/// <summary>
/// Returns a suitable audio resample context according to sample format, number of channels, and sample rate.
/// The context returned will be cached until this function is called again with different arguments.
/// </summary>
/// <param name="sampleFormat">Sample format of the output.</param>
/// <param name="channels">Number of channels of the output.</param>
/// <param name="sampleRate">Sample rate of the output (Hz).</param>
/// <returns>Cached or created resample context.</returns>
internal SwrContext *GetSuitableResampleContext(AVSampleFormat sampleFormat, int channels, int sampleRate)
{
EnsureNotDisposed();
Trace.Assert(channels > 0 && sampleRate > 0);
Trace.Assert(channels == FFmpegHelper.RequiredChannels);
var resampleContext = _resampleContext;
if (resampleContext == null || sampleFormat != _lastSampleFormat || channels != _lastChannels || sampleRate != _lastSampleRate)
{
if (resampleContext != null)
{
ffmpeg.swr_close(resampleContext);
ffmpeg.swr_free(&resampleContext);
_resampleContext = null;
}
var codecContext = CodecContext;
var srcChannelLayout = codecContext->channels == ffmpeg.av_get_channel_layout_nb_channels(codecContext->channel_layout) ? codecContext->channel_layout : (ulong)ffmpeg.av_get_default_channel_layout(codecContext->channels);
int dstChannelLayout;
if (channels == 1)
{
dstChannelLayout = ffmpeg.AV_CH_LAYOUT_MONO;
}
else if (channels == 2)
{
dstChannelLayout = ffmpeg.AV_CH_LAYOUT_STEREO;
}
else
{
// There are more kinds of layouts; but we don't actually need them.
dstChannelLayout = ffmpeg.AV_CH_LAYOUT_SURROUND;
}
// There is another function swr_alloc_setopts(), but re-creating doesn't matter,
// since changing parameters is not frequent.
resampleContext = ffmpeg.swr_alloc();
if (resampleContext == null)
{
Dispose();
throw new FFmpegException("Failed to init audio resample context.");
}
// Set options.
FFmpegHelper.Verify(ffmpeg.av_opt_set_int(resampleContext, "in_channel_layout", (int)srcChannelLayout, 0), Dispose);
FFmpegHelper.Verify(ffmpeg.av_opt_set_int(resampleContext, "in_sample_rate", codecContext->sample_rate, 0), Dispose);
FFmpegHelper.Verify(ffmpeg.av_opt_set_sample_fmt(resampleContext, "in_sample_fmt", codecContext->sample_fmt, 0), Dispose);
FFmpegHelper.Verify(ffmpeg.av_opt_set_int(resampleContext, "out_channel_layout", dstChannelLayout, 0), Dispose);
FFmpegHelper.Verify(ffmpeg.av_opt_set_int(resampleContext, "out_sample_rate", sampleRate, 0), Dispose);
FFmpegHelper.Verify(ffmpeg.av_opt_set_sample_fmt(resampleContext, "out_sample_fmt", sampleFormat, 0), Dispose);
// Don't forget to initialize the context. This is the difference between SWR and SWS (sws_getContext).
FFmpegHelper.Verify(ffmpeg.swr_init(resampleContext));
_resampleContext = resampleContext;
_lastSampleFormat = sampleFormat;
_lastChannels = channels;
_lastSampleRate = sampleRate;
}
return(_resampleContext);
}