Claimed unintentional channel layout overwritten resulting no sound

[solomoncyj]

Summary of the bug:
when i was encoding a dts 5.1 ch audio that had the layout of C L R Ls Rs LFE , file using ffmpeg via libopus , i found that the resulting file did not play in vlc, due to the channel layout being rewritten to L R C Lb Rb LFE. however if downmixed to stereo, there would be sound. FFmpeg should retain the channel layout, or copy the data in the channel to the correct one in the output file

How to reproduce:

% ffmpeg -i out.dts -c:a libopus -b:a 160k -af "channelmap=channel_layout=5.1" out.opus
ffmpeg version 6.1.1
built on dnf rpm fusion

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.

[solomoncyj]

logs

[solomoncyj]

errors were produce by the library (presumably) when i did not remap the channels

`
[libopus @ 0x561fbbfb0740] Invalid channel layout 5.1(side) for specified mapping family -1.
[aost#0:0/libopus @ 0x561fbbfb0480] Error while opening encoder - maybe incorrect parameters such as bit_rate, rate, width or height.
Error while filtering: Invalid argument
[out#0/opus @ 0x561fbbfad6c0] Nothing was written into output file, because at least one of its streams received no packets.
`

[MasterQuestionable]

͏    `-af "channelmap=channel_layout=5.1"`..?
͏    Supposedly you instructed it?
͏    "channel layout being rewritten"

͏    See also: https://ffmpeg.org/ffmpeg-filters.html#channelmap
͏    And I guess irrelevant of Opus.

[solomoncyj]

well, it did not rewrite it correctly, as it should have mapped channel L -> L for example, however it mapped L -> C

[MasterQuestionable]

͏    Make sure you specify the filter correctly, as intended.
͏    I scarcely use anything other than Mono/Stereo...