docs/manual/advanced-autoplugging.xml

<chapter id="chapter-autoplugging">
  <title>Autoplugging</title>
  <para>
    In <xref linkend="chapter-helloworld"/>, you've learned to build a
    simple media player for Ogg/Vorbis files. By using alternative elements,
    you are able to build media players for other media types, such as
    Ogg/Speex, MP3 or even video formats. However, you would rather want
    to build an application that can automatically detect the media type
    of a stream and automatically generate the best possible pipeline
    by looking at all available elements in a system. This process is called
    autoplugging, and &GStreamer; contains high-quality autopluggers. If
    you're looking for an autoplugger, don't read any further and go to
    <xref linkend="chapter-playback-components"/>. This chapter will explain the
    <emphasis>concept</emphasis> of autoplugging and typefinding. It will
    explain what systems &GStreamer; includes to dynamically detect the
    type of a media stream, and how to generate a pipeline of decoder
    elements to playback this media. The same principles can also be used
    for transcoding. Because of the full dynamicity of this concept,
    &GStreamer; can be automatically extended to support new media types
    without needing any adaptations to its autopluggers.
  </para>
  <para>
    We will first introduce the concept of Media types as a dynamic and
    extendible way of identifying media streams. After that, we will introduce
    the concept of typefinding to find the type of a media stream. Lastly,
    we will explain how autoplugging and the &GStreamer; registry can be
    used to setup a pipeline that will convert media from one mediatype to
    another, for example for media decoding.
  </para>

  <sect1 id="section-media">
    <title>Media types as a way to identify streams</title>
    <para>
      We have previously introduced the concept of capabilities as a way
      for elements (or, rather, pads) to agree on a media type when
      streaming data from one element to the next (see <xref
      linkend="section-caps"/>). We have explained that a capability is
      a combination of a media type and a set of properties. For most
      container formats (those are the files that you will find on your
      hard disk; Ogg, for example, is a container format), no properties
      are needed to describe the stream. Only a media type is needed. A
      full list of media types and accompanying properties can be found
      in <ulink type="http"
      url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/section-types-definitions.html">the
      Plugin Writer's Guide</ulink>.
    </para> 
    <para>
      An element must associate a media type to its source and sink pads
      when it is loaded into the system. &GStreamer; knows about the
      different elements and what type of data they expect and emit through
      the &GStreamer; registry. This allows for very dynamic and extensible
      element creation as we will see.
    </para> 

    <para>
      In <xref linkend="chapter-helloworld"/>, we've learned to build a
      music player for Ogg/Vorbis files. Let's look at the media types
      associated with each pad in this pipeline. <xref
      linkend="section-mime-img"/> shows what media type belongs to each
      pad in this pipeline.
    </para>

    <figure float="1" id="section-mime-img">
      <title>The Hello world pipeline with media types</title>
      <mediaobject>
        <imageobject>
          <imagedata scale="75" fileref="images/mime-world.&image;" format="&IMAGE;"/>
        </imageobject>
      </mediaobject>
    </figure>

    <para>
      Now that we have an idea how &GStreamer; identifies known media
      streams, we can look at methods &GStreamer; uses to setup pipelines
      for media handling and for media type detection.
    </para>
  </sect1>

  <sect1 id="section-typefinding">
    <title>Media stream type detection</title>
    <para>
      Usually, when loading a media stream, the type of the stream is not
      known. This means that before we can choose a pipeline to decode the
      stream, we first need to detect the stream type. &GStreamer; uses the
      concept of typefinding for this. Typefinding is a normal part of a
      pipeline, it will read data for as long as the type of a stream is
      unknown. During this period, it will provide data to all plugins
      that implement a typefinder. When one of the typefinders recognizes
      the stream, the typefind element will emit a signal and act as a
      passthrough module from that point on. If no type was found, it will
      emit an error and further media processing will stop.
    </para>
    <para>
      Once the typefind element has found a type, the application can
      use this to plug together a pipeline to decode the media stream.
      This will be discussed in the next section.
    </para>
    <para>
      Plugins in &GStreamer; can, as mentioned before, implement typefinder
      functionality. A plugin implementing this functionality will submit
      a media type, optionally a set of file extensions commonly used for this
      media type, and a typefind function. Once this typefind function inside
      the plugin is called, the plugin will see if the data in this media
      stream matches a specific pattern that marks the media type identified
      by that media type. If it does, it will notify the typefind element of
      this fact, telling which mediatype was recognized and how certain we
      are that this stream is indeed that mediatype. Once this run has been
      completed for all plugins implementing a typefind functionality, the
      typefind element will tell the application what kind of media stream
      it thinks to have recognized.
    </para>
    <para>
      The following code should explain how to use the typefind element.
      It will print the detected media type, or tell that the media type
      was not found. The next section will introduce more useful behaviours,
      such as plugging together a decoding pipeline.
    </para>
    <programlisting><!-- example-begin typefind.c a -->
#include &lt;gst/gst.h&gt;
<!-- example-end typefind.c a -->
[.. my_bus_callback goes here ..]<!-- example-begin typefind.c b --><!--
static gboolean
my_bus_callback (GstBus     *bus,
                 GstMessage *message,
                 gpointer    data)
{
  GMainLoop *loop = data;

  switch (GST_MESSAGE_TYPE (message)) {
    case GST_MESSAGE_ERROR: {
      GError *err;
      gchar *debug;

      gst_message_parse_error (message, &amp;err, &amp;debug);
      g_print ("Error: %s\n", err-&gt;message);
      g_error_free (err);
      g_free (debug);

      g_main_loop_quit (loop);
      break;
    }
    case GST_MESSAGE_EOS:
      /* end-of-stream */
      g_main_loop_quit (loop);
      break;
    default:
      break;
  }

  /* remove from queue */
  return TRUE;
}
--><!-- example-end typefind.c b -->
<!-- example-begin typefind.c c -->
static gboolean
idle_exit_loop (gpointer data)
{
  g_main_loop_quit ((GMainLoop *) data);

  /* once */
  return FALSE;
}

static void
cb_typefound (GstElement *typefind,
	      guint       probability,
	      GstCaps    *caps,
	      gpointer    data)
{
  GMainLoop *loop = data;
  gchar *type;

  type = gst_caps_to_string (caps);
  g_print ("Media type %s found, probability %d%%\n", type, probability);
  g_free (type);

  /* since we connect to a signal in the pipeline thread context, we need
   * to set an idle handler to exit the main loop in the mainloop context.
   * Normally, your app should not need to worry about such things. */
  g_idle_add (idle_exit_loop, loop);
}

gint 
main (gint   argc,
      gchar *argv[])
{
  GMainLoop *loop;
  GstElement *pipeline, *filesrc, *typefind, *fakesink;
  GstBus *bus;

  /* init GStreamer */
  gst_init (&amp;argc, &amp;argv);
  loop = g_main_loop_new (NULL, FALSE);

  /* check args */
  if (argc != 2) {
    g_print ("Usage: %s &lt;filename&gt;\n", argv[0]);
    return -1;
  }

  /* create a new pipeline to hold the elements */
  pipeline = gst_pipeline_new ("pipe");

  bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
  gst_bus_add_watch (bus, my_bus_callback, NULL);
  gst_object_unref (bus);

  /* create file source and typefind element */
  filesrc = gst_element_factory_make ("filesrc", "source");
  g_object_set (G_OBJECT (filesrc), "location", argv[1], NULL);
  typefind = gst_element_factory_make ("typefind", "typefinder");
  g_signal_connect (typefind, "have-type", G_CALLBACK (cb_typefound), loop);
  fakesink = gst_element_factory_make ("fakesink", "sink");

  /* setup */
  gst_bin_add_many (GST_BIN (pipeline), filesrc, typefind, fakesink, NULL);
  gst_element_link_many (filesrc, typefind, fakesink, NULL);
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_PLAYING);
  g_main_loop_run (loop);

  /* unset */
  gst_element_set_state (GST_ELEMENT (pipeline), GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
}
    <!-- example-end typefind.c c --></programlisting>
    <para>
      Once a media type has been detected, you can plug an element (e.g. a
      demuxer or decoder) to the source pad of the typefind element, and
      decoding of the media stream will start right after.
    </para>
  </sect1>

  <sect1 id="section-dynamic">
    <title>Dynamically autoplugging a pipeline</title>
    <para>
      See <xref linkend="chapter-playback-components"/> for using the high
      level object that you can use to dynamically construct pipelines.
    </para>
  </sect1>
</chapter>