[FFmpeg-user] Why histogram filter mode levels cannot use h|vstack filter inputed rgb(a) source?

[nicolab]

I am using  histogram <https://ffmpeg.org/ffmpeg-filters.html#histogram>  
filter mode levels is error but color, color2 and waveform mode is OK.

ffmpeg-20150928-git-1d0487f-win64-static\bin>ffplay -f lavfi -i mandelbrot
-vf split[1][2];\
[1]histogram=mode=levels[1a];[2]histogram=mode=levels[2a];[1a][2a]hstack

ffplay version N-75573-g1d0487f Copyright (c) 2003-2015 the FFmpeg
developers
  built with gcc 4.9.3 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads
--enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r
--enable-gnutls --enable-iconv --enable-libass --enable-libbluray
--enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype
--enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug
--enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb
--enable-libopenjpeg --enable-libopus --enable-librtmp
--enable-libschroedinger --enable-libsoxr --enable-libspeex
--enable-libtheora --enable-libtwolame --enable-libvidstab
--enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis
--enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264
--enable-libx265 --enable-libxavs --enable-libxvid --enable-lzma
--enable-decklink --enable-zlib
  libavutil      55.  2.100 / 55.  2.100
  libavcodec     57.  3.100 / 57.  3.100
  libavformat    57.  2.100 / 57.  2.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6.  9.100 /  6.  9.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, lavfi, from 'mandelbrot':0KB vq=    0KB sq=    0B f=0/0
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo (RGBA / 0x41424752), rgba, 640x480 [SAR 1:1
DAR 4:3], 25 tbr, 25 tbn, 25 tbc
[AVFilterGraph @ 00000083ab043620] The following filters could not choose
their formats: Parsed_histogram_1, Parsed_histogram_2
Consider inserting the (a)format filter near their input or output

These commands are OK.
ffplay -f lavfi -i color -vf
split[1][2];[1]histogram=mode=levels[1a];[2]histogram=mode=levels[2a];[1a][2a]hstack
ffplay -f lavfi -i mandelbrot -vf
format=yuv444p,split[1][2];[1]histogram=mode=levels[1a];[2]histogram=mode=levels[2a];[1a][2a]hstack

Thanks in advance.



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