// sets up libavformat state: creates the AVFormatContext, the frames, etc. to start decoding, but does not actually start the decodingLoop
private void prepareDecoding()
{
const int context_buffer_size = 4096;
// the first call to FFmpeg will throw an exception if the libraries cannot be found
// this will be safely handled in StartDecoding()
var fcPtr = ffmpeg.avformat_alloc_context();
formatContext = fcPtr;
contextBuffer = (byte *)ffmpeg.av_malloc(context_buffer_size);
managedContextBuffer = new byte[context_buffer_size];
readPacketCallback = readPacket;
seekCallback = seek;
formatContext->pb = ffmpeg.avio_alloc_context(contextBuffer, context_buffer_size, 0, (void *)handle.Handle, readPacketCallback, null, seekCallback);
int openInputResult = ffmpeg.avformat_open_input(&fcPtr, "dummy", null, null);
inputOpened = openInputResult >= 0;
if (!inputOpened)
{
throw new InvalidOperationException($"Error opening file or stream: {getErrorMessage(openInputResult)}");
}
int findStreamInfoResult = ffmpeg.avformat_find_stream_info(formatContext, null);
if (findStreamInfoResult < 0)
{
throw new InvalidOperationException($"Error finding stream info: {getErrorMessage(findStreamInfoResult)}");
}
var nStreams = formatContext->nb_streams;
for (var i = 0; i < nStreams; ++i)
{
stream = formatContext->streams[i];
codecParams = *stream->codecpar;
if (codecParams.codec_type == AVMediaType.AVMEDIA_TYPE_VIDEO)
{
duration = stream->duration <= 0 ? formatContext->duration : stream->duration;
timeBaseInSeconds = stream->time_base.GetValue();
var codecPtr = ffmpeg.avcodec_find_decoder(codecParams.codec_id);
if (codecPtr == null)
{
throw new InvalidOperationException($"Couldn't find codec with id: {codecParams.codec_id}");
}
int openCodecResult = ffmpeg.avcodec_open2(stream->codec, codecPtr, null);
if (openCodecResult < 0)
{
throw new InvalidOperationException($"Error trying to open codec with id {codecParams.codec_id}: {getErrorMessage(openCodecResult)}");
}
break;
}
}
prepareFilters();
}