[FFmpeg-trac] #8919(avfilter:new): Setting 'fscale=log' together w/ 'start>0' will mess up the scaling of the y-axis in showspectrum(pic)

[FFmpeg]

#8919: Setting 'fscale=log' together w/ 'start>0' will mess up the scaling of the
y-axis in showspectrum(pic)
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             Reporter:  rkleinert    |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avfilter     |                  Version:  git-
             Keywords:               |  master
  showspectrum, showspectrumpic      |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 == Summary of the bug: ==
 In the 'showspectrum' and 'showspectrumpic'-functions you can choose to
 scale the y-axis (frequencies) either linear or logarithmic (parameter
 'fscale'). Also you can define the range of the y-axis (parameters 'start'
 and 'stop'). If you choose logarithmic scaling and start at y=0Hz
 ('start=0') the y-axis is scaled correctly; i.e. the proportions from one
 mark on the axis to the next in equal distances stays constant. If you
 however choose anything else than 'start=0' the scaling is entirely wrong.
 The ratio between the individual, equidistant marks on the y-axis will
 start somewhere above 2 at the higher values and decrease to 1 further
 down at the bottom of the axis.

 == TL;DR: ==
 Setting parameter 'fscale=log' together with 'start>0'will mess up the
 scaling of the y-axis in showspectrum(pic)

 == How to reproduce: ==
 The exact way of calling the function doesn't seem to matter. It's just
 about the situation explained above. I used this example to verify:

 {{{
 #!div style="font-size: 80%"
 Sample Code:
   ffmpeg -i input.wav -filter_complex
 "showspectrumpic=size=700x400:mode=combined:color=rainbow:scale=sqrt:fscale=log:legend=1:start=500:stop=20000"
 output.png
 }}}

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Ticket URL: <https://trac.ffmpeg.org/ticket/8919>
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