Dynamic pipelines

<chapter id="cha-dynamic">
  <title>Dynamic pipelines</title>
  <para> 
    this chapter we will see how you can create a dynamic pipleine. A dynamic
    pipeline is a pipeline that is updated or created while media is flowing 
    through it. We will create a partial pipeline first and add more elements
    while the pipeline is playing. Dynamic pipelines cause all sorts of
    scheduling issues and will remain a topic of research for a long time in
    GStreamer.
  </para>
  <para> 
    We will show how to create an mpeg1 video player using dynamic pipelines.
    As you have seen in the pad section, we can attach a signal to an element
    when a pad is created. We will use this to create our MPEG1 player.
  </para>

  <para> 
    We'll start with a simple main function:
  </para>
  <programlisting>
#include &lt;gnome.h&gt;
#include &lt;gst/gst.h&gt;

void eof(GstElement *src) {
  g_print("have eos, quitting\n");
  exit(0);
}

gboolean 
idle_func (gpointer data) 
{
  gst_bin_iterate (GST_BIN (data));
  return TRUE;
}

int 
main(int argc, char *argv[]) 
{
  GstElement *pipeline, *src, *demux;
  struct poptOption *gst_table;

  gst_init (&amp;argc, &amp;argv);

  pipeline = gst_pipeline_new ("pipeline");
  g_return_val_if_fail (pipeline != NULL, -1);

  src = gst_element_factory_make ("filesrc", "src");
  g_return_val_if_fail (src != NULL, -1);
  g_object_set (G_OBJECT (src), "location", argv[1], NULL);

  demux = gst_element_factory_make ("mpegdemux", "demux");
  g_return_val_if_fail (demux != NULL, -1);

  gst_bin_add_many (GST_BIN (pipeline), src, demux, NULL);

  g_signal_connect (G_OBJECT (demux), "new_pad",
                     G_CALLBACK (new_pad_created), pipeline);

  g_signal_connect (G_OBJECT (src), "eos",
                     G_CALLBACK (eof), NULL);

  gst_element_connect (src, parse);

  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);

  g_idle_add (idle_func, pipeline);

  gdk_threads_enter ();
  gst_main ();
  gdk_threads_leave ();

  return 0;
}

  </programlisting>
  <para>
    We create two elements: a file source and an MPEG demuxer.. We also add an EOS (End Of Stream)
    signal to the filesrc so that we will be notified when the file has ended. There's nothing
    special about this piece of code except for the signal 'new_pad' that we connected to the
    mpegdemux using:
  </para>
  <programlisting>
  g_signal_connect (G_OBJECT (demux), "new_pad",
                     G_CALLBACK (new_pad_created), pipeline);
  </programlisting>
  <para> 
    When an elementary stream has been detected in the system stream,
    mpegdemux will create a new pad that will provide the data of the
    elementary stream. A function 'new_pad_created' will be called when
    the pad is created:
  </para>
  <programlisting>
void 
new_pad_created (GstElement *parse, GstPad *pad, GstElement *pipeline) 
{
  GstElement *decode_audio, *parse_video, *decode_video, *play, *videoscale, *show;
  GstElement *audio_queue, *video_queue;
  GstElement *audio_thread, *video_thread;

  g_print ("***** a new pad &percnt;s was created\n", gst_pad_get_name (pad));
  
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PAUSED);

  // connect to audio pad
  if (strncmp (gst_pad_get_name (pad), "audio_", 6) == 0) {

    // construct internal pipeline elements
    decode = gst_element_factory_make ("mad", "decode_audio");
    g_return_if_fail (decode != NULL);
    play = gst_element_factory_make ("osssink", "play_audio");
    g_return_if_fail (play != NULL);

    // create the thread and pack stuff into it
    audio_thread = gst_thread_new ("audio_thread");
    g_return_if_fail (audio_thread != NULL);
    gst_bin_add_many (GST_BIN (audio_thread), decode_audio, play, NULL);

    // set up pad connections
    gst_element_add_ghost_pad (GST_ELEMENT (audio_thread),
                               gst_element_get_pad (decode_audio, "sink"));
    gst_element_connect (decode, play);

    // construct queue and connect everything in the main pipelie
    audio_queue = gst_element_factory_make ("queue", "audio_queue");

    gst_bin_add_many (GST_BIN (pipeline), audio_queue, audio_thread, NULL);

    gst_pad_connect (pad, gst_element_get_pad (audio_queue, "sink"));
    gst_element_connect (audio_queue, audio_thread);

    // set up thread state and kick things off
    g_print ("setting to READY state\n");
    gst_element_set_state (GST_ELEMENT (audio_thread), GST_STATE_READY);

  } 
  else if (strncmp (gst_pad_get_name (pad), "video_", 6) == 0) {

    // construct internal pipeline elements
    decode_video = gst_element_factory_make ("mpeg2dec", "decode_video");
    g_return_if_fail (decode_video != NULL);

    show = gst_element_factory_make ("xvideosink", "show");
    g_return_if_fail (show != NULL);

    // create the thread and pack stuff into it
    video_thread = gst_thread_new ("video_thread");
    g_return_if_fail (video_thread != NULL);
    gst_bin_add_many (GST_BIN (video_thread), decode_video, show, NULL);

    // set up pad connections
    gst_element_add_ghost_pad (GST_ELEMENT (video_thread),
                               gst_element_get_pad (parse_video, "sink"));
    gst_element_connect (decode_video, show);

    // construct queue and connect everything in the main pipeline
    video_queue = gst_element_factory_make ("queue", "video_queue");

    gst_bin_add_many (GST_BIN (pipeline), video_queue, video_thread);

    gst_pad_connect (pad, gst_element_get_pad (video_queue, "sink"));
    gst_element_connect (video_queue, video_thread);

    // set up thread state and kick things off
    g_print ("setting to READY state\n");
    gst_element_set_state (GST_ELEMENT (video_thread), GST_STATE_READY);
  }
  g_print("\n");
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
}
  </programlisting>
  <para>
    In the above example, we created new elements based on the name of the newly created pad. We
    then added them to a new thread. There are other possibilities to check the type of the pad, for
    example by using the MIME type and the properties of the pad.
  </para>
</chapter>