// 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();
        }