On secondary transforms for prediction residual

In this paper, we present a transform scheme that utilizes a secondary transform after the conventional DCT for intra as well as inter prediction residues. Our approach is applicable to any block-based video codec that employs transforms along the horizontal and vertical direction separably. The secondary transform is applied to the lower K (K=4 or 8) frequency coefficients of the output of conventional DCT at block with dimensions 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 whether to use the secondary transform is solely based on the prediction direction for intra residue, and transform unit location in the prediction unit for the inter residue. The scheme 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 video sequences in the ongoing HEVC standardization.