Complexity Reduction Opportunities in the Future VVC Intra Encoder

The Joint Video Expert Team (JVET) is developing the next-generation video coding standard called Versatile Video Coding (VVC) and their ultimate goal is to double the coding efficiency over the current state-of-the-art standard HEVC without letting complexity get out of hand. This work addresses the complexity of the VVC reference encoder called VVC Test Model (VTM) under All Intra coding configuration. The VTM3.0 is able to improve intra coding efficiency by 21% over the latest HEVC reference encoder HM16.19. This coding gain primarily stems from three new coding tools. First, the HEVC Quad-Tree (QT) structure extension with Multi-Type Tree (MTT) partitioning. Second, the duplication of intra prediction modes from 35 to 67. And third, the Multiple Transform Selection (MTS) scheme with two new discrete cosine/sine transforms (DCT-VIII and DST-VII). However, these new tools also play an integral part in making VTM intra encoding around 20 times as complex as that of HM. The purpose of this work is to analyze these tools individually and specify theoretical upper limits for their complexity reduction. According to our evaluations, the complexity reduction opportunity of block partitioning is up to 97%, i.e., the encoding complexity would drop down to 3% for the same coding efficiency if the optimal block partitioning could be directly predicted. The respective percentages for intra mode reduction and MTS optimization are 65% and 55%. We believe these results motivate VVC codec designers to develop techniques that are able to take most out of these opportunities.