Disclaimer: This analysis is merely an objective analysis of the coding features the encoder uses and not an analysis of the subjective or objective picture quality of the encoder.
Early stage encoders like the one used in the BBC World Cup UHD trial are interesting in that they provide an insight into the development process of a encoder and what coding tools encoder manufacturers have decided to use first (often with limited processing power). This information usually remains under NDA but public use of the encoder means anyone can perform analysis on it.
A very good introduction to HEVC coding tools can be found here: http://forum.doom9.org/showthread.php?t=167081
Thanks to the help provided by Parabola Research in producing this post. You can download a bitstream analysis report from Parabola Explorer Pro 3.0 below. This report helped produce the analysis below.
In no particular order:
- The GOP structure is pretty standard IBBPBBPBBP. It appears not to adapt. It’s quite similar to MPEG-2 in that it only keeps a maximum of one frame in L0 and L1.
- CTU size is 64×64; 9 slices in total.
- Intra prediction modes are very limited. Almost always using DC, horizontal or vertical and very rarely using the other 32 modes.
- No use of Asymmetric Prediction Units; quite similar to MPEG-4 / AVC.
- Constant Quantiser – no use of (variance based) adaptive quantisation yet to improve visual quality.
- There’s no use of Sample Adaptive Offset and no use of Weighted Prediction.
- There’s no use of BI prediction at all. It’s either L0 or L1.
All in all, not really a surprise. At this early stage it’s no real surprise that people like Netflix are saying “We’re not seeing efficiency gains being claimed by HEVC encoding vendors”1 and such a limited use of the toolkit is the main reason why.
The report from Parabola Explorer Pro can be found here.
