strategies are implemented to mitigate the complexity, such as pass-cancellation and fast CTU partitioning from one pass to another.