Video content dependent directional transform for intra frame coding

The mode-dependent directional transform (MDDT) employed Karhunen-Loève Transform (KLT) for compressing directional residue signal of intra prediction along its direction. The transform bases were derived from the singular value decomposition (SVD) of residue signals coming from all kinds of video sequences, which were expected to be efficient for most of video sequences. However, the advantage of KLT comes from the concept of a “signal content dependent transform”. MDDT and its variants failed to exploit such a concept, so they did not fully exploit the efficiency of KLT. In this paper, a video content feature is firstly defined as the histogram of the residue produced by intra prediction. Secondly, one KLT basis is computed for each feature of each mode from off-line experiments. Thus, multiple KLT bases identified by their features are provided to each mode instead of only one basis in MDDT. One of them is selected during encoding process by matching the feature of signal being processed to the predefined features. The experiments show that the average improvement of 0.17dB PSNR and 2.23% bits saving can be achieved by the proposed video content dependent directional transform (CDDT) comparing to the state-of-the-art MDDT.