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.

Lightweight video compression has effectively alleviated the tension between growing transmission demands and expensive integration upgrades. Effective rate control algorithms are believed to be the crucial bottleneck for quality improvement during those ultra-high throughput coding processes. This paper proposes a novel rate control (RC) scheme that constructs a contextual adaptive bit estimation model through clustering historical compression information into block-gradient complexity categories. A buffer-aware tuning method and a flexible quantization parameter (QP) mapping algorithm are designed to determine the Luma/Chroma QP distribution where a simplified Lagrangian multiplier is further defined to preserve the stability of the overall compression process. As a result, the constant-bitrate compression towards low-latency interoperable ASICs is implemented with a promising RC performance.

This paper proposes a novel quantization parameter (QP) derivation module that constructs several spatiotemporal characteristics into key-frame QP determination through an efficient pre-analysis progress. A series of simplified prediction modes and a histogram statistic are employed to model the reference quality that key-frames provide to subsequent frames. An adaptive delta-QP value is generated to address the conflict between the low compression efficiency of intra-only frames and the critical predictive basis of temporal-underlying frames. The experimental result shows that the proposed method reduces 41.91% of peak-to-valley bitrate difference while leading to a 0.02% BDBR performance change, indicates that high-quality key-frames may not be indispensable in nowadays video compression frameworks. The proposed method has shown that the pre-analysis based QP optimization for intra-only frames is promising for the enhancement of transmission bandwidth utilization, which may hopefully provide new inspiration for bit allocation and rate control designs.