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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.

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 and sRGB displays to start with, but eventually, P3, rec2020 and HDR displays.
  • 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. 

Overall workflow

For this paper, we are assuming that we are encoding from a file-sequence of frames into a movie (rather than re-encoding a movie), but we are also assuming almost all of the colorspace work would be done outside of ffmpeg, with tools using the OCIO library. Examples could include nuke and oiio. Once we get to ffmpeg, the goal being that the pixel data we get in, should be as close as possible to the data we get out of ffmpeg. However, there are still quite a few areas that a ffmpeg user could go wrong, which we break down below. 

TODO: Find examples of overall workflow. 

Codec Selection.

To start with we would be exploring the following codecs.

...

x264rgb

...

Prores

DnxHD

...

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

• 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

...

Description

...

-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

...

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.

...

-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

...

-pix_fmt yuv422p10le -c:v prores_ks -profile:v 3 -vendor ap10 -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

...

Note the -vendor ap10 part below is only needed if working with Final Cut, but it does no harm otherwise.

-profile:v 3 is equivalent to -profile:v hq

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.

https://github.com/Netflix/vmaf

https://jina-liu.medium.com/a-practical-guide-for-vmaf-481b4d420d9c

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

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

RGB/YCrVb Colorspace Conversion  

As a rule of thumb, we would like ffmpeg to do as little as possible in terms of color space conversion. i.e. what comes in goes out. The problem is that most of the codecs are doing some sort of RGB to YUV conversion (technically YCrCb). The notable exception is x264rgb (see above). For more information, see: https://trac.ffmpeg.org/wiki/colorspace and 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 and sRGB displays to start with, but eventually, P3, rec2020 and HDR displays.
  • 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. 

Presentation Links

Overall workflow

For this paper, we are assuming that we are encoding from a file-sequence of frames into a movie (rather than re-encoding a movie), but we are also assuming almost all of the colorspace work would be done outside of ffmpeg, with tools using the OCIO library. Examples could include nuke and oiio. Once we get to ffmpeg, the goal being that the pixel data we get in, should be as close as possible to the data we get out of ffmpeg. However, there are still quite a few areas that a ffmpeg user could go wrong, which we break down below. 

TODO: Find examples of overall workflow. 

Codec Selection.

h264

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 - see also: https://superuser.com/questions/564402/explanation-of-x264-tune I suspect that we would want to use one of:
  1. -tune film    good for live action content.
  2. -tune animation good for animated content with areas of flat colors.
  3. -tune grain   good for live action content where you want to preserve the grain as much as possible.
• -qscale:v 1 - Generic quality scale flag: https://www.ffmpeg.org/ffmpeg.html#toc-Main-options - not sure if its needed?

-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

h265/HVEC

Support: https://caniuse.com/hevc (or https://www.chromium.org/audio-video ) currently no support for h265 on chrome or chromium based browsers.

links:

• https://codecalamity.com/encoding-uhd-4k-hdr10-videos-with-ffmpeg/
• https://codecalamity.com/encoding-settings-for-hdr-4k-videos-using-10-bit-x265/
• https://support.frame.io/en/articles/4305241-creating-hdr-files-for-frame-io
• https://stackoverflow.com/questions/69251960/how-can-i-encode-rgb-images-into-hdr10-videos-in-ffmpeg-command-line
• https://brandur.org/fragments/ffmpeg-h265

HDR

• https://developer.apple.com/av-foundation/High-Dynamic-Range-Metadata-for-Apple-Devices.pdf
• https://www.avsforum.com/threads/open-source-video-testing-calibration-patterns.2944378/
• https://github.com/test-full-band/tfb-video/releases
• https://trev16.hatenablog.com/entry/2021/07/23/145725 – good site for HDR test sites.

ProRes

There are four Prores encoders, Prores, Prores_ks, Prores_aw and now with ffmpeg 5 VideoToolBox Prores, which is a hardware based OSX M1 encoder/decoder. 

From https://trac.ffmpeg.org/wiki/Encode/VFX the recommendation is to use Prores_ks with -profile:v 3 and the qscale of 11

Options that can be used include:

• -profile:v values can be one of.
  • proxy (0)
  • lt (1)
  • standard (2)
  • hq (3)
  • 4444 (4)
  • 4444xq (5)
• -qscale:v between values of 9 - 13 give a good result, 0 being best.
• -vendor apl0 - tricks the codec into believing its from an Apple codec.

Using this with the usual color space flags, seems to work well with the exception of ffmpeg itself, which needs the flags:-vf scale=in_color_matrix=bt709:out_color_matrix=bt709 added to the command to ensure the right input colorspace is recognised, e.g.:

ffmpeg.exe -i INPUTFILE.mov -compression_level 10 -pred mixed -pix_fmt rgba64be -sws_flags spline+accurate_rnd+full_chroma_int -vframes 1 -vf scale=in_color_matrix=bt709:out_color_matrix=bt709 OUTPUTFILE.png

However, other encoders seem to be recognised correctly, so there is clearly some metadata missing. I did try using the prores_metadata filter to try adding some additional parameters, but it didnt seem to help.

ffmpeg.exe -i ./chip-chart-yuvconvert\basicnclc.mov -c copy -bsf:v prores_metadata=color_primaries=bt709:color_trc=bt709:colorspace=bt709 chip-chart-yuvconvert\basicnclcmetadata.mov

TODO:

• Figure out the missing metadata.
• Wedge qscale values
• Do some colorspace tests with different qscale values to see where color breaks down.

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.

https://github.com/Netflix/vmaf

https://jina-liu.medium.com/a-practical-guide-for-vmaf-481b4d420d9c

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

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

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.

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.

RGB/YCrCb Colorspace Conversion  

As a rule of thumb, we would like ffmpeg to do as little as possible in terms of color space conversion. i.e. what comes in goes out. The problem is that most of the codecs are doing some sort of RGB to YUV conversion (technically YCrCb). The notable exception is x264rgb (see above). For more information, see: https://trac.ffmpeg.org/wiki/colorspace 

For examples comparing these see: https://richardssam.github.io/ffmpeg-tests/tests/chip-chart-yuvconvert/compare.html

colormatrix filter

ffmpeg -y -i ../sourceimages/chip-chart-1080-noicc.png -sws_flags spline+accurate_rnd+full_chroma_int -vf "colormatrix=bt470bg:bt709" -c:v libx264 -preset placebo -qp 0 -x264-params "keyint=15:no-deblock=1" -pix_fmt yuv444p10le -qscale:v 1 -color_range 1 -colorspace 1 -color_primaries 1 -color_trc 1 ./chip-chart-yuvconvert/spline444colormatrix2.mp4

This is the most basic colorspace filtering. bt470bg is essentially part of the bt601 spec.  See: https://www.ffmpeg.org/ffmpeg-filters.html#colormatrix

colorspace filter

ffmpeg -y -i ../sourceimages/chip-chart-1080-noicc.png -sws_flags spline+accurate_rnd+full_chroma_int -vf "colorspace=bt709:iall=bt601-6-625:fast=1" -c:v libx264 -preset placebo -qp 0 -x264-params "keyint=15:no-deblock=1" -pix_fmt yuv444p10le -qscale:v 1 -color_range 1 -colorspace 1 -color_primaries 1 -color_trc 1 ./chip-chart-yuvconvert/spline444colorspace.mp4

Using colorspace filter, better quality filter, SIMD so faster too, can support 10-bit too.  The second part -vf "colorspace=bt709:iall=bt601-6-625:fast=1" encodes for the output being bt709, rather than the default bt601 matrix. iall=bt601-6-625 says to treat all the input (colorspace, primaries and transfer function) with the bt601-6-625 label). fast=1 skips gamma/primary conversion in a mathematically correct way.  See:  https://ffmpeg.org/ffmpeg-filters.html#colorspace

While it is possible to do the encoding outside of ffmpeg (see later). Its easier if you do the encoding inside, using flags like:

libswscale filter

ffmpeg -y -i ../sourceimages/chip-chart-1080-noicc.png -sws_flags spline+accurate_rnd

...

Metadata flags -sws_flags spline+accurate_rnd+full_chroma_int helps with the YCrCb conversion. Its using the swscale filter, which has a number of options:

TODO: why spline?   - Filtering applied to the decimation filtering to go from RGB to 444 to 422.

accurate_rnd - enables accurate rounding

full_chroma_int - Enable full chroma interpolation.

full_chroma_p?

The second part -vf "colorspace=bt709:iall=bt601-6-625:fast=1" encodes for the output being bt709, rather than the default bt601 matrix. iall=bt601-6-625 says to treat all the input (colorspace, primaries and transfer function) with the bt601-6-625 label). fast=1 skips gamma/primary conversion in a mathematically correct way. 

TODO, CONFIRM - THIS IS TO GET THE CONVERSION TO TREAT THE RESULTING DATA AS BT709 rather than BT601? Why does fast=1 work? Surely I do want it to do all the flags?

Color Metadata

...

+full_chroma_int+full_chroma_inp -vf "scale=in_range=full:in_color_matrix=bt709:out_range=tv:out_color_matrix=bt709" -c:v libx264 -preset placebo -qp 0 -x264-params "keyint=15:no-deblock=1" -pix_fmt yuv444p10le -qscale:v 1 -color_range 1 -colorspace 1 -color_primaries 1 -color_trc 1 ./chip-chart-yuvconvert/spline444out_color_matrix.mp4

Using the libswscale library. Seems similar to colorspace, but with image resizing, and levels built in.  https://www.ffmpeg.org/ffmpeg-filters.html#scale-1

This is the recommended filter.

Color Metadata NCLC/NCLX

The above gets the underlying data stored correctly, but there are additional 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.  This is defined as a ISO spec here (see https://www.iso.org/standard/73412.html). The numbers below are part of the definition.

NCLC stands for Non-Consistent Luminance Coding, a brief overview of its history is here. For MP4 files, its also known as NCLX. Additionally this metadata can also be represented in the h264 metadata stream in the video usability Information (VUI) block. 

You can read the metadata using mp4box.js which is a visual browser of the mp4 metadata, and look at moov/trak/mdia/minf/stbl/stsd/avc1/colr

NOTE: None of the flags below affect the encoding of the source imagery, they are meant to be used to guide how the mp4 file is decoded.

...

The docs are pretty sparse for this, some of the better info is FFmpeg/pixfmt.h at master · FFmpeg/FFmpeg · GitHub

links:

Color Range

Uses the flag -color_range e.g. -color_range 1 or -color_range tv

Color Space

This defines the YUV colorspace type, as defined by ISO/IEC 23091-2_2019 subclause 8.3

...

Color Primaries

Chromaticity coordinates of the source primaries. These values match the ones defined by ISO/IEC 23091-2_2019 subclause 8.1 and ITU-T H.273.

...

Color Transfer Characteristic aka color_trc

These values match the ones defined by ISO/IEC 23091-2_2019 subclause 8.2.

...

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 is defined by bt1886. In order to get a gamma 2.4, you will need to use a quicktime hack (see below), but this only works on OSX. However, we suspect that chrome ignores the setting (see the following tests).

...

This second test highlights that better, by giving a source image that is designed so that when the images are displayed with the TRC settings they should match with a gamma 2.2 monitor.

https://taurich.org/encodingTests/ICCTest/greyramp-rev-ps/compare.html  https://taurichwww.color.org/encodingTests/ICCTest/greyramp-rev2/compare.htmlversion4html.xalter

Again, you will notice the bt709 (color-trc=1) is wildly off.

...

• https://vimeo.com/349868875 
•  https://developer.apple.com/documentation/avfoundation/media_assets_and_metadata/sample-level_reading_and_writing/tagging_media_with_video_color_information 
• https://www.iso.org/standard/73412.html - Note this has a link to the download of the earlier version of the doc, the latest and paywalled version is here: https://www.iso.org/standard/57794.html
• 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.

Web Browser Deliverables

How should we be encoding content for a web browser.

...

Gamma 2.4

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

...

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) 

...