[FFmpeg-trac] #7949(undetermined:new): libopus ocl=downmix fails

[FFmpeg]

#7949: libopus ocl=downmix fails
-------------------------------------+-------------------------------------
             Reporter:  omniplex     |                     Type:
                                     |  enhancement
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:  4.1
  undetermined                       |
             Keywords:  OPUS         |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug: unlike libtheora the libopus default
 [http://ffmpeg.org/ffmpeg-codecs.html#libopus -mapping_family -1] does not
 permit [https://ffmpeg.org/ffmpeg-utils.html#channel-layout-syntax
 ocl=downmix]. How to reproduce:

 {{{
 % ffmpeg -i input -vn -filter:a
 aresample=48000:resampler=soxr:precision=28:ocl=downmix:cheby=1:matrix_encoding=dolby:tsf=s32p
 -c:a libopus -b:a 25KiB output

 ffmpeg version 4.1.3 built with gcc 8.3.1 (GCC) 20190414
 }}}

 The same command with [http://ffmpeg.org/ffmpeg-codecs.html#libopus
 -mapping_family 255] works.

 Maybe it is only me, but I expected [https://ffmpeg.org/ffmpeg-utils.html
 #channel-layout-syntax ocl=downmix] to work for ordinary MP3 stereo input
 while upgrading a script from VP8/libtheora to VP9/libopus. It's no bug,
 but had a [https://en.wikipedia.org/wiki/Principle_of_least_astonishment
 high astonishment factor].

--
Ticket URL: <https://trac.ffmpeg.org/ticket/7949>
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