PNG produced from BT.709 video should be tagged as gAMA 2.4

[Balling]

Summary of the bug:

EOTF of BT.709 OETF is 2.4 gamma, not 2.2 gamma (Chrome hack) and not what it is now, 1.961 gamma, that is what OETF is. Gamma chunk in PNG signals EOTF, not like in JPEG XL that signals OETF or in AVIF.

[Balling]

Related. ​https://patchwork.ffmpeg.org/project/ffmpeg/patch/20230116200139.232178-1-leo.izen@gmail.com/

[Balling]

Oh, of course. Mastering is what is outside of gamut, cHRM is just primaries. How did I not complain about it? Ha.

​https://patchwork.ffmpeg.org/project/ffmpeg/patch/20230117185051.475157-2-leo.izen@gmail.com/

2548c32cc10f256cc0ff90457631ef7ef94070af GENIUS!!

If before we had

color_range=pc
color_space=unknown
color_primaries=unknown
color_transfer=unknown
chroma_location=unspecified
TAG:gamma=45455/100000
[/FRAME]

now we have sRGB overwriting gamma as it should and

color_range=pc
color_space=gbr
color_primaries=bt709
color_transfer=iec61966-2-1
chroma_location=unspecified
[/FRAME]

[Balling]

Even after f7bab37c8e66f3c77a5fbb5b5b72c10b0fb4ca5a it still writes Gamma 1.961, that is incorrect. It must be 2.4 gamma. At least it writes PNG cICP now, so mpv should work correctly now, YES!! But AFAIK, fallback for BT.709 must be 2.4 gamma, but I cannot see that in the spec here ​https://w3c.github.io/PNG-spec/#example-4

Command line

ffmpeg -i C:\Users\ZAQU\Downloads\example.h264 -v:frames 1 mcasa.png

ffmpeg.exe -v verbose -i C:\Users\ZAQU\Downloads\example.h264 -v:frames 1 mcasa.png
ffmpeg version N-109662-g2c3107c3e9-20230125 Copyright (c) 2000-2023 the FFmpeg developers

built with gcc 12.2.0 (crosstool-NG 1.25.0.90_cf9beb1)
configuration: --prefix=/ffbuild/prefix --pkg-config-flags=--static --pkg-config=pkg-config --cross-prefix=x86_64-w64-mingw32- --arch=x86_64 --target-os=mingw32 --enable-gpl --enable-version3 --disable-debug --enable-shared --disable-static --disable-w32threads --enable-pthreads --enable-iconv --enable-libxml2 --enable-zlib --enable-libfreetype --enable-libfribidi --enable-gmp --enable-lzma --enable-fontconfig --enable-libvorbis --enable-opencl --disable-libpulse --enable-libvmaf --disable-libxcb --disable-xlib --enable-amf --enable-libaom --enable-libaribb24 --enable-avisynth --enable-chromaprint --enable-libdav1d --enable-libdavs2 --disable-libfdk-aac --enable-ffnvcodec --enable-cuda-llvm --enable-frei0r --enable-libgme --enable-libkvazaar --enable-libass --enable-libbluray --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librist --enable-libssh --enable-libtheora --enable-libvpx --enable-libwebp --enable-lv2 --disable-libmfx --enable-libvpl --enable-openal --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --enable-librav1e --enable-librubberband --enable-schannel --enable-sdl2 --enable-libsoxr --enable-libsrt --enable-libsvtav1 --enable-libtwolame --enable-libuavs3d --disable-libdrm --disable-vaapi --enable-libvidstab --enable-vulkan --enable-libshaderc --enable-libplacebo --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libzimg --enable-libzvbi --extra-cflags=-DLIBTWOLAME_STATIC --extra-cxxflags= --extra-ldflags=-pthread --extra-ldexeflags= --extra-libs=-lgomp --extra-version=20230125
libavutil 57. 44.100 / 57. 44.100
libavcodec 59. 57.100 / 59. 57.100
libavformat 59. 36.100 / 59. 36.100
libavdevice 59. 8.101 / 59. 8.101
libavfilter 8. 54.100 / 8. 54.100
libswscale 6. 8.112 / 6. 8.112
libswresample 4. 9.100 / 4. 9.100
libpostproc 56. 7.100 / 56. 7.100

PS C:\Users\XXXX> exiftool.exe mcasa.png
ExifTool Version Number : 12.44
File Name : mcasa.png
Directory : C:/Users/XXXX
File Size : 87 kB
File Modification Date/Time : 2023:01:26 12:59:24+03:00
File Access Date/Time : 2023:01:26 12:59:42+03:00
File Creation Date/Time : 2023:01:26 12:59:24+03:00
File Permissions : -rw-rw-rw-
File Type : PNG
File Type Extension : png
MIME Type : image/png
Image Width : 1280
Image Height : 720
Bit Depth : 8
Color Type : RGB
Compression : Deflate/Inflate
Filter : Adaptive
Interlace : Noninterlaced
Pixels Per Unit X : 1
Pixels Per Unit Y : 1
Pixel Units : Unknown
White Point X : 0.3127
White Point Y : 0.329
Red X : 0.64
Red Y : 0.33
Green X : 0.3
Green Y : 0.6
Blue X : 0.15
Blue Y : 0.06
Gamma : 1.961
Image Size : 1280x720
Megapixels : 0.922

Sample is here: ​https://github.com/wang-bin/QtAV/files/1132086/example.zip

[Balling]

Hey, I added samples and command line, please fix the gAMA chunk. It is low priority, since all modern apps should just support cICP.

[richardssam]

I think I understand where the gamma 1.98 value is coming from. In quicktime and mp4 containers the rec709 ETOF definition is for the "Camera rec709" value, not bt1886.
There is not any explicit setting (particularly in mp4's) which allow you to set bt1886.

Both camera-rec709 (~1.98) and sRGB (~2.2) are approximations, which cannot be correctly represented by a single gamma value, so its somewhat scary for this to be applied automatically.

I dont know what the right answer here is, although we should at least have the ability to override what it uses (particularly if we know the resulting image is bt1886 and does have a proper gamma of 2.4).

[Balling]

I think I understand where the gamma 1.98 value is coming from.

It is just BT.709 OETF.

I dont know what the right answer here is,

EOTF for OETF BT.709 is 2.4 gamma only. It is clear as day, it should be 2.4 gamma.

[Balling]

From ​https://github.com/mpv-player/mpv/issues/13438

The PNG specification recommends that PNG writers write gAMA chunks even if it writes other color chunks that override it as fallback metadata to poorly color managed PNG viewers. The actual file is tagged correctly with cICP chunk, which overrides the fallback gAMA chunk.

NO, the PNG SPEC v3 does not specify any such chunk for BT.709. So this is an open issue with a standard. Also, Mac does not use 1.9 gamma for EOTF, it also use close to 2.4.