For each generation of video coding standard, increasing the block partitioning flexibility has been very efficient. From fixed-size square blocks to rectangle-sized blocks, the new block shapes have significantly improved the video compression efficiency. During the development of Versatile Video Coding (VVC), the latest video standard developed by the Moving Picture Experts Group (MPEG), it was shown that the flexible partitioning, adding the Binary Tree (BT) and the Ternary Tree (TT) splits on top of the Quadtree (QT) splits, was the tool bringing the highest compression gains over previous standard High Efficiency Video Coding (HEVC). BT and TT splits brought a lot of flexibility to the block partitioning mechanism. During the VVC development, more flexible partitioning tools have also been studied, namely Asymmetric Binary Tree (ABT). In this paper, we show how increasing partition flexibility on top of newest video codec still brings additional gains.
Transform and partitioning represent core components of the video coding architectures. Compared with HEVC, VVC is characterized by higher number of transform types, additional transform level (LFNST) and more flexible partitioning via the binary tree and ternary tree. This flexibility in transform and partitioning provides about 2% and 10% coding gain. Nevertheless, the current design is not ultimately optimized for the highest coding gain, but rather for the compromise with the design complexity. That is, the potential of combining higher transform and partitioning diversity is higher than the current state in VVC. This can be demonstrated by utilizing some early transform and partitioning proposals in the context of VVC development, which were not adopted due to the complexity concerns. In this paper, we revisit these designs targeting the maximum bitrate saving. This is to establish a new state of the art anchor for the post VVC development.
Recently, the advances in transform coding have contributed to significant bitrate saving for the next generation of video coding. In particular, the combination of different discrete trigonometric transforms (DTT’s) was adopted in the Joint Video Exploration Team (JVET) solution, as well as the Bench-Mark Set (BMS) of the future video coding standard (Versatile Video Coding), to efficiently model the residual signal statistics and to improve the overall coding efficiency. However, this combination of transforms necessitates an increase in the memory requirement as well as coding complexity, which could potentially limit their practical use. In this paper, we solve both memory and complexity issues by reducing the number of transforms, where some of the transforms are generated from other by simple mathematical operations, like sign changing and reverse ordering. The simulation results showed that this approach achieves competitive results with substantial simplification of the transform design.
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