gstreamer/docs/manual/autoplugging.xml

<chapter id="cha-autoplug">
  <title>Autoplugging</title>
  <para> 
    <application>GStreamer</application> provides an API to automatically
    construct complex pipelines based on source and destination capabilities.
    This feature is very useful if you want to convert type X to type Y but
    don't care about the plugins needed to accomplish this task. The 
    autoplugger will consult the plugin repository, select and link the
    elements needed for the conversion.
  </para>
  <para> 
    The autoplugger API is implemented in an abstract class. Autoplugger
    implementations reside in plugins and are therefore optional and can be
    optimized for a specific task.  Two types of autopluggers exist: renderer
    ones and non-renderer ones. The renderer autopluggers will not have any
    source pads while the non-renderer ones do. The renderer autopluggers are
    mainly used for media playback while the non renderer ones are used for
    arbitrary format conversion.
  </para>

  <sect1>
    <title>Using autoplugging</title>
    <para> 
      You first need to create a suitable autoplugger with gst_autoplug_factory_make().
      The name of the autoplugger must be one of the registered autopluggers..
    </para>
    <para>
      A list of all available autopluggers can be obtained with gst_autoplug_factory_get_list().
    </para>
    <para>
      If the autoplugger supports the RENDERER API, use the
      gst_autoplug_to_renderers() function to create a bin that links
      the source caps to the specified render elements. You can then add
      the bin to a pipeline and run it.

      <programlisting>

        GstAutoplug *autoplug;
	GstElement  *element;
	GstElement  *sink;

	/* create a static autoplugger */
	autoplug = gst_autoplug_factory_make ("staticrender");

	/* create an osssink */
	sink = gst_element_factory_make ("osssink", "our_sink");

	/* create an element that can play audio/mp3 through osssink */
	element = gst_autoplug_to_renderers (autoplug,
	                                     gst_caps_new (
	                                       "sink_audio_caps",
	                                       "audio/mp3",
	                                       NULL
	                                     ),
	                                     sink,
	                                     NULL);

	/* add the element to a bin and link the sink pad */
	...
      </programlisting>
    </para>
    <para>
      If the autoplugger supports the CAPS API, use the gst_autoplug_to_caps()
      function to link the source caps to the destination caps. The created
      bin will have source and sink pads compatible with the provided caps.

      <programlisting>

        GstAutoplug *autoplug;
	GstElement  *element;

	/* create a static autoplugger */
	autoplug = gst_autoplug_factory_make ("static");

	/* create an element that converts audio/mp3 to audio/raw */
	element = gst_autoplug_to_caps (autoplug,
	                                gst_caps_new (
	                                  "sink_audio_caps",
	                                  "audio/mp3",
	                                  NULL
	                                ),
	                                gst_caps_new (
	                                  "src_audio_caps",
	                                  "audio/raw",
	                                  NULL
	                                ),
	                                NULL);

	/* add the element to a bin and link the src/sink pads */
	...
      </programlisting>
    </para>
  </sect1>
  <sect1>
    <title>Using the <classname>GstAutoplugCache</classname> element</title>
    <para>
      The <classname>GstAutoplugCache</classname> element is used to cache the 
      media stream when performing typedetection. As we have have seen in the
      previous chapter (typedetection), the type typefind function consumes a 
      buffer to determine the media type of it. After we have set up the pipeline
      to play the media stream we should be able to 'replay' the previous buffer(s).
      This is where the autoplugcache is used for.
    </para>
    <para>
      The basic usage pattern for the autoplugcache in combination with the typefind 
      element is like this:
      <orderedlist>
        <listitem>
          <para>
            Add the autoplugcache element to a bin and link the sink pad
            to the source pad of an element with unknown caps.
          </para>
        </listitem>
        <listitem>
          <para>
            Link the source pad of the autoplugcache to the sink pad of
            the typefind element.
          </para>
        </listitem>
        <listitem>
          <para>
	    Iterate the pipeline until the typefind element has found a type. 
          </para>
        </listitem>
        <listitem>
          <para>
            Remove the typefind element and add the plugins needed to play
            back the discovered media type to the autoplugcache source pad.
          </para>
        </listitem>
        <listitem>
          <para>
	    Reset the cache to start playback of the cached data. Connect to the 
	    "cache_empty" signal.
          </para>
        </listitem>
        <listitem>
          <para>
	    In the cache_empty signal callback function, remove the autoplugcache and
	    relink the pads.
          </para>
        </listitem>
      </orderedlist>
    </para>
    <para>
      In the next chapter we will create a new version of our helloworld example using the
      autoplugger, the autoplugcache and the typefind element.
    </para>
  </sect1>
  <sect1>
    <title>Another approach to autoplugging</title>
    <para>
      The autoplug API is interesting, but often impractical. It is static;
      it cannot deal with dynamic pipelines (insert ref here). What you
      often want is just an element to stick into a pipeline that will DWIM
      (Do What I Mean)(ref). Enter the spider.
    </para>
    <sect2>
      <title>The spider element</title>
      <para>
        The spider element is a generalized autoplugging element. At this point (April 2002), it's
        the best we've got; it can be inserted anywhere within a pipeline to perform caps
        conversion, if possible. Consider the following gst-launch line:
        <programlisting>
          $ gst-launch filesrc location=my.mp3 ! spider ! osssink
        </programlisting>
        The spider will detect the type of the stream, autoplug it to the osssink's caps, and play
        the pipeline. It's neat.
      </para>
    </sect2>
    <sect2>
      <title>Spider features</title>
      <para>
        <orderedlist>
          <listitem>
            <para>
              Automatically typefinds the incoming stream.
            </para>
          </listitem>
         <listitem>
            <para>
              Has request pads on the source side. This means that it can
              autoplug one source stream into many sink streams. For example,
              an MPEG1 system stream can have audio as well as video; that
              pipeline would be represented in gst-launch syntax as

              <programlisting>
                $ gst-launch filesrc location=my.mpeg1 ! spider ! { queue ! osssink } spider.src_%d!
                             { queue ! xvideosink }
              </programlisting>
            </para>
          </listitem>
        </orderedlist>
      </para>
    </sect2>
  </sect1>
</chapter>