// 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(); }