gstreamer/docs/manual/elements.xml

<chapter id="cha-elements">
  <title>Elements</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 an element ?</title>
    <para> 
      An element is the basic building block for the media pipeline.
      All the different high-level 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" (see <xref linkend="cha-pads"/>), or link points. This terminology arises from soldering; 
      pads are where wires can be attached.
    </para>
  </sect1>

  <sect1 id="sec-elements-types">
    <title>Types of elements</title>
 
    <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>
        <xref linkend="sec-element-srcimg"/> shows how we will visualise 
        a source element.
	We always draw a source 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.&image;" format="&IMAGE;" />
            </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 source pad. A source
        pad can only generate data.
      </para>
    </sect2>

    <sect2 id="sec-elements-filter">
      <title>Filters and codecs</title>
      <para>
        Filter elements have both input and output pads. They operate on
        data they receive in their sink pads and produce data on their source
        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.&image;" format="&IMAGE;" />
            </imageobject>
          </mediaobject>
      </figure>
      <para>
        <xref linkend="sec-element-filterimg"/> shows how we will visualise 
        a filter element.
        This element has one sink (input) pad and one source (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.&image;" 
                       format="&IMAGE;" />
          </imageobject>
        </mediaobject>
      </figure>
      <para>
        <xref linkend="sec-element-filterimg"/> shows the visualisation of a filter element with
        more than one output pad. An example of such a filter is the AVI
        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 link an arbitrary element to the newly created pad.
      </para>
    </sect2>
  
    <sect2 id="sec-elements-sink">
      <title>Sink elements</title>
      <para>
        Sink elements are end 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.
        <xref linkend="sec-element-sinkimg"/> shows a sink element.
      </para>
      <figure float="1" id="sec-element-sinkimg">
        <title>Visualisation of a sink element</title>
        <mediaobject>  
          <imageobject>
            <imagedata fileref="images/sink-element.&image;" format="&IMAGE;" />
          </imageobject>
        </mediaobject>
      </figure>
    </sect2>
  </sect1>
</chapter>