[FFmpeg-user] HDR bt.2020 to SDR bt.709 conversion

[pehache]

Hello,

I have trouble converting a 4K HDR video into a HD SDR one. Basically it 
plays much too dark and reddish in VLC (on macOS). The ffprobe output is 
this video is at the bottom of the post

I first tried applying a curve filter to boost the dark tones. I works 
not too bad, but the reddish shift is still, if not amplified.

Then I browsed the web and found a filter set that looks popular (I mean 
copied/pasted on many different pages):

-vf zscale=t=linear:npl=100,format=gbrpf32le,zscale=p=bt709, \
tonemap=tonemap=hable,zscale=t=bt709:m=bt709:r=tvformat=yuv420p

But, well, it doesn't solve anything... Using "gamma" instead of "hable" 
gives a higher luminosity, but there's still a strong red shift.

I tried many different things with the zscale filter, but the problem is 
that I don't fully understand the effect of the different parameters, 
although I'm not completly unfamiliar with the color profiles. It seems 
to me that all of this deals mainly the luminosity, and little with the 
chrominance.

I have also found a page that describes how to convert from the bt.2020 
to bt709 profile, with a link to download a LUT file.
https://www.binarytides.com/color-grading-hlg-videos-with-ffmpeg/
https://nopixels.net/articles/hlg-to-rec709/

So I tried the filter lut3d=nopixels_net_hlg2020_to_rec709.cube, with 
various combinations of the zscale filter, but could not obtain anything 
correct at the end. I always have reddish, or very reddish, or pinkish, 
color shifts...


I'm a bit lost, now... Any help appreciated !



===============================================================
*****$ ffprobe in0.mkv
ffprobe version 4.4.4 Copyright (c) 2007-2023 the FFmpeg developers
   built with Apple LLVM version 9.1.0 (clang-902.0.39.2)
   configuration: --prefix=/opt/local --cc=/usr/bin/clang 
--mandir=/opt/local/share/man --enable-audiotoolbox --disable-indev=jack 
--disable-libjack --disable-libopencore-amrnb 
--disable-libopencore-amrwb --disable-libxcb --disable-libxcb-shm 
--disable-libxcb-xfixes --enable-opencl --disable-outdev=xv 
--enable-sdl2 --disable-securetransport --enable-videotoolbox 
--enable-avfilter --enable-avresample --enable-fontconfig 
--enable-gnutls --enable-libass --enable-libbluray --enable-libdav1d 
--enable-libfreetype --enable-libfribidi --enable-libmodplug 
--enable-libmp3lame --enable-libopenjpeg --enable-libopus 
--enable-librsvg --enable-libsoxr --enable-libspeex --enable-libtheora 
--enable-libvorbis --enable-libvpx --enable-libzimg --enable-libzvbi 
--enable-lzma --enable-pthreads --enable-shared --enable-swscale 
--enable-zlib --enable-libaom --enable-libsvtav1 --arch=x86_64 
--enable-x86asm --enable-gpl --enable-libvidstab --enable-libx264 
--enable-libx265 --enable-libxvid --enable-postproc
   libavutil      56. 70.100 / 56. 70.100
   libavcodec     58.134.100 / 58.134.100
   libavformat    58. 76.100 / 58. 76.100
   libavdevice    58. 13.100 / 58. 13.100
   libavfilter     7.110.100 /  7.110.100
   libavresample   4.  0.  0 /  4.  0.  0
   libswscale      5.  9.100 /  5.  9.100
   libswresample   3.  9.100 /  3.  9.100
   libpostproc    55.  9.100 / 55.  9.100
Input #0, matroska,webm, from 'in0.mkv':
   Metadata:
     encoder         : libebml v1.4.0 + libmatroska v1.6.2
     creation_time   : 2021-03-09T20:53:48.000000Z
     Writing frontend: StaxRip v1.7.0.3
   Duration: 00:49:10.18, start: 0.000000, bitrate: 4963 kb/s
   Chapters:
[...]
   Stream #0:0: Video: hevc (Main 10), yuv420p10le(tv, 
bt2020nc/bt2020/smpte2084), 3840x2160, SAR 1:1 DAR 16:9, 23.98 fps, 
23.98 tbr, 1k tbn, 23.98 tbc (default)
     Metadata:
       BPS-eng         : 4003363
       DURATION-eng    : 00:49:09.197000000
       NUMBER_OF_FRAMES-eng: 70710
       NUMBER_OF_BYTES-eng: 1475838552
       _STATISTICS_WRITING_APP-eng: mkvmerge v50.0.0 ('Awakenings') 64-bit
       _STATISTICS_WRITING_DATE_UTC-eng: 2021-03-09 20:53:48
       _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES
[...]