On secondary transforms for intra prediction residual

In this paper, we present a secondary transform that is applied in a codec after the conventional DCT for all the video-coding intra prediction modes. Our approach is applicable to any block-based intra prediction scheme that employs transforms along the horizontal and vertical direction separably. The secondary transform is applied to the lower 8×8 frequency coefficients of the output of conventional DCT at block sizes 8×8 and higher. The proposed transform scheme has low complexity as it is applied only to the top-left portion of DCT output, especially in the context of large blocks such as 32×32 where an alternate transform of size 32×32 other than DCT would be expensive to be implemented in hardware. The proposed technique works in single-pass, and the choice of when to use the secondary transform is solely based on the intra prediction mode and requires no additional signaling information or R-D search. Our simulation results show that the proposed transform scheme provides significant BD-Rate improvement over the conventional DCT-based coding scheme for intra prediction of video sequences in the ongoing HEVC standardization.