[FFmpeg-trac] #2859(swresample:open): -ac downmix fails.

[FFmpeg]

#2859: -ac downmix fails.
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             Reporter:  Mista_D     |                    Owner:
                 Type:  defect      |                   Status:  open
             Priority:  important   |                Component:  swresample
              Version:  git-master  |               Resolution:
             Keywords:  regression  |               Blocked By:
             Blocking:              |  Reproduced by developer:  1
Analyzed by developer:  0           |
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Comment (by cehoyos):

 Replying to [comment:2 Cigaes]:
 > I confirm the downmixing is at fault, not the sample.

 (Any doubts?)

 > It can be observed using the following command:
 > {{{./tools/make_chlayout_test FL+FR+FC+LFE+BL+BR+DL+DR -ac 2
 /tmp/2ch.wav}}}
 > The resulting file says "front left, front right", but should be saying
 "downmix left, downmix right".

 Thank you for pointing to make_chlayout_test (I haven't used it so far), I
 still have a slight preference for real-world samples.
 But assuming make_chlayout_test calls swresample, I wonder if you are
 right:
 {{{$ ffmpeg -i 5.1plusdownmix.wav -ac 2 out.wav}}} should definitely
 output FL+FR imo (not DL+DR).

 > The commit you are pointing seems to be doing half the work: when
 converting to stereo from a layout that has DL+DR, the matrix should just
 map DL to FL and DR to FR and drop the rest.

 Imo, it should only do that if mono or stereo output was requested (and
 the user should be able to either force or forbid using DL+DR to output
 FL+FR or FC instead of 5.1).
 The original commit was meant for DL+DR output, I still wonder what the
 exact use-case was. (Probably {{{ffmpeg -i input -channel_layout DL+DR
 out.wav}}} but I wonder if this is really useful.)

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Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2859#comment:3>
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