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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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TODO: Find examples of overall workflow. 

Codec Selection.

To start with we would be exploring the following codecs.

x264

Key flags (see https://trac.ffmpeg.org/wiki/Encode/H.264 ) 

• -crf 23 - This is the constant rate factor, controlling the default quality (see: https://slhck.info/video/2017/02/24/crf-guide.html ) where -crf 0 is uncompressed. By default this is set to 23, which is probably good enough for our needs.
• -qp 23 - Quantization Parameter - it is recommended that you do not use this, in preference to -crf above (see: https://slhck.info/video/2017/03/01/rate-control.html )
• -preset slower - https://trac.ffmpeg.org/wiki/Encode/H.264#FAQ
• -tune film - Optionally use the tune option to change settings based on specific inputs - https://trac.ffmpeg.org/wiki/Encode/H.264#FAQ

-preset slow -crf 18   level 4 profile high.

RV - 10bit YUV444.

Better to stick with yuv – 

FFMPEG.

TODO:

• Suggestions for max-bitrate?
• Suggestions for preset - ? slow
• Suggestions for tune

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

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x264

Key flags (see https://trac.ffmpeg.org/wiki/Encode/H.264 ) 

• -crf 23 - This is the constant rate factor, controlling the default quality (see: https://slhck.info/video/2017/02/24/crf-guide.html ) where -crf 0 is uncompressed. By default this is set to 23, which is probably good enough for our needs.
• -qp 23 - Quantization Parameter - it is recommended that you do not use this, in preference to -crf above (see: https://slhck.info/video/2017/03/01/rate-control.html )
• -preset slower - 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
• Encode/H.264#FAQ
• -tune film - Optionally use the tune option to change settings based on specific inputs - https://docstrac.nvidiaffmpeg.comorg/video-technologies/video-codec-sdk/ffmpeg-with-nvidia-gpu/
• https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.1886-0-201103-I!!PDF-E.pdf - the BT1886 spec, esentially gamma 2.4 rec709 primaries.
• https://github.com/bbc/qtff-parameter-editor - A BBC open source app, for setting quicktime NCLC attributes.
• https://vimeo.com/349868875 - Video from baselight, reviewing setting the right NCLC tags for apple colorsync.

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

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Description

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-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

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The sws_flags are needed for the RGB to YUV conversion.

-color_range 1   # mpeg see: FFmpeg/pixfmt.h at master · FFmpeg/FFmpeg · GitHub

-colorspace 1 # BT709   FFmpeg/pixfmt.h at master · FFmpeg/FFmpeg · GitHub

-color_primaries 1 # BT709 FFmpeg/pixfmt.h at master · FFmpeg/FFmpeg · GitHub

-color_trc 1 # bt709 FFmpeg/pixfmt.h at master · FFmpeg/FFmpeg · GitHub - Color Transfer Characteristics.

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-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 

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Compression quality

Testing Methodology

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• wiki/Encode/H.264#FAQ

-preset slow -crf 18   level 4 profile high.

RV - 10bit YUV444.

Better to stick with yuv – 

FFMPEG.

TODO:

• Suggestions for max-bitrate?
• Suggestions for preset - ? slow
• Suggestions for tune

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://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.1886-0-201103-I!!PDF-E.pdf - the BT1886 spec, esentially gamma 2.4 rec709 primaries.
• https://github.com/bbc/qtff-parameter-editor - A BBC open source app, for setting quicktime NCLC attributes.
• https://vimeo.com/349868875 - Video from baselight, reviewing setting the right NCLC tags for apple colorsync.

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

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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