gstreamer/docs/manual/basics-elements.xml

<chapter id="cha-elements">
  <title>GstElement</title>
  <para> 
    The most important object in <application>GStreamer</application> for the 
    application programmer is the <classname>GstElement</classname> object. 
  </para>

  <sect1 id="sec-elements-design">
    <title>What is a GstElement</title>
    <para> 
      <classname>GstElement</classname> is the basic building block for the
      media pipeline. All the different components you are going to use are
      derived from <classname>GstElement</classname>.  This means that a
      lot of functions you are going to use operate on objects of this class.
    </para>
    <para>
      Elements, from the perspective of GStreamer, are viewed as "black boxes" 
      with a number of different aspects. One of these aspects is the presence 
      of "pads", or connection points. This terminology arises from soldering; 
      pads are where wires can be attached.
    </para>

    <sect2 id="sec-elements-src">
      <title>Source elements</title>
      <para>
        Source elements generate data for use by a pipeline, for example 
        reading from disk or from a sound card.
      </para>
      <para>
        Below you see how we will visualize the element.
	We always draw a src pad to the right of the element.
      </para>
      <figure float="1" id="sec-element-srcimg">
        <title>Visualisation of a source element</title>
          <mediaobject>  
            <imageobject>
              <imagedata fileref="images/src-element.&magic;" format="&magic;" />
            </imageobject>
          </mediaobject>
      </figure>
      <para>
        Source elements do not accept data, they only generate data. You can
        see this in the figure because it only has a src pad. A src pad can
        only generate data.
      </para>
    </sect2>

    <sect2 id="sec-elements-filter">
      <title>Filters and codecs</title>
      <para>
        Filter elements both have input and output pads. They operate on
        data they receive in their sink pads and produce data on their src
        pads. For example, MPEG decoders and volume filters would fall into
        this category.
      </para>
      <para>
        Elements are not constrained as to the number of pads they might have;
        for example, a video mixer might have two input pads (the images of 
        the two different video streams) and one output pad.
      </para>
      <figure float="1" id="sec-element-filterimg">
        <title>Visualisation of a filter element</title>
          <mediaobject>  
            <imageobject>
              <imagedata fileref="images/filter-element.&magic;" format="&magic;" />
            </imageobject>
          </mediaobject>
      </figure>
      <para>
        The above figure shows the visualisation of a filter element.
        This element has one sink (input) pad and one src (output) pad. 
        Sink pads are drawn on the left of the element.
      </para> 
      <figure float="1" id="sec-element-multifilterimg">
        <title>Visualisation of a filter element with
	  more than one output pad</title>
        <mediaobject>  
          <imageobject>
            <imagedata fileref="images/filter-element-multi.&magic;" 
                       format="&magic;" />
          </imageobject>
        </mediaobject>
      </figure>
      <para>
        The above figure shows the visualisation of a filter element with
        more than one output pad. An example of such a filter is the AVI
        splitter (demultiplexer). This element will parse the input data and
        extract the audio and video data. Most of these filters dynamically
        send out a signal when a new pad is created so that the application
        programmer can connect an arbitrary element to the newly created pad.
      </para>
    </sect2>
  
    <sect2 id="sec-elements-sink">
      <title>Sink elements</title>
      <para>
        Sink elements are terminal points in a media pipeline. They accept 
        data but do not produce anything. Disk writing, soundcard playback, 
        and video output would all be implemented by sink elements.
      </para>
      <figure float="1" id="sec-element-sinkimg">
        <title>Visualisation of a sink element</title>
        <mediaobject>  
          <imageobject>
            <imagedata fileref="images/sink-element.&magic;" format="&magic;" />
          </imageobject>
        </mediaobject>
      </figure>
    </sect2>
  </sect1>
  <sect1 id="sec-elements-create">
    <title>Creating a GstElement</title>
    <para> 
      A <classname>GstElement</classname> object is created from
      a factory.  To create an element, you have to get access to a
      <classname>GstElementFactory</classname> object using a unique
      factory name.
    </para> 
    <para> 
      The following code example is used to get a factory that can be used 
      to create the 'mad' element, an mp3 decoder.
    </para> 
    <programlisting>
 GstElementFactory *factory;

 factory = gst_element_factory_find ("mad");
    </programlisting>
    <para> 
      Once you have the handle to the element factory, you can create a 
      real element with the following code fragment:
    </para> 
    <programlisting>
 GstElement *element;

 element = gst_element_factory_create (factory, "decoder");
    </programlisting>
    <para>
      <function>gst_element_factory_create</function> will use the element
      factory to create an element with the given name. The name of the
      element is something you can use later on to look up the element in
      a bin, for example. You can pass <symbol>NULL</symbol> as the name
      argument to get a unique, default name.
    </para>
    <para>
      A simple shortcut exists for creating an element from a factory. The
      following example creates an element named "decoder" from the element
      factory named "mad". This convenience function is most widely used to
      create an element.
    </para>
    <programlisting>
 GstElement *element;

 element = gst_element_factory_make ("mad", "decoder");
    </programlisting>
    <para> 
      When you don't need the element anymore, you need to unref it, as shown in the following
      example. 
    </para> 
    <programlisting>
 GstElement *element;

  ...
 gst_element_unref (element);
    </programlisting>
  </sect1>
  <sect1 id="sec-elements-properties">
    <title>GstElement properties</title>
    <para> 
      A <classname>GstElement</classname> can have several properties
      which are implemented using standard <classname>GObject</classname>
      properties. The usual <classname>GObject</classname> methods to query,
      set and get property values and <classname>GParamSpecs</classname>
      are therefore supported.
    </para> 
    <para> 
      Every <classname>GstElement</classname> inherits at least
      one property of its parent <classname>GstObject</classname>:
      the "name" property. This is the name you provide to the
      functions <function>gst_element_factory_make</function> or
      <function>gst_element_factory_create</function>. You can get and set 
      this property using the functions 
      <function>gst_object_set_name</function>
      and <function>gst_object_get_name</function> or use the
      <classname>GObject</classname> property mechanism as shown below.
    </para> 
    <programlisting>
 GstElement *element;
 GValue value = { 0, }; /* initialize the GValue for g_object_get() */

 element = gst_element_factory_make ("mad", "decoder");
 g_object_set (G_OBJECT (element), "name", "mydecoder", NULL);
 ...

 g_value_init (&amp;value, G_TYPE_STRING);
 g_object_get_property (G_OBJECT (element), "name", &amp;value);
 ...
    </programlisting>
    <para> 
      Most plugins provide additional properties to provide more information
      about their configuration or to configure the element. 
      <command>gst-inspect</command> is a useful tool to query the properties
      of a particular element, it will also use property introspection to give
      a short explanation about the function of the property and about the
      parameter types and ranges it supports.
    </para> 
    <para> 
      For more information about <classname>GObject</classname>
      properties we recommend you read the <ulink
      url="http://developer.gnome.org/doc/API/2.0/gobject/index.html"
      type="http">GObject manual</ulink>.
    </para> 
  </sect1>
  
  <sect1 id="sec-elements-signals">
    <title>GstElement signals</title>
    <para> 
      A <classname>GstElement</classname> also provides various 
      <classname>GObject</classname> signals that can be used as a flexible
      callback mechanism.
    </para> 
  </sect1>
  
  <sect1 id="sec-elements-factories">
    <title>More about GstElementFactory</title>
    <para> 
      We talk some more about the GstElementFactory object.
    </para> 
    
    <sect2 id="sec-elements-factories-details">
      <title>Getting information about an element using the factory details</title>
      <para> 
      </para> 
    </sect2>
    
    <sect2 id="sec-elements-factories-padtemplates">
      <title>Finding out what pads an element can contain</title>
      <para> 
      </para> 
    </sect2>
    
    <sect2 id="sec-elements-factories-query">
      <title>Different ways of querying the factories</title>
      <para> 
      </para> 
    </sect2>
  </sect1>
</chapter>