Link dynamic pads of demuxer


Demuxers do not have any pads till they receive the buffers to parse. As data is available to parse, pads are dynamically added based on the streams available.

The pad-added signal

The pad-added signal can be used to attach new elements to the pipeline once a new pad gets added. Use the g_signal_connect function to listen for pad-added. In the callback function, you can add new elements to the pipeline and link them to the demuxer based on the name of the pad. If the pad name starts with audio, for instance, you can link the element for audio playback. The state of these new elements needs to set to GST_STATE_PLAYING.

Here’s how you can register a callback for pad-added

g_signal_connect (demux, "pad-added", (GCallback)demux_pad_added, NULL);

Here’s a sample callback function for the matroskademux element

void
demux_pad_added (GstElement* demux, GstPad* pad, gpointer user_data)
{
  char* name;
  GstPad *sinkpad;
  GstElement *tee, *sink;

  name = gst_pad_get_name(pad);

  if (strncmp(name, "audio", 5) == 0)
  {
    // link audio sink pad of demuxer to src pad of audio tee
    tee = gst_element_factory_make ("tee", "audiotee");
    sink = gst_element_factory_make ("fakesink", "audiosink");
    gst_bin_add_many (GST_BIN (pipeline), tee, sink, NULL);
    gst_element_link (tee, sink);
    sinkpad = gst_element_get_static_pad(tee, "sink");
    gst_pad_link(pad, sinkpad);
    gst_object_unref (sinkpad);
    gst_element_set_state(tee, GST_STATE_PLAYING);
    gst_element_set_state(sink, GST_STATE_PLAYING);
    g_print ("Linked pad %s of demuxer\n", name);
  }
  else if (strncmp(name, "video", 5) == 0)
  {
    // link src pad of demuxer to sink pad of video tee
    tee = gst_element_factory_make ("tee", "videotee");
    sink = gst_element_factory_make ("fakesink", "videosink");
    gst_bin_add_many (GST_BIN (pipeline), tee, sink, NULL);
    gst_element_link (tee, sink);
    sinkpad = gst_element_get_static_pad(tee, "sink");
    gst_pad_link(pad, sinkpad);
    gst_object_unref (sinkpad);
    gst_element_set_state(tee, GST_STATE_PLAYING);
    gst_element_set_state (sink, GST_STATE_PLAYING);
    g_print ("Linked pad %s of demuxer\n", name);
  }

  g_free (name);
}

The no-more-pads signal

Another signal that can be used is no-more-pads. You can check for its existence with your version of GStreamer by using gst-inspect e.g. gst-inspect avidemux. In the callback of that signal you can link new elements to the demuxer using gst_element_link_filtered. Call the function once for each caps. The caps parameter required by the function can be created using gst_caps_new_simple e.g. gst_caps_new_simple ("video/x-vp8", NULL). Again, the state of these new elements needs to set to GST_STATE_PLAYING.

Here’s how you can register a callback for no-more-pads

  g_signal_connect (demux, "no-more-pads", (GCallback)demux_no_more_pads, NULL);

Here’s a sample callback function.

void
demux_no_more_pads (GstElement* demux, gpointer user_data)
{
  GstCaps *caps;
  GstElement *tee, *sink;

  tee = gst_element_factory_make ("tee", "videotee");
  sink = gst_element_factory_make ("fakesink", "videosink");
  gst_bin_add_many (GST_BIN (pipeline), tee, sink, NULL);
  gst_element_link (tee, sink);
  caps = gst_caps_new_simple ("video/x-vp8", NULL);
  gst_element_link_filtered (demux, tee, caps);
  gst_element_set_state(tee, GST_STATE_PLAYING);
  gst_element_set_state(sink, GST_STATE_PLAYING);

  tee = gst_element_factory_make ("tee", "audiotee");
  sink = gst_element_factory_make ("fakesink", "audiosink");
  gst_bin_add_many (GST_BIN (pipeline), tee, sink, NULL);
  gst_element_link (tee, sink);
  caps = gst_caps_new_simple ("audio/x-vorbis", NULL);
  gst_element_link_filtered (demux, tee, caps);
  gst_element_set_state(tee, GST_STATE_PLAYING);
  gst_element_set_state(sink, GST_STATE_PLAYING);
}

Debugging

As usual, if you have any issues you need to troubleshoot with your pipeline, you can try setting the environment variable GST_DEBUG to 5. GStreamer and its elements will print copious amounts of information as they execute.

export GST_DEBUG=5

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