The Decoder-side Secondary Transform Derivation (DSTD) method has been adopted by the exploration software of the fourth generation Audio Video coding Standard (AVS4), Exploration Video Model (EVM). Although DSTD can significantly improve the coding performance, it increases the encoding complexity simultaneously. To reduce the encoding complexity of DSTD, three enhanced DSTD methods are proposed in this paper, which consists of Deleting Mode Optimization (DMO), Intra Mode Dependent Optimization (IMDO) and Interleaved Intra Mode Dependent Optimization (IIMDO). The process of DMO is similar to that of the original DSTD method, but all the contents related to diagonal flipping have been removed. IMDO divides intra prediction modes into three areas based on the angle of the intra prediction mode, as shown in Fig. 1. Each area will correspondingly reduce a secondary transform type. IIMDO divides intra prediction modes into four areas, as shown in Fig. 2. The experimental results demonstrate that the proposed methods can effectively reduce the encoding complexity with negligible coding performance loss. The first enhanced method, Deleting Mode Optimization, has been adopted by EVM.

Secondary transform was adopted into the third generation Audio Video coding Standard (AVS3) to improve the intra-coded residual coding by applying a 4×4 secondary transform kernel. However, the adaptability of the single 4×4 transform kernel is limited for various residual data. In order to achieve higher residual coding gains, we propose a Decoder-side Secondary Transform Derivation (DSTD) method. Specifically, DSTD expands the maximum range of secondary transform from 4×4 to 8×8, where an 8×8 size transform kernel is introduced to further enhance the capability of compacting residuals. In particularly, three flipped secondary transform types are employed to extend transform candidates, including horizontal, vertical and diagonal flipping types. The boundary continuity is utilized to derive the transform type. Experimental results show that the proposed method can achieve 0.51% and 0.18% BD-rate savings on average under All Intra (AI) and Random Access (RA) configurations, respectively. DSTD has been adopted into the Exploration Video Model (EVM) for AVS4.