Low-complexity mode-dependent KLT for block-based intra coding

Applying mode-dependent separable transforms, e.g., mode- dependent directional transform (MDDT), is an effective method for improving transform coding of intra prediction residuals. However, two transform matrices typically need to be stored for each intra prediction mode. By using a simple image correlation mode, we have previously derived and proposed a simplified mode-dependent separable transforms scheme that uses a combination of two well-known trans- forms: Discrete Cosine Transform (DCT) and Discrete Sine Transform (DST). In this paper, we propose an orthogonal 4-point integer DST that has a multiplier-less implementation consisting of only adds and bit-shifts. We also propose a simple set of mode-dependent scans for coefficient coding that can be used on top of mode-dependent transforms. Our experimental results on the current HEVC reference software show that in terms of coding efficiency, our proposed approach has comparable performance to MDDT. More importantly, compared to MDDT, our approach requires no training and has lower computational and storage costs.