source: trunk/third/gstreamer/docs/manual/highlevel-components.xml @ 21448

<chapter id="chapter-components">
  <title>Components</title>

  <para>
    &GStreamer; includes several higher-level components to simplify your
    applications life. All of the components discussed here (for now) are
    targetted at media playback. The idea of each of these components is
    to integrate as closely as possible with a &GStreamer; pipeline, but
    to hide the complexity of media type detection and several other
    rather complex topics that have been discussed in <xref
    linkend="part-advanced"/>.
  </para>

  <para>
    We currently recommend people to use either playbin (see <xref
    linkend="section-components-playbin"/>) or decodebin (see <xref
    linkend="section-components-decodebin"/>), depending on their needs. The
    other components discussed here are either outdated or deprecated. The
    documentation is provided for legacy purposes. Use of those other
    components is not recommended.
  </para>

  <sect1 id="section-components-playbin">
    <title>Playbin</title>

    <para>
      Playbin is an element that can be created using the standard &GStreamer;
      API (e.g. <function>gst_element_factory_make ()</function>). The factory
      is conveniently called <quote>playbin</quote>. By being a
      <classname>GstElement</classname>, playbin automatically supports all
      of the features of this class, including error handling, tag support,
      state handling, getting stream positions, seeking, and so on.
    </para>

    <para>
      Setting up a playbin pipeline is as simple as creating an instance of
      the playbin element, setting a file location (this has to be a valid
      URI, so <quote>&lt;protocol&gt;://&lt;location&gt;</quote>, e.g.
      file:///tmp/my.ogg or http://www.example.org/stream.ogg) using the
      <quote>uri</quote> property on playbin, and then setting the element
      to the <classname>GST_STATE_PLAYING</classname> state. Internally,
      playbin uses threads, so there's no need to iterate the element or
      anything. However, one thing to keep in mind is that signals fired
      by playbin might come from another than the main thread, so be sure
      to keep this in mind in your signal handles. Most application
      programmers will want to use a function such as <function>g_idle_add
      ()</function> to make sure that the signal is handled in the main
      thread.
    </para>

    <programlisting><!-- example-begin playbin.c -->
#include &lt;gst/gst.h&gt;

static void
cb_eos (GstElement *play,
        gpointer    data)
{
  gst_main_quit ();
}

static void
cb_error (GstElement *play,
          GstElement *src,
          GError     *err,
          gchar      *debug,
          gpointer    data)
{
  g_print ("Error: %s\n", err->message);
}

gint
main (gint   argc,
      gchar *argv[])
{
  GstElement *play;

  /* init GStreamer */
  gst_init (&amp;argc, &amp;argv);

  /* make sure we have a URI */
  if (argc != 2) {
    g_print ("Usage: %s &lt;URI&gt;\n", argv[0]);
    return -1;
  }

  /* set up */
  play = gst_element_factory_make ("playbin", "play");
  g_object_set (G_OBJECT (play), "uri", argv[1], NULL);
  g_signal_connect (play, "eos", G_CALLBACK (cb_eos), NULL);
  g_signal_connect (play, "error", G_CALLBACK (cb_error), NULL);
  if (gst_element_set_state (play, GST_STATE_PLAYING) != GST_STATE_SUCCESS) {
    g_print ("Failed to play\n");
    return -1;
  }

  /* now run */
  gst_main ();

  /* also clean up */
  gst_element_set_state (play, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (play));

  return 0;
}
    <!-- example-end playbin.c --></programlisting>

    <para>
      Playbin has several features that have been discussed previously:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Settable video and audio output (using the <quote>video-sink</quote>
          and <quote>audio-sink</quote> properties).
        </para>
      </listitem>
      <listitem>
        <para>
          Mostly controllable and trackable as a
          <classname>GstElement</classname>, including error handling, eos
          handling, tag handling, state handling, media position handling and
          seeking.
        </para>
      </listitem>
      <listitem>
        <para>
          Buffers network-sources.
        </para>
      </listitem>
      <listitem>
        <para>
          Supports visualizations for audio-only media.
        </para>
      </listitem>
      <listitem>
        <para>
          Supports subtitles, both in the media as well as from separate
          files.
        </para>
      </listitem>
      <listitem>
        <para>
          Supports stream selection and disabling. If your media has
          multiple audio or subtitle tracks, you can dynamically choose
          which one to play back, or decide to turn it off alltogther
          (which is especially useful to turn off subtitles).
        </para>
      </listitem>
    </itemizedlist>
  </sect1>

  <sect1 id="section-components-decodebin">
    <title>Decodebin</title>

    <para>
      Decodebin is the actual autoplugger backend of playbin, which was
      discussed in the previous section. Decodebin will, in short, accept
      input from a source that is linked to its sinkpad and will try to
      detect the media type contained in the stream, and set up decoder
      routines for each of those. It will automatically select decoders.
      For each decoded stream, it will emit the <quote>new-decoded-pad</quote>
      signal, to let the client know about the newly found decoded stream.
      For unknown streams (which might be the whole stream), it will emit
      the <quote>unknown-type</quote> signal. The application is then
      responsible for reporting the error to the user.
    </para>

    <para>
      The example code below will play back an audio stream of an input
      file. For readability, it does not include any error handling of
      any sort.
    </para>

    <programlisting><!-- example-begin decodebin.c -->
#include &lt;gst/gst.h&gt;

GstElement *pipeline, *audio;
GstPad *audiopad;

static void
cb_newpad (GstElement *decodebin,
           GstPad     *pad,
           gboolean    last,
           gpointer    data)
{
  GstCaps *caps;
  GstStructure *str;

  /* only link audio; only link once */
  if (GST_PAD_IS_LINKED (audiopad))
    return;
  caps = gst_pad_get_caps (pad);
  str = gst_caps_get_structure (caps, 0);
  if (!g_strrstr (gst_structure_get_name (str), "audio"))
    return;

  /* link'n'play */
  gst_pad_link (pad, audiopad);
  gst_bin_add (GST_BIN (pipeline), audio);
  gst_bin_sync_children_state (GST_BIN (pipeline));
}

gint
main (gint   argc,
      gchar *argv[])
{
  GstElement *src, *dec, *conv, *scale, *sink;

  /* init GStreamer */
  gst_init (&amp;argc, &amp;argv);

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

  /* setup */
  pipeline = gst_pipeline_new ("pipeline");
  src = gst_element_factory_make ("filesrc", "source");
  g_object_set (G_OBJECT (src), "location", argv[1], NULL);
  dec = gst_element_factory_make ("decodebin", "decoder");
  g_signal_connect (dec, "new-decoded-pad", G_CALLBACK (cb_newpad), NULL);
  audio = gst_bin_new ("audiobin");
  conv = gst_element_factory_make ("audioconvert", "aconv");
  audiopad = gst_element_get_pad (conv, "sink");
  scale = gst_element_factory_make ("audioscale", "scale");
  sink = gst_element_factory_make ("alsasink", "sink");
  gst_bin_add_many (GST_BIN (audio), conv, scale, sink, NULL);
  gst_element_link_many (conv, scale, sink, NULL);
  gst_bin_add_many (GST_BIN (pipeline), src, dec, NULL);
  gst_element_link (src, dec);

  /* run */
  gst_element_set_state (audio, GST_STATE_PAUSED);
  gst_element_set_state (pipeline, GST_STATE_PLAYING);
  while (gst_bin_iterate (GST_BIN (pipeline))) ;

  /* cleanup */
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (GST_OBJECT (pipeline));

  return 0;
}
    <!-- example-end decodebin.c --></programlisting>

    <para>
      Decodebin, similar to playbin, supports the following features:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Can decode an unlimited number of contained streams to decoded
          output pads.
        </para>
      </listitem>
      <listitem>
        <para>
          Is handled as a <classname>GstElement</classname> in all ways,
          including tag or error forwarding and state handling.
        </para>
      </listitem>
    </itemizedlist>
    <para>
      Although decodebin is a good autoplugger, there's a whole lot of
      things that it does not do and is not intended to do:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Taking care of input streams with a known media type (e.g. a DVD,
          an audio-CD or such).
        </para>
      </listitem>
      <listitem>
        <para>
          Selection of streams (e.g. which audio track to play in case of
          multi-language media streams).
        </para>
      </listitem>
      <listitem>
        <para>
          Overlaying subtitles over a decoded video stream.
        </para>
      </listitem>
    </itemizedlist>
    <para>
      Decodebin can be easily tested on the commandline, e.g. by using the
      command <command>gst-launch-0.8 filesrc location=file.ogg ! decodebin
      ! audioconvert ! audioscale ! alsasink</command>.
    </para>
  </sect1>

  <sect1 id="section-components-spider">
    <title>Spider</title>

    <para>
      Spider is an autoplugger that looks and feels very much like decodebin.
      On the commandline, you can literally switch between spider and
      decodebin and it'll mostly just work. Try, for example,
      <command>gst-launch-0.8 filesrc location=file.ogg ! spider !
      audioconvert ! audioscale ! alsasink</command>. Although the two may
      seem very much alike from the outside, they are very different from
      the inside. Those internal differences are the main reason why spider
      is currently considered deprecated (along with the fact that it was
      hard to maintain).
    </para>
    <para>
      As opposed to decodebin, spider does not decode pads and emit signals
      for each detected stream. Instead, you have to add output sinks to
      spider by create source request pads and connecting those to sink
      elements. This means that streams decoded by spider cannot be dynamic.
      Also, spider uses many loop-based elements internally, which is rather
      heavy scheduler-wise.
    </para>
    <para>
      Code for using spider would look almost identical to the code of
      decodebin, and is therefore omitted. Also, featureset and limitations
      are very much alike, except for the above-mentioned extra limitations
      for spider with respect to decodebin.
    </para>
  </sect1>

  <sect1 id="section-components-gst-play">
    <title>GstPlay</title>
    <para> 
      GstPlay is a GtkWidget with a simple API to play, pause and stop a media file.
    </para>
      
  </sect1>

  <sect1 id="section-components-gst-editor">
    <title>GstEditor</title>
    <para> 
      GstEditor is a set of widgets to display a graphical representation of a 
      pipeline.
    </para> 
  </sect1>

</chapter>