Question

I am currently developing an application that needs to decode a UDP multicast RTSP stream. At the moment, I can view the RTP stream using ffplay via

ffplay -rtsp_transport udp_multicast rtsp://streamURLGoesHere

However, I am trying to use FFMPEG to open the UDP stream via (error checking and cleanup code removed for the sake of brevity).

AVFormatContext* ctxt = NULL;
av_open_input_file(
    &ctxt,
    urlString,
    NULL,
    0,
    NULL
);

av_find_stream_info(ctxt);

AVCodecContext* codecCtxt;

int videoStreamIdx = -1;
for (int i = 0; i < ctxt->nb_streams; i++)
{
    if (ctxt->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
    {
        videoStreamIdx = i;
        break;
    }
}

AVCodecContext* codecCtxt = ctxt->streams[videoStreamIdx]->codec;
AVCodec* codec = avcodec_fine_decoder(codecCtxt->codec_id);
avcodec_open(codecCtxt, codec);

AVPacket packet;
while(av_read_frame(ctxt, &packet) >= 0)
{
    if (packet.stream_index == videoStreamIdx)
    {
        /// Decoding performed here
        ...
    }
}

...

This approach works fine with file inputs that consist of a raw encoded video stream, but for UDP multicast RTSP streams, it fails any error checking performed on av_open_input_file(). Please advise…

Answer 1

It turns out that opening a multicast UDP RTSP stream can be performed via the following:

AVFormatContext* ctxt = avformat_alloc_context();

AVDictionary* options = NULL;
av_dict_set(&options, "rtsp_transport", "udp_multicast", 0);
avformat_open_input(
    &ctxt,
    urlString,
    NULL,
    &options
);

...

avformat_free_context(ctxt);

Using avformat_open_input() in this manner instead of av_open_input_file() results in the desired behavior. I’m guessing that av_open_input_file() is either deprecated or was never intended to be used in this manner — more than likely the latter 😉