[FFmpeg-trac] #9195(avfilter:new): vibrance (video filter) incorrect defaults for luma

[FFmpeg]

#9195: vibrance (video filter) incorrect defaults for luma
-------------------------------------+-------------------------------------
             Reporter:  Jim          |                     Type:  defect
  DeLaHunt                           |
               Status:  new          |                 Priority:  normal
            Component:  avfilter     |                  Version:  git-
                                     |  master
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug: The "vibrance" video filter appears to have incorrect
 default values for the luma coefficient parameters "rlum" and "blum". They
 appear to be swapped.

 How to reproduce:
 {{{
 % ffmpeg -h filter=vibrance
 ffmpeg version 4.4 Copyright (c) 2000-2021 the FFmpeg developers
   built with Apple LLVM version 10.0.0 (clang-1000.10.44.4)
   configuration: --prefix=/opt/local --enable-swscale --enable-avfilter
 --enable-avresample --enable-libmp3lame --enable-libvorbis --enable-
 libopus --enable-librsvg --enable-libtheora --enable-libopenjpeg --enable-
 libmodplug --enable-libvpx --enable-libsoxr --enable-libspeex --enable-
 libass --enable-libbluray --enable-lzma --enable-gnutls --enable-
 fontconfig --enable-libfreetype --enable-libfribidi --disable-libjack
 --disable-libopencore-amrnb --disable-libopencore-amrwb --disable-
 indev=jack --enable-opencl --disable-outdev=xv --enable-audiotoolbox
 --enable-videotoolbox --enable-sdl2 --disable-securetransport
 --mandir=/opt/local/share/man --enable-shared --enable-pthreads
 --cc=/usr/bin/clang --enable-libdav1d --arch=x86_64 --enable-x86asm
 --enable-libx265 --enable-gpl --enable-postproc --enable-libx264 --enable-
 libxvid
   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
 Filter vibrance
   Boost or alter saturation.
     slice threading supported
     Inputs:
        #0: default (video)
     Outputs:
        #0: default (video)
 vibrance AVOptions:
   intensity         <float>      ..FV.....T. set the intensity value (from
 -2 to 2) (default 0)
   rbal              <float>      ..FV.....T. set the red balance value
 (from -10 to 10) (default 1)
   gbal              <float>      ..FV.....T. set the green balance value
 (from -10 to 10) (default 1)
   bbal              <float>      ..FV.....T. set the blue balance value
 (from -10 to 10) (default 1)
   rlum              <float>      ..FV.....T. set the red luma coefficient
 (from 0 to 1) (default 0.072186)
   glum              <float>      ..FV.....T. set the green luma
 coefficient (from 0 to 1) (default 0.715158)
   blum              <float>      ..FV.....T. set the blue luma coefficient
 (from 0 to 1) (default 0.212656)
   alternate         <boolean>    ..FV.....T. use alternate colors (default
 false)

 This filter has support for timeline through the 'enable' option.
 }}}

 The vibrance filter is documented at  https://ffmpeg.org/ffmpeg-
 filters.html#vibrance .

 It looks like coefficients "rlum", "glum", and "blum" are supposed to
 match the luma computation as defined in BT.709-6
 (https://en.wikipedia.org/wiki/Rec._709) or something similar. Wikipedia
 gives this formula as:

 > R’G’B’ coefficients 0.2126, 0.7152, and 0.0722 (together they add to 1)

 So, `luma = 0.2126 * r + 0.7152 * g + 0.0722 * b`, a value in [0.0, 1.0]
 for r, g, b in [0.0, 1.0]

 But the default values I see in the FFmpeg vibrance filter are (per output
 above):

 > rlum (default 0.072186), glum (default 0.715158), blum (default
 0.212656)

 Thus it looks like the default values for rlum and blum are swapped.

 Vibrance would still compute a luma a value in a range of [0.0, 1.0], but
 it wouldn't be the correct value per BT.709. Since the vibrance
 adjustments are relative to luma, they might be slightly off.

 I believe the fix would be the simple, obvious exchange of default values
 in ffmpeg/libavfilter/vf_vibrance.c:372-374. But I haven't tried this.

 Present code:
 {{{
     { "rlum", "set the red luma coefficient",   OFFSET(lcoeffs[2]),
 AV_OPT_TYPE_FLOAT, {.dbl=0.072186}, 0,  1, VF },
     { "glum", "set the green luma coefficient", OFFSET(lcoeffs[0]),
 AV_OPT_TYPE_FLOAT, {.dbl=0.715158}, 0,  1, VF },
     { "blum", "set the blue luma coefficient",  OFFSET(lcoeffs[1]),
 AV_OPT_TYPE_FLOAT, {.dbl=0.212656}, 0,  1, VF },
 }}}

 Code with possible fix:
 {{{
     { "rlum", "set the red luma coefficient",   OFFSET(lcoeffs[2]),
 AV_OPT_TYPE_FLOAT, {.dbl=0.212656}, 0,  1, VF },
     { "glum", "set the green luma coefficient", OFFSET(lcoeffs[0]),
 AV_OPT_TYPE_FLOAT, {.dbl=0.715158}, 0,  1, VF },
     { "blum", "set the blue luma coefficient",  OFFSET(lcoeffs[1]),
 AV_OPT_TYPE_FLOAT, {.dbl=0.072186}, 0,  1, VF },
 }}}
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9195>
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