Check Vorbis channel maps.

[icculus]

Apparently Vorbis uses a different channel layout than SDL; check the state of this.

[sezero]

Maybe wavpack too, while you're there..

[dkosmari]

I made a nice big table, with links to references, in remap_channels.c.

I'm going to paste here the note I wrote about WavPack:

Note: WavPack documentation claims that ALL MS/USB channels (up to the max) must be
present. So to contain all the 7.1 channels, a .wv file must have 11 channels, with
silent FLC, FRC, BC/RC channels.

I have not verified if that's the case in practice, with any WavPack-compatible software.

MS Wave/USB audio requires the channels to appear in a specific order, but any number of channels can be absent. So a wave file could have two channels, with the dwChannelMask bits set to SPEAKER_BACK_LEFT | SPEAKER_BACK_RIGHT, and those must be mapped to rear left/right, not front left/right.

There's also the issue of FLAC and Vorbis having FC when SDL expects LFE, in some modes. Should the LFE just be silent? Should the FC be remixed to FL/FR, or discarded?

It's better to first focus on only supporting automatic remapping for mono, stereo, 5.1 and 7.1 modes, since at least all file formats agree on what channels should be present, even if in a different order. That will cover 99% of the use cases anyway.

[icculus]

There's also the issue of FLAC and Vorbis having FC when SDL expects LFE, in some modes. Should the LFE just be silent? Should the FC be remixed to FL/FR, or discarded?

To answer this, this is only a problem on 3 and 5 channel audio files, and here's what we do: LFE is silent, we tell it we have 5.1 audio, map FL, FR, FC appropriately, mark the other channels as unused. Since libvorbis gives us planar data, we just give it the three|five channel arrays and SDL knows to throw the rest away without interpolating it into the audio stream. Everything else either works without channel mapping or has a direct mapping.

(This doesn't work for the Tremor decoder, which gives us interpolated data that we would have to weave silence into, but my current attitude is that these files are likely rare in the first place, and doubly so if you needed to use Tremor on a low-powered device. In those cases, we currently feed the wrong channel order through. It has a FIXME. Every channel layout but 3 and 5 channels works with Tremor, though.)

[icculus]

WAV support is in. This was messy (and I suspect, probably unnecessary in the real world), but now we're more robust.

Considering calling this Good Enough for 3.2.0 and revisiting this in 3.2.2 for WavPack, FLAC, and stb_vorbis.