Content Adaptive Directional Transform for High Efficiency Video Coding

HEVC is an emerging new standard for digital video compression, which is regarded as a successor to H.264/AVC standard. It still belongs to block-based hybrid video coding framework. The block patterns range from 4x4 to 64x64 blocks, and DCT is extended from 4x4 to 32x32. 2D-DCT for image is performed along the vertical and horizontal directions, so it is good at the energy compaction of residual block with vertical or horizontal edges. However, the edges are usually neither horizontal nor vertical for most cases, such as neither vertical nor horizontal intra prediction, so directional transform was explored in the past several years. In this paper, a directional transform adaptive to image content is proposed. Firstly, the prediction residual blocks are collected from coding a number of video sequences with plenty of image content. Secondly, for each kind of image content, the residual blocks are clustered to form the given number of clusters. Thirdly, each cluster contributes a transform basis after Singular Value Decomposition (SVD). The experimental results in terms of PSNR gains demonstrate the efficiency of the proposed algorithm with the comparison with the standard HM software.