Tomas Chlebek Before a few days Netflix has finally enabled downloading content for offline viewing. One of the main reasons why this option came only now was said to be problems with finding a suitable format and quality.
Two quality levels are offered for download - "Standard" and "Higher". It is not known what specific resolutions and bitrates they have, which is due to them varying according to content. Netflix wanted to provide the best possible ratio between the quality and the size of the downloaded file.
The result is better quality at a smaller size
He has been using variable data flow for streaming for a long time, but he wanted to come up with an even more economical solution for downloading. Thus, while streaming has so far used the H.264/AVC Main profile (AVCMain) codec (data compression type), Netflix for mobile has introduced support for two others – H.264/AVC High profile (AVCHi) and VP9, the former being used by iOS devices and a second Android device.
VP9 is better in terms of the ratio between quality and data rate; but while it's available for free, Apple doesn't support this Google-created codec, and it doesn't look like that's going to change anytime soon. That's why Netflix chose AVCHi. He decided to use a new method for data compression. This consists in analyzing individual scenes and determining their image complexity (e.g. a calm scene with a minimum of movement vs. an action scene with many moving objects).
According to her, the entire film/series is then "sliced" into parts of between one and three minutes in length, and for each part the resolution and data flow required to achieve the required quality are individually calculated. This approach was then also used for the VP9 codec, and Netflix plans to apply it to its complete library and use it not only for downloading, but also for streaming.
Different codecs and compression methods have two consequences: reducing the data flow while maintaining the original quality, or increasing the quality while maintaining the same data flow. Specifically, files with objectively the same image quality can require 19% less space with the AVCHi codec and up to 35,9% less space with the VP9 codec. Video quality with the same data stream (post on the Netflix blog gives an example for 1 Mb/s) compared to AVCMain increased by 7 points for AVCHi according to the test standard VMAF, with VP9 then by 10 points. "These increases provide noticeably better picture quality for mobile streaming," the blog says.
