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FFmpeg is frequently used by different studios for encoding their media, however the documentation for ffmpeg is often poor, or cryptic so its often harder than it should be to come up with a good starting point. We are aiming to come up with recommendations for different scenarios as well as document what the different flags are doing with the aim to make this easier to get to a good baseline.

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VMAF

I did explore using VMAF - Video Multi-Method Assessment Fusion as a way to quantify the compression, the notes for setting this up are below, however I think we are going with a fairly high compression factor , so I think this is probably not really going to help us much.

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The above gets the underlying data stored correctly, but there are additonal metadata flags that can be set that are interpreted by some players, these are the NCLC color tags for color primaries, transfer function and conversion matrix. (See:   This is defined as a ISO spec here (https://developerwww.appleiso.comorg/documentation/avfoundation/media_assets_and_metadata/sample-level_reading_and_writing/tagging_media_with_video_color_information )standard/57794.html which sadly is paywalled). The numbers below are part of the definition.

" -color_range 1 -colorspace 1 -color_primaries 1 -color_trc 1 "

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NOTE: -color_trc 1 - is not bt1886, but is actually the camera gamma, so has a gamma of ~1.95 rather than the 2.4 that bt1886 has.

Web Browser Deliverables

How should we be encoding content for a web browser.

Most windows laptops and most monitors typically default to a sRGB color space, the tricky part is that sRGB is sometimes interpreted as having exactly a 2.2 gamma, and sometimes a hybrid curve (based on the spec), for more details on this, see: Widget Connectorwidth300urlhttpsIn order to get a gamma 2.4, you will need to use a quicktime hack (see below), but this only works on OSX.

For more information on this I recommend:

• https://vimeo.com/

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Questions to be answered:

• On windows do any of the browsers read the color management settings flags for each monitor?
• Why does everybody set -color_trc 1 ? - it seems completely meaningless?
• Find the details about the firefox plugin for color management?
• What do ICC profiles for stills do on windows/linux boxes? - Are there situations where this is replicated for movies.
• Color shift on Chrome, reported349868875 
•  https://developer.apple.com/documentation/avfoundation/media_assets_and_metadata/sample-level_reading_and_writing/tagging_media_with_video_color_information 

TODO:

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• See if we can get ISO spec: https://bugswww.chromiumiso.org/p/chromium/issues/detail?id=1262622#makechanges

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• standard/57794.html

Web Browser Deliverables

How should we be encoding content for a web browser.

Most windows laptops and most monitors typically default to a sRGB color space, the tricky part is that sRGB is sometimes interpreted as having exactly a 2.2 gamma, and sometimes a hybrid curve (based on the spec), for more details on this, see:

Questions to be answered:

• On windows do any of the browsers read the color management settings flags for each monitor?
• Why does everybody set -color_trc 1 ? - it seems completely meaningless?
• Find the details about the firefox plugin for color management?
• What do ICC profiles for stills do on windows/linux boxes? - Are there situations where this is replicated for movies.
• Color shift on Chrome, reported: https://bugs.chromium.org/p/chromium/issues/detail?id=1262622#makechanges

Gamma 2.4

Gamma 2.4

There is not a color_trc flag for gamma 2.4, the only option that exists for OSX is a cheat 

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ffmpeg -y -i chip-chart-1080.png -c:v libx264 -pix_fmt yuv444p -qscale:v 1 -sws_flags spline+accurate_rnd+full_chroma_int -vf "colorspace=bt709:iall=bt601-6-625:fast=1" -color_range 1 -colorspace 1 -color_primaries 1 -color_trc 2 -movflags write_colr+write_gama -mov_gamma 2.4 test2-h264-ffmpeg-yuv444p-gamma24.mp4

Full range vs. legal range

Typically x264 (and other codecs) are following the video standard that lumance is scaled to the range 16-235. This has a history from early signaling where 236-255 were used for signaling and 0-15 to avoid any noise in the low end (some of the logic was derived from analog video) (TODO find reference). 

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gama -mov_gamma 2.4 test2-h264-ffmpeg-yuv444p-gamma24.mp4

Full range vs. legal range

Typically x264 (and other codecs) are following the video standard that lumance is scaled to the range 16-235. This has a history from early signaling where 236-255 were used for signaling and 0-15 to avoid any noise in the low end (some of the logic was derived from analog video) (TODO find reference). 

However, that means that when we do the conversion, we can end up with 235-16 = 219 luminance values, rather than 255 (14% less levels). This is actually supported in web browsers, e.g.: chrome, firefox, safari.

The following web page demonstrates the resulting differences:

https://taurich.org/encodingTests/ICCTest/greyramp-fulltv/compare.html

Note, that you should be setting the NCLC tag -color_range 2, just in case somebody does start looking there.

TODO:

• Do tests of what happens when ffmpeg then converts the resulting file format, to ensure that the correct range is read.
• This yuvj420p pixel format is listed as depreciated, but its not clear what the alternative, the code has the comment "deprecated in favor of AV_PIX_FMT_YUV420P and setting color_range" which doesn't apparently work. It does appear to be widely used, so could use some further investigation.

References:

• https://news.ycombinator.com/item?id=20036710

Other links

https://github.com/bbc/qtff-parameter-editor