Movie Compression Usages

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.

Overview

We are looking for recommendations for the following:

• Best color preservation for output to:
  • Web, OSX, IOS and Windows.
  • Common applications: e.g. RV, Nuke.
  • Rec709, P3, rec2020 and HDR.
  • Web browser - Firefox reviewing mp4. - use firefox plugin.
  • RV
• Codec recommendations for:
  • Proxy H264 playback (e.g. web streaming), should be setup for web streaming.
  • Animation/Modelling/Layout movie playback. - somewhat lower quality playback, but should always provide smooth motion.
  • Lookdev/lighting/compositing movie playback - should have excellent color fidelity and minimal encoding artifacts
  • • Should any filmlook be baked in, or should we assume that is always applied during viewing.
    • How much should we be able to adjust color and have the image hold up? (Or rely on exr's for that?).
  • Export to editorial.
  • High-resolution or frame rate - e.g. 4k, 8k, 60fps, 120fps.
  • Stereo or VR.
• Q: Which container should we be considering: mov, mp4, mxf.

Where ffmpeg arguments, it would be great to document why we are using them, rather than ending up with a recipe. 

Color Preservation

Testing Methodology

Converting SMPTE color bars to the compressed movie, using ffmpeg to expand and then compare with OIIO. NOTE, for compression schemes that are not 444 we may need to mask the transitions.

Testing loading the compressed movie in to RV, Firefox, VLC, Avid, resolve, , to compare the resulting color transformation - not sure if there is a procedural way to run this?

For the tests below we are assuming that other tools are being used (e.g. oiiotool) to convert the rendered frames into an intermediate file (e.g. PNG) in the target color-space. 

Q: Currently focusing just on color matching in vs. out, but should also do EXR ACEScg in to resulting movie. Feels like we should also bless full pipeline, e.g.: Reference "Dailies script" https://github.com/jedypod/generate-dailies

Test Sources

SMPTE test chart: https://commons.wikimedia.org/wiki/File:SMPTE_Color_Bars_16x9.svg

Download image sequence from: https://senkorasic.com/testmedia/ - 

Explore netflix: https://opencontent.netflix.com/

Test Results:

taurich.org/encodingTests/results.html

Links

• An excellent starting point for this is: https://trac.ffmpeg.org/wiki/colorspace
• https://github.com/RxLaboratory/DuME/blob/master/src/FFmpeg_COLORS.md
• https://medium.com/invideo-io/talking-about-colorspaces-and-ffmpeg-f6d0b037cc2f
• https://docs.nvidia.com/video-technologies/video-codec-sdk/ffmpeg-with-nvidia-gpu/
• https://github.com/bbc/qtff-parameter-editor

Notes

The big issue here is that by default if you start converting images to another format, and ffmpeg cannot determine the colorspace  it will default to bt601. So many of the flags below are to:

A: Tell ffmpeg that the source media is in fact bt709

B: Add the metadata to the output, so that other future conversions also know how to convert it back.

C: Do as clean a conversion from RGB to YUV as possible.

Example Usages

Need to specify the build of ffmpeg. – ? - and specify build flags. -loglevel trace

-pix_fmt yuv444p10le -sws_flags spline+accurate_rnd+full_chroma_int -vf "colormatrix=bt470bg:bt709" -color_range 1 -colorspace 1 -color_primaries 1 -color_trc 1

-pix_fmt yuv444p10le -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 1 

Compression quality

Testing Methodology

Explore using VMAF:

https://netflixtechblog.com/toward-a-practical-perceptual-video-quality-metric-653f208b9652

https://github.com/Netflix/vmaf

https://ottverse.com/vmaf-ffmpeg-ubuntu-compilation-installation-usage-guide/ - building VMAF on ubuntu.