Support Opus Ambisonics playback

media/ffmpeg/ffmpeg_common.cc

Index: media/ffmpeg/ffmpeg_common.cc
diff --git a/media/ffmpeg/ffmpeg_common.cc b/media/ffmpeg/ffmpeg_common.cc
index 950b9c0fd103672103d65048a7ad47ab48b89ee7..4453b4cef9cc1977aa777cf83629ed74f7f151df 100644
--- a/media/ffmpeg/ffmpeg_common.cc
+++ b/media/ffmpeg/ffmpeg_common.cc
@@ -324,8 +324,16 @@ bool AVCodecContextToAudioDecoderConfig(
   SampleFormat sample_format = AVSampleFormatToSampleFormat(
       codec_context->sample_fmt, codec_context->codec_id);
 
-  ChannelLayout channel_layout = ChannelLayoutToChromeChannelLayout(
-      codec_context->channel_layout, codec_context->channels);
+  bool is_opus_discrete = false;
+  if (codec == kCodecOpus && codec_context->extradata_size >= 19) {
+    int mapping_family = codec_context->extradata[18];
+    is_opus_discrete = mapping_family == 2;
+  }
+  ChannelLayout channel_layout =
+      is_opus_discrete && codec_context->channels > 8

[DaleCurtis, 2017/05/24 00:49:58]

Did you ever figure out if this > 8 check could be dropped? Or does it not
matter for your use case?

[flim-chromium, 2017/05/25 00:04:25]

I'm not familiar enough with the potential problems (e.g. in OSX that you
pointed out before) to drop this. 

Anyway, I think it's actually useful to keep this such that for <= 8ch, the
default up/downmixing is applied if a webaudio renderer isn't attached. It
wouldn't sound spatially correct but that's to be expected, and at least we'd
get to preview the audio content. Potentially a future CL could look at some
saner up/downmixing matrices, and also for >8 ch ambisonics. wdyt?

[DaleCurtis, 2017/05/25 00:29:16]

Leaving sgtm for now, though I think WebAudio doesn't want channel mixing, so if
we could always skip it when WebAudio is connected that is fine.

+          ? CHANNEL_LAYOUT_DISCRETE
+          : ChannelLayoutToChromeChannelLayout(codec_context->channel_layout,
+                                               codec_context->channels);
 
   int sample_rate = codec_context->sample_rate;
   switch (codec) {
@@ -371,6 +379,7 @@ bool AVCodecContextToAudioDecoderConfig(
   config->Initialize(codec, sample_format, channel_layout, sample_rate,
                      extra_data, encryption_scheme, seek_preroll,
                      codec_context->delay);
+  config->SetChannelsForDiscrete(codec_context->channels);

[DaleCurtis, 2017/05/24 00:49:58]

Probably you don't want to always do this. channels from ffmpeg is not reliable.
Only with layout == discrete should this be called I think.

[flim-chromium, 2017/05/25 00:04:26]

Right, I ran into this problem with the ffmpeg tests, hence the change in
SetChannelsForDiscrete. Your way makes sense, I've changed it back now.

 
 #if BUILDFLAG(ENABLE_AC3_EAC3_AUDIO_DEMUXING)
   // These are bitstream formats unknown to ffmpeg, so they don't have
@@ -419,8 +428,7 @@ void AudioDecoderConfigToAVCodecContext(const AudioDecoderConfig& config,
 
   // TODO(scherkus): should we set |channel_layout|? I'm not sure if FFmpeg uses
   // said information to decode.
-  codec_context->channels =
-      ChannelLayoutToChannelCount(config.channel_layout());
+  codec_context->channels = config.channels();
   codec_context->sample_rate = config.samples_per_second();
 
   if (config.extra_data().empty()) {