gstreamer/docs/manual/factories.xml

<chapter id="cha-factories">
  <title>More on factories</title>
  <para> 
    The small application we created in the previous chapter used the
    concept of a factory to create the elements. In this chapter we will
    show you how to use the factory concepts to create elements based
    on what they do instead of what they are called.
  </para>

  <para> 
    We will first explain the concepts involved before we move on
    to the reworked helloworld example using autoplugging.
  </para>
  <sect1>
    <title>The problems with the helloworld example</title>
    <para> 
      If we take a look at how the elements were created in the previous
      example we used a rather crude mechanism:
    </para>

    <programlisting>
  ...    
  /* now it's time to get the parser */
  decoder = gst_element_factory_make ("mad", "decoder");
  ...
    </programlisting>

    <para>
      While this mechanism is quite effective it also has some big problems:
      The elements are created based on their name. Indeed, we create an
      element, mad, by explicitly stating the mad element's name.  Our little
      program therefore always uses the mad decoder element to decode
      the MP3 audio stream, even if there are three other MP3 decoders in the
      system. We will see how we can use a more general way to create an
      MP3 decoder element.
    </para>
    <para>
      We have to introduce the concept of MIME types and capabilities 
      added to the source and sink pads.
    </para>
  </sect1>

  <sect1>
    <title>More on MIME Types</title>
    <para> 
      GStreamer uses MIME types to identify the different types of data
      that can be handled by the elements. They are the high level
      mechanisms to make sure that everyone is talking about the right
      kind of data. 
    </para> 
    <para> 
      A MIME (Multipurpose Internet Mail Extension) type is a pair of
      strings that denote a certain type of data. Examples include:
      <itemizedlist>
        <listitem>
          <para>
	    audio/raw : raw audio samples
          </para>
        </listitem>
        <listitem>
          <para>
	    audio/mpeg : MPEG audio
          </para>
        </listitem>
        <listitem>
          <para>
	    video/mpeg : MPEG video
          </para>
        </listitem>
      </itemizedlist>
    </para> 
    <para> 
      An element must associate a MIME 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.
      This allows for very dynamic and extensible element creation as we
      will see.
    </para> 
    <para> 
      As we have seen in the previous chapter, MIME types are added
      to the Capability structure of a pad.
    </para> 

    <para>
      In our helloworld example the elements we constructed would have the
      following MIME types associated with their source and sink pads:
    </para>
    <figure float="1" id="sec-mime-img">
      <title>The Hello world pipeline with MIME types</title>
      <mediaobject>
        <imageobject>
          <imagedata fileref="images/mime-world.&magic;" format="&magic;" />
        </imageobject>
      </mediaobject>  

    </figure>
    <para>
      We will see how you can create an element based on the MIME types
      of its source and sink pads. This way the end-user will have the
      ability to choose his/her favorite audio/mpeg decoder without
      you even having to care about it.
    </para>
    <para>
      The typing of the source and sink pads also makes it possible to
      'autoplug' a pipeline. We will have the ability to say: "construct
      me a pipeline that does an audio/mpeg to audio/raw conversion".
    </para>
    <note>
      <para>
        The basic GStreamer library does not try to solve all of your 
	autoplug problems. It leaves the hard decisions to the application
	programmer, where they belong. 
      </para>
    </note>
    
  </sect1>

  <sect1>
    <title>GStreamer types</title>
    <para> 
      GStreamer assigns a unique number to all registered MIME types.
      GStreamer also keeps a reference to
      a function that can be used to determine if a given buffer is of
      the given MIME type.
    </para>
    <para>
      There is also an association between a MIME type and a file extension,
      but the use of typefind functions (similar to file(1)) is preferred.
    </para>
    <para> 
      The type information is maintained in a list of 
      <classname>GstType</classname>. The definition of a 
      <classname>GstType</classname> is like:
    </para>
    <programlisting>
typedef GstCaps (*GstTypeFindFunc) (GstBuffer *buf,gpointer *priv);
 
typedef struct _GstType GstType;

struct _GstType {
  guint16 id;                   /* type id (assigned) */

  gchar *mime;                  /* MIME type */
  gchar *exts;                  /* space-delimited list of extensions */

  GstTypeFindFunc typefindfunc; /* typefind function */
};
    </programlisting>
    <para> 
      All operations on <classname>GstType</classname> occur
      via their <classname>guint16 id</classname> numbers, with
      the <classname>GstType</classname> structure private to the GStreamer
      library.
    </para> 

    <sect2>
    <title>MIME type to id conversion</title>

      <para> 
        We can obtain the id for a given MIME type
        with the following piece of code:
      </para> 
      <programlisting>
  guint16 id;
  
  id = gst_type_find_by_mime ("audio/mpeg");
      </programlisting>
      <para> 
        This function will return 0 if the type was not known.
      </para> 
    </sect2>

    <sect2>
    <title>id to <classname>GstType</classname> conversion</title>
      <para> 
        We can obtain the <classname>GstType</classname> for a given id 
        with the following piece of code:
      </para> 
      <programlisting>
  GstType *type;
  
  type = gst_type_find_by_id (id);
      </programlisting>
      <para> 
        This function will return NULL if the id was not associated with
        any known <classname>GstType</classname>
      </para> 
    </sect2>

    <sect2>
    <title>extension to id conversion</title>
      <para> 
        We can obtain the id for a given file extension
        with the following piece of code:
      </para> 
      <programlisting>
  guint16 id;
  
  id = gst_type_find_by_ext (".mp3");
      </programlisting>
      <para> 
        This function will return 0 if the extension was not known.
      </para> 
    </sect2>
    <para>
      For more information, see <xref linkend="cha-autoplug"/>.
    </para>
  </sect1>

  <sect1>
    <title>Creating elements with the factory</title>
    <para>
      In the previous section we described how you could obtain
      an element factory using MIME types. One the factory has been
      obtained, you can create an element using:
    </para>
    <programlisting>
  GstElementFactory *factory;
  GstElement *element;

  // obtain the factory
  factory = ... 

  element = gst_element_factory_create (factory, "name");
    </programlisting>
    <para>
      This way, you do not have to create elements by name which
      allows the end-user to select the elements he/she prefers for the
      given MIME types.
    </para>
  </sect1>

  <sect1>
    <title>GStreamer basic types</title>
    <para>
      GStreamer only has two builtin types:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          audio/raw : raw audio samples
        </para>
      </listitem>
      <listitem>
        <para>
          video/raw and image/raw : raw video data
        </para>
      </listitem>
    </itemizedlist>
    <para>
      All other MIME types are maintained by the plugin elements.
    </para>

  </sect1>
</chapter>