Extended Transform Skip Mode and Fast Multiple Transform Set Selection in VVC

The Versatile Video Coding (VVC) development has adopted the possibility to bypass the transform when the transform block size is equal to 4×4 from its predecessor High Efficiency Video Coding (HEVC). This so-called Transform Skip Mode (TSM) results in increased encoding time when extending it to transform block sizes up to 32×32 while the compression efficiency improvement is for screen content only. This paper presents the so-called Unified MTS scheme that makes TSM for luma transform block sizes up to 32×32 possible without the disadvantage of excessive encoding time increase by incorporating the TSM with the existing Multiple Transform Set (MTS) technique. The Unified MTS scheme achieves compression efficiency improvements, in terms of BD-rate, of about -5.0% in the All-Intra configuration and -5.4% in the Random-Access configuration, respectively, for the screen content sequences of the used test set. Compared to the straightforward extension of TSM to transform block sizes up to 32×32, the encoding time is about 25% less in the All-Intra configuration and about 21% less in the Random-Access configuration, respectively, whereas the compression efficiency improvements are only 0.04% less in the All-Intra configuration, and 0.09% less in the Random-Access configuration, respectively. Relative to the anchor using TSM for 4×4 transform blocks only, the encoding time is the same for natural content and 3% higher for screen content.