[FFmpeg-devel] Channel layouts (was: [PATCH] lavfi: store and propagate number of channels information in audio buffer properties)

Nicolas George nicolas.george at normalesup.org
Sun Nov 11 21:10:37 CET 2012

On 2012-11-11, I wrote:

Short version:

For streams with strange channel layouts, we need an immediate workaround
and a long-term solution.

Immediate workaround I just thought of:

#define AV_CH_STRANGE_UNKNOWN        0x0000800000000000ULL

(this bit is far away from any other already defined channel)

With the rule: if this channel is set in a channel layout, then the channel
layout as a whole is "strange/unknown". If we need to define a n-channels
strange/unknown layout, we just make one using this bit and n-1 other bits
(using preferably unused bits).

Then we can have:

int av_fix_channel_layout(void *log, int count, uint64_t *layout)
    if (count != av_get_channel_layout_nb_channels(*layout)) {
	av_log(log, AV_LOG_WARNING, "...");
	*layout = 0;
    if (!*layout)
	layout = av_make_strange_channel_layout(count);

and use it all over the place.

Clearly, it is a hack, but I believe it is a robust one that does not cause
any problem.

And it is not a long-term solution, since it can not express some
information that the demuxer or decoder can have, like "these 2 channels are
not stereo, they are mono/Japanese + mono/English". Looks like we will need
per-channel metadata, yay!


  Nicolas George
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