Netflix optimizes its 4K films and saves 50% data

[15:07 Sun,6.September 2020 by Thomas Richter]

Netflix offers 4K movies and series not only as an 8-bit stream, but also as premium content with 10-bit, HFR (High Frame Rate) and/or HDR (High Dynamic Range).

Until now, 4K content has been encoded at fixed bit rates of 8, 10, 12 and 16 Mbps and rigid presets (how many Mbps per defined resolution) - regardless of the content. Since then, however, Netflix has developed new techniques, such as specific encoding per movie and later even per setting, which enable optimized compression. However, these techniques and variable keyframe or Group-of-Pictures (GoPs) compression have not yet been used for previously encoded 4K premium content.

Netflix has a strong interest in saving on the bit rate of its videos because bit rates also cause high data volumes and thus large infrastructure costs for Netflix itself and again for the users (in mobile playback). Netflix has therefore been driving the development of new and better encoding methods for years.

Now Netflix has recompressed all these old 4K movies using the latest technology: each individual camera shot of a movie has been recompressed using the Netflix VMAF method with the optimal bitrate for each shot.

dynamically encoded. The VMAF (Video Multi-Method Assessment Fusion) algorithm evaluates the visual image quality of videos in a similar way as humans do, allowing automatic optimization of maximum image quality at minimum bitrate when testing videos compressed at different bitrate and parameters.

Additional optimization potentials resulted from improvements in the codec itself. In the

linked article, Netflix engineers describe how they have recompressed SD and 4K 10-bit material and use a few image examples to show how better image quality was achieved at a lower bit rate. On average it was possible to achieve the same image quality by dynamic compression as before with a 50% lower bitrate, so now a maximum of 8 Mbps is usually sufficient to display 4K videos well - before it was 16 Mbps.

In the following graphic you can see how the new method ("optimized ladder") can be used to achieve the same bitrate with a higher resolution or a lower bitrate with the same picture quality than the old method ("fixed ladder"). The episode of a thriller drama was used here as an example material.

Advantages for the users

Over the next few months, Netflix will recompress all 4K versions, and then the HDR versions of all movies using the new methods. For Netflix high-bandwidth subscribers, this means they get the same quality at on average half the bit rate. Users with limited bandwidth will benefit from higher quality at the same (or even lower) bitrate. For example, viewers who were limited to 720p by their network can now watch 1080p or higher resolution instead. The new encoding also greatly reduces the number of annoying playback pauses caused by rebuffering (by 65% on average).

Furthermore, all viewers should benefit from the fact that most films start with a higher initial quality. If the first frames are to be played back quickly when a new film is played back, then - depending on the available bandwidth - the picture quality will be briefly reduced before there is enough data for optimum playback. Due to improvements in DRM and the lower bit rate, the
Playback delay of new movies reduced by about 10%.

Netflix&s progress will also be reflected in other streaming providers, as Netflix is very open about documenting its technical developments and providing open source tools such as VMAF.
more infos at bei netflixtechblog.com

deutsche Version dieser Seite: Netflix optimiert seine 4K Filme und spart 50% Daten