docs/pwg/building-boiler.xml - mtk-gstreamer-debian - Git at Google

 <!-- ############ chapter ############# -->

 <chapter id="chapter-building-boiler" xreflabel="Constructing the Boilerplate">
   <title>Constructing the Boilerplate</title>
   <para>
     In this chapter you will learn how to construct the bare minimum code for a
     new plugin. Starting from ground zero, you will see how to get the
     &GStreamer; template source. Then you will learn how to use a few basic
     tools to copy and modify a template plugin to create a new plugin. If you
     follow the examples here, then by the end of this chapter you will have a
     functional audio filter plugin that you can compile and use in &GStreamer;
     applications.
   </para>

   <!-- ############ sect1 ############# -->

   <sect1 id="section-boiler-source" xreflabel="Getting the GStreamer Plugin Templates">
     <title>Getting the GStreamer Plugin Templates</title>
     <para>
       There are currently two ways to develop a new plugin for &GStreamer;: You
       can write the entire plugin by hand, or you can copy an existing plugin
       template and write the plugin code you need. The second method is by far
       the simpler of the two, so the first method will not even be described
       here. (Errm, that is, <quote>it is left as an exercise to the
       reader.</quote>)
     </para>
     <para>
       The first step is to check out a copy of the
       <filename>gst-template</filename> git module to get an important tool and
       the source code template for a basic &GStreamer; plugin. To check out the
       <filename>gst-template</filename> module, make sure you are connected to
       the internet, and type the following commands at a command console:
     </para>
     <screen>
 <prompt>shell $ </prompt><userinput>git clone git://anongit.freedesktop.org/gstreamer/gst-template.git</userinput>
 Initialized empty Git repository in /some/path/gst-template/.git/
 remote: Counting objects: 373, done.
 remote: Compressing objects: 100% (114/114), done.
 remote: Total 373 (delta 240), reused 373 (delta 240)
 Receiving objects: 100% (373/373), 75.16 KiB | 78 KiB/s, done.
 Resolving deltas: 100% (240/240), done.
     </screen>
     <para>
       This command will check out a series of files and directories into
       <filename class="directory">gst-template</filename>. The template you
       will be using is in the
       <filename class="directory">gst-template/gst-plugin/</filename>
       directory. You should look over the files in that directory to get a
       general idea of the structure of a source tree for a plugin.
     </para>
     <para>
       If for some reason you can't access the git repository, you can also
       <ulink type="http"
       url="http://cgit.freedesktop.org/gstreamer/gst-template/commit/">
       download a snapshot of the latest revision</ulink> via the cgit web
       interface.
     </para>
   </sect1>

   <!-- ############ sect1 ############# -->

   <sect1 id="section-boiler-project-stamp" xreflabel="Using the Project Stamp">
     <title>Using the Project Stamp</title>
     <para>
       The first thing to do when making a new element is to specify some basic
       details about it: what its name is, who wrote it, what version number it
       is, etc. We also need to define an object to represent the element and to
       store the data the element needs. These details are collectively known as
       the <emphasis>boilerplate</emphasis>.
     </para>
     <para>
       The standard way of defining the boilerplate is simply to write some code,
       and fill in some structures. As mentioned in the previous section, the
       easiest way to do this is to copy a template and add functionality
       according to your needs. To help you do so, there is a tool in the
       <filename class="directory">./gst-plugin/tools/</filename> directory.
       This tool, <filename>make_element</filename>, is a command line utility
       that creates the boilerplate code for you.
     </para>
     <para>
       To use <command>make_element</command>, first open up a terminal window.
       Change to the <filename class="directory">gst-template/gst-plugin/src</filename>
       directory, and then run the <command>make_element</command> command. The
       arguments to the <command>make_element</command> are:
     </para>
     <orderedlist>
       <listitem>
         <para>the name of the plugin, and</para>
       </listitem>
       <listitem>
         <para>
           the source file that the tool will use. By default,
           <filename>gstplugin</filename> is used.
         </para>
       </listitem>
     </orderedlist>
     <para>
       For example, the following commands create the MyFilter plugin based on
       the plugin template and put the output files in the
       <filename class="directory">gst-template/gst-plugin/src</filename>
       directory:
     </para>
     <screen>
 <prompt>shell $ </prompt><userinput>cd gst-template/gst-plugin/src</userinput>
 <prompt>shell $ </prompt><userinput>../tools/make_element MyFilter</userinput>
     </screen>
     <note>
       <para>
         Capitalization is important for the name of the plugin. Keep in mind
         that under some operating systems, capitalization is also important
         when specifying directory and file names in general.
       </para>
     </note>
     <para>
       The last command creates two files:
       <filename>gstmyfilter.c</filename> and
       <filename>gstmyfilter.h</filename>.
     </para>
     <note>
       <para>
         It is recommended that you create a copy of the <filename
 	class="directory">gst-plugin</filename>
 	directory before continuing.
       </para>
     </note>
     <para>
       Now one needs to adjust the <filename>Makefile.am</filename> to use the
       new filenames and run <filename>autogen.sh</filename> from the parent
       directory to bootstrap the build environment. After that, the project
       can be built and installed using the well known
       <userinput>make &amp;&amp; sudo make install</userinput> commands.
     </para>
     <note>
       <para>
         Be aware that by default <filename>autogen.sh</filename> and
         <filename>configure</filename> would choose <filename class="directory">/usr/local</filename>
         as a default location. One would need to add
         <filename class="directory">/usr/local/lib/gstreamer-1.0</filename>
         to <symbol>GST_PLUGIN_PATH</symbol> in order to make the new plugin
         show up in a gstreamer that's been installed from packages.
       </para>
     </note>
     <note>
       <para>
         FIXME: this section is slightly outdated. gst-template is still useful
         as an example for a minimal plugin build system skeleton. However, for
         creating elements the tool gst-element-maker from gst-plugins-bad is
         recommended these days.
       </para>
     </note>
   </sect1>

   <!-- ############ sect1 ############# -->

   <sect1 id="section-boiler-examine">
     <title>Examining the Basic Code</title>
     <para>
       First we will examine the code you would be likely to place in a header
       file (although since the interface to the code is entirely defined by the
       plugin system, and doesn't depend on reading a header file, this is not
       crucial.)
     </para>

     <example id="ex-boiler-examine-h">
       <title>Example Plugin Header File</title>
       <programlisting><!-- example-begin filter.h a -->
 #include &lt;gst/gst.h&gt;

 /* Definition of structure storing data for this element. */
 typedef struct _GstMyFilter {
   GstElement element;

   GstPad *sinkpad, *srcpad;

   gboolean silent;
 <!-- example-end filter.h a -->
 <!-- example-begin filter.h b --><!--
   gint samplerate, channels;
   gint from_samplerate, to_samplerate;
   gboolean passthrough;
   guint64 offset;
 --><!-- example-end filter.h b -->
 <!-- example-begin filter.h c -->
 } GstMyFilter;

 /* Standard definition defining a class for this element. */
 typedef struct _GstMyFilterClass {
   GstElementClass parent_class;
 } GstMyFilterClass;

 /* Standard macros for defining types for this element.  */
 #define GST_TYPE_MY_FILTER (gst_my_filter_get_type())
 #define GST_MY_FILTER(obj) \
   (G_TYPE_CHECK_INSTANCE_CAST((obj),GST_TYPE_MY_FILTER,GstMyFilter))
 #define GST_MY_FILTER_CLASS(klass) \
   (G_TYPE_CHECK_CLASS_CAST((klass),GST_TYPE_MY_FILTER,GstMyFilterClass))
 #define GST_IS_MY_FILTER(obj) \
   (G_TYPE_CHECK_INSTANCE_TYPE((obj),GST_TYPE_MY_FILTER))
 #define GST_IS_MY_FILTER_CLASS(klass) \
   (G_TYPE_CHECK_CLASS_TYPE((klass),GST_TYPE_MY_FILTER))

 /* Standard function returning type information. */
 GType gst_my_filter_get_type (void);
       <!-- example-end filter.h c --></programlisting>
     </example>
     <para>
       Using this header file, you can use the following macro to setup
       the <classname>GObject</classname> basics in your source file so
       that all functions will be called appropriately:
     </para>
     <programlisting><!-- example-begin boilerplate.c a -->
 #include "filter.h"

 G_DEFINE_TYPE (GstMyFilter, gst_my_filter, GST_TYPE_ELEMENT);
     <!-- example-end boilerplate.c a --></programlisting>
   </sect1>

   <!-- ############ sect1 ############# -->

   <sect1 id="section-boiler-details">
     <title>Element metadata</title>
     <para>
       The Element metadata provides extra element information. It is configured
       with <function>gst_element_class_set_metadata</function> or
       <function>gst_element_class_set_static_metadata</function> which takes the
       following parameters:
     </para>
     <itemizedlist>
       <listitem><para>
         A long, English, name for the element.
       </para></listitem><listitem><para>
         The type of the element, see the docs/design/draft-klass.txt document
         in the GStreamer core source tree for details and examples.
       </para></listitem><listitem><para>
         A brief description of the purpose of the element.
       </para></listitem><listitem><para>
         The name of the author of the element, optionally followed by a contact
         email address in angle brackets.
       </para></listitem>
     </itemizedlist>
     <para>
       For example:
     </para>
     <programlisting>
 gst_element_class_set_static_metadata (klass,
   "An example plugin",
   "Example/FirstExample",
   "Shows the basic structure of a plugin",
   "your name &lt;your.name@your.isp&gt;");
     </programlisting>
     <para>
       The element details are registered with the plugin during
       the <function>_class_init ()</function> function, which is part of
       the GObject system. The <function>_class_init ()</function> function
       should be set for this GObject in the function where you register
       the type with GLib.
     </para>
     <programlisting><!-- example-begin boilerplate.c c -->
 static void
 gst_my_filter_class_init (GstMyFilterClass * klass)
 {
   GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
 <!-- example-end boilerplate.c c -->
 [..]<!-- example-begin boilerplate.c d -->
   gst_element_class_set_static_metadata (element_klass,
     "An example plugin",
     "Example/FirstExample",
     "Shows the basic structure of a plugin",
     "your name &lt;your.name@your.isp&gt;");
 <!-- example-end boilerplate.c d -->
 }
     </programlisting>
   </sect1>

   <!-- ############ sect1 ############# -->

   <sect1 id="section-boiler-padtemplates">
     <title>GstStaticPadTemplate</title>
     <para>
       A GstStaticPadTemplate is a description of a pad that the element will
       (or might) create and use. It contains:
     </para>
     <itemizedlist>
       <listitem>
         <para>A short name for the pad.</para>
       </listitem>
       <listitem>
         <para>Pad direction.</para>
       </listitem>
       <listitem>
         <para>
           Existence property. This indicates whether the pad exists always (an
           <quote>always</quote> pad), only in some cases (a
           <quote>sometimes</quote> pad) or only if the application requested
           such a pad (a <quote>request</quote> pad).
         </para>
       </listitem>
       <listitem>
         <para>Supported types by this element (capabilities).</para>
       </listitem>
     </itemizedlist>
     <para>
       For example:
     </para>
     <programlisting><!-- example-begin boilerplate.c e -->
 static GstStaticPadTemplate sink_factory =
 GST_STATIC_PAD_TEMPLATE (
   "sink",
   GST_PAD_SINK,
   GST_PAD_ALWAYS,
   GST_STATIC_CAPS ("ANY")
 );
 <!-- example-end boilerplate.c e -->
 <!-- example-begin boilerplate.c f --><!--
 static GstStaticPadTemplate src_factory =
 GST_STATIC_PAD_TEMPLATE (
   "src",
   GST_PAD_SRC,
   GST_PAD_ALWAYS,
   GST_STATIC_CAPS ("ANY")
 );
 --><!-- example-end boilerplate.c f -->
       </programlisting>
     <para>
       Those pad templates are registered during the
       <function>_class_init ()</function> function with the
       <function>gst_element_class_add_pad_template ()</function>. For this
       function you need a handle the <classname>GstPadTemplate</classname>
       which you can create from the static pad template with
       <function>gst_static_pad_template_get ()</function>. See below for more
       details on this.
     </para>
     <para>
       Pads are created from these static templates in the element's
       <function>_init ()</function> function using
       <function>gst_pad_new_from_static_template ()</function>.
       In order to create a new pad from this
       template using <function>gst_pad_new_from_static_template ()</function>, you
       will need to declare the pad template as a global variable. More on
       this subject in <xref linkend="chapter-building-pads"/>.
     </para>
     <programlisting>
 static GstStaticPadTemplate sink_factory = [..],
     src_factory = [..];

 static void
 gst_my_filter_class_init (GstMyFilterClass * klass)
 {
   GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
 [..]
 <!-- example-begin boilerplate.c g -->
   gst_element_class_add_pad_template (element_class,
 	gst_static_pad_template_get (&amp;src_factory));
   gst_element_class_add_pad_template (element_class,
 	gst_static_pad_template_get (&amp;sink_factory));
 }
 <!-- example-end boilerplate.c g -->
 <!-- example-begin boilerplate.c h --><!--
 static void
 gst_my_filter_init (GstMyFilter * filter)
 {
 }

 #include "register.func"
     --><!-- example-end boilerplate.c h --></programlisting>
     <para>
       The last argument in a template is its type
       or list of supported types. In this example, we use 'ANY', which means
       that this element will accept all input. In real-life situations, you
       would set a media type and optionally a set of properties to make sure
       that only supported input will come in. This representation should be
       a string that starts with a media type, then a set of comma-separates
       properties with their supported values. In case of an audio filter that
       supports raw integer 16-bit audio, mono or stereo at any samplerate, the
       correct template would look like this:
     </para>
     <programlisting>
 <![CDATA[
 static GstStaticPadTemplate sink_factory =
 GST_STATIC_PAD_TEMPLATE (
   "sink",
   GST_PAD_SINK,
   GST_PAD_ALWAYS,
   GST_STATIC_CAPS (
     "audio/x-raw, "
       "format = (string) " GST_AUDIO_NE (S16) ", "
       "channels = (int) { 1, 2 }, "
       "rate = (int) [ 8000, 96000 ]"
   )
 );
 ]]>
     </programlisting>
     <para>
       Values surrounded by curly brackets (<quote>{</quote> and
       <quote>}</quote>) are lists, values surrounded by square brackets
       (<quote>[</quote> and <quote>]</quote>) are ranges.
       Multiple sets of types are supported too, and should be separated by
       a semicolon (<quote>;</quote>). Later, in the chapter on pads, we will
       see how to use types to know the exact format of a stream:
       <xref linkend="chapter-building-pads"/>.
     </para>
   </sect1>

   <!-- ############ sect1 ############# -->

   <sect1 id="section-boiler-constructors">
     <title>Constructor Functions</title>
     <para>
       Each element has two functions which are used for construction of an
       element. The <function>_class_init()</function> function,
       which is used to initialise the class only once (specifying what signals,
       arguments and virtual functions the class has and setting up global
       state); and the <function>_init()</function> function, which is used to
       initialise a specific instance of this type.
     </para>
   </sect1>

   <!-- ############ sect1 ############# -->

   <sect1 id="section-boiler-plugininit">
     <title>The plugin_init function</title>
     <para>
       Once we have written code defining all the parts of the plugin, we need to
       write the plugin_init() function. This is a special function, which is
       called as soon as the plugin is loaded, and should return TRUE or FALSE
       depending on whether it loaded initialized any dependencies correctly.
       Also, in this function, any supported element type in the plugin should
       be registered.
     </para>
     <programlisting>
 <!-- example-begin register.func -->
 <![CDATA[
 static gboolean
 plugin_init (GstPlugin *plugin)
 {
   return gst_element_register (plugin, "my_filter",
 			       GST_RANK_NONE,
 			       GST_TYPE_MY_FILTER);
 }

 GST_PLUGIN_DEFINE (
   GST_VERSION_MAJOR,
   GST_VERSION_MINOR,
   my_filter,
   "My filter plugin",
   plugin_init,
   VERSION,
   "LGPL",
   "GStreamer",
   "http://gstreamer.net/"
 )
 ]]>
 <!-- example-end register.func -->
     </programlisting>
     <para>
       Note that the information returned by the plugin_init() function will be
       cached in a central registry. For this reason, it is important that the
       same information is always returned by the function: for example, it
       must not make element factories available based on runtime conditions.
       If an element can only work in certain conditions (for example, if the
       soundcard is not being used by some other process) this must be reflected
       by the element being unable to enter the READY state if unavailable,
       rather than the plugin attempting to deny existence of the plugin.
     </para>
   </sect1>
 </chapter>
	<!-- ############ chapter ############# -->

	<chapter id="chapter-building-boiler" xreflabel="Constructing the Boilerplate">
	<title>Constructing the Boilerplate</title>
	<para>
	In this chapter you will learn how to construct the bare minimum code for a
	new plugin. Starting from ground zero, you will see how to get the
	&GStreamer; template source. Then you will learn how to use a few basic
	tools to copy and modify a template plugin to create a new plugin. If you
	follow the examples here, then by the end of this chapter you will have a
	functional audio filter plugin that you can compile and use in &GStreamer;
	applications.
	</para>

	<!-- ############ sect1 ############# -->

	<sect1 id="section-boiler-source" xreflabel="Getting the GStreamer Plugin Templates">
	<title>Getting the GStreamer Plugin Templates</title>
	<para>
	There are currently two ways to develop a new plugin for &GStreamer;: You
	can write the entire plugin by hand, or you can copy an existing plugin
	template and write the plugin code you need. The second method is by far
	the simpler of the two, so the first method will not even be described
	here. (Errm, that is, <quote>it is left as an exercise to the
	reader.</quote>)
	</para>
	<para>
	The first step is to check out a copy of the
	<filename>gst-template</filename> git module to get an important tool and
	the source code template for a basic &GStreamer; plugin. To check out the
	<filename>gst-template</filename> module, make sure you are connected to
	the internet, and type the following commands at a command console:
	</para>
	<screen>
	<prompt>shell $ </prompt><userinput>git clone git://anongit.freedesktop.org/gstreamer/gst-template.git</userinput>
	Initialized empty Git repository in /some/path/gst-template/.git/
	remote: Counting objects: 373, done.
	remote: Compressing objects: 100% (114/114), done.
	remote: Total 373 (delta 240), reused 373 (delta 240)
	Receiving objects: 100% (373/373), 75.16 KiB \| 78 KiB/s, done.
	Resolving deltas: 100% (240/240), done.
	</screen>
	<para>
	This command will check out a series of files and directories into
	<filename class="directory">gst-template</filename>. The template you
	will be using is in the
	<filename class="directory">gst-template/gst-plugin/</filename>
	directory. You should look over the files in that directory to get a
	general idea of the structure of a source tree for a plugin.
	</para>
	<para>
	If for some reason you can't access the git repository, you can also
	<ulink type="http"
	url="http://cgit.freedesktop.org/gstreamer/gst-template/commit/">
	download a snapshot of the latest revision</ulink> via the cgit web
	interface.
	</para>
	</sect1>

	<!-- ############ sect1 ############# -->

	<sect1 id="section-boiler-project-stamp" xreflabel="Using the Project Stamp">
	<title>Using the Project Stamp</title>
	<para>
	The first thing to do when making a new element is to specify some basic
	details about it: what its name is, who wrote it, what version number it
	is, etc. We also need to define an object to represent the element and to
	store the data the element needs. These details are collectively known as
	the <emphasis>boilerplate</emphasis>.
	</para>
	<para>
	The standard way of defining the boilerplate is simply to write some code,
	and fill in some structures. As mentioned in the previous section, the
	easiest way to do this is to copy a template and add functionality
	according to your needs. To help you do so, there is a tool in the
	<filename class="directory">./gst-plugin/tools/</filename> directory.
	This tool, <filename>make_element</filename>, is a command line utility
	that creates the boilerplate code for you.
	</para>
	<para>
	To use <command>make_element</command>, first open up a terminal window.
	Change to the <filename class="directory">gst-template/gst-plugin/src</filename>
	directory, and then run the <command>make_element</command> command. The
	arguments to the <command>make_element</command> are:
	</para>
	<orderedlist>
	<listitem>
	<para>the name of the plugin, and</para>
	</listitem>
	<listitem>
	<para>
	the source file that the tool will use. By default,
	<filename>gstplugin</filename> is used.
	</para>
	</listitem>
	</orderedlist>
	<para>
	For example, the following commands create the MyFilter plugin based on
	the plugin template and put the output files in the
	<filename class="directory">gst-template/gst-plugin/src</filename>
	directory:
	</para>
	<screen>
	<prompt>shell $ </prompt><userinput>cd gst-template/gst-plugin/src</userinput>
	<prompt>shell $ </prompt><userinput>../tools/make_element MyFilter</userinput>
	</screen>
	<note>
	<para>
	Capitalization is important for the name of the plugin. Keep in mind
	that under some operating systems, capitalization is also important
	when specifying directory and file names in general.
	</para>
	</note>
	<para>
	The last command creates two files:
	<filename>gstmyfilter.c</filename> and
	<filename>gstmyfilter.h</filename>.
	</para>
	<note>
	<para>
	It is recommended that you create a copy of the <filename
	class="directory">gst-plugin</filename>
	directory before continuing.
	</para>
	</note>
	<para>
	Now one needs to adjust the <filename>Makefile.am</filename> to use the
	new filenames and run <filename>autogen.sh</filename> from the parent
	directory to bootstrap the build environment. After that, the project
	can be built and installed using the well known
	<userinput>make && sudo make install</userinput> commands.
	</para>
	<note>
	<para>
	Be aware that by default <filename>autogen.sh</filename> and
	<filename>configure</filename> would choose <filename class="directory">/usr/local</filename>
	as a default location. One would need to add
	<filename class="directory">/usr/local/lib/gstreamer-1.0</filename>
	to <symbol>GST_PLUGIN_PATH</symbol> in order to make the new plugin
	show up in a gstreamer that's been installed from packages.
	</para>
	</note>
	<note>
	<para>
	FIXME: this section is slightly outdated. gst-template is still useful
	as an example for a minimal plugin build system skeleton. However, for
	creating elements the tool gst-element-maker from gst-plugins-bad is
	recommended these days.
	</para>
	</note>
	</sect1>

	<!-- ############ sect1 ############# -->

	<sect1 id="section-boiler-examine">
	<title>Examining the Basic Code</title>
	<para>
	First we will examine the code you would be likely to place in a header
	file (although since the interface to the code is entirely defined by the
	plugin system, and doesn't depend on reading a header file, this is not
	crucial.)
	</para>

	<example id="ex-boiler-examine-h">
	<title>Example Plugin Header File</title>
	<programlisting><!-- example-begin filter.h a -->
	#include <gst/gst.h>

	/* Definition of structure storing data for this element. */
	typedef struct _GstMyFilter {
	GstElement element;

	GstPad sinkpad, srcpad;

	gboolean silent;
	<!-- example-end filter.h a -->
	<!-- example-begin filter.h b --><!--
	gint samplerate, channels;
	gint from_samplerate, to_samplerate;
	gboolean passthrough;
	guint64 offset;
	--><!-- example-end filter.h b -->
	<!-- example-begin filter.h c -->
	} GstMyFilter;

	/* Standard definition defining a class for this element. */
	typedef struct _GstMyFilterClass {
	GstElementClass parent_class;
	} GstMyFilterClass;

	/* Standard macros for defining types for this element. */
	#define GST_TYPE_MY_FILTER (gst_my_filter_get_type())
	#define GST_MY_FILTER(obj) \
	(G_TYPE_CHECK_INSTANCE_CAST((obj),GST_TYPE_MY_FILTER,GstMyFilter))
	#define GST_MY_FILTER_CLASS(klass) \
	(G_TYPE_CHECK_CLASS_CAST((klass),GST_TYPE_MY_FILTER,GstMyFilterClass))
	#define GST_IS_MY_FILTER(obj) \
	(G_TYPE_CHECK_INSTANCE_TYPE((obj),GST_TYPE_MY_FILTER))
	#define GST_IS_MY_FILTER_CLASS(klass) \
	(G_TYPE_CHECK_CLASS_TYPE((klass),GST_TYPE_MY_FILTER))

	/* Standard function returning type information. */
	GType gst_my_filter_get_type (void);
	<!-- example-end filter.h c --></programlisting>
	</example>
	<para>
	Using this header file, you can use the following macro to setup
	the <classname>GObject</classname> basics in your source file so
	that all functions will be called appropriately:
	</para>
	<programlisting><!-- example-begin boilerplate.c a -->
	#include "filter.h"

	G_DEFINE_TYPE (GstMyFilter, gst_my_filter, GST_TYPE_ELEMENT);
	<!-- example-end boilerplate.c a --></programlisting>
	</sect1>

	<!-- ############ sect1 ############# -->

	<sect1 id="section-boiler-details">
	<title>Element metadata</title>
	<para>
	The Element metadata provides extra element information. It is configured
	with <function>gst_element_class_set_metadata</function> or
	<function>gst_element_class_set_static_metadata</function> which takes the
	following parameters:
	</para>
	<itemizedlist>
	<listitem><para>
	A long, English, name for the element.
	</para></listitem><listitem><para>
	The type of the element, see the docs/design/draft-klass.txt document
	in the GStreamer core source tree for details and examples.
	</para></listitem><listitem><para>
	A brief description of the purpose of the element.
	</para></listitem><listitem><para>
	The name of the author of the element, optionally followed by a contact
	email address in angle brackets.
	</para></listitem>
	</itemizedlist>
	<para>
	For example:
	</para>
	<programlisting>
	gst_element_class_set_static_metadata (klass,
	"An example plugin",
	"Example/FirstExample",
	"Shows the basic structure of a plugin",
	"your name <your.name@your.isp>");
	</programlisting>
	<para>
	The element details are registered with the plugin during
	the <function>_class_init ()</function> function, which is part of
	the GObject system. The <function>_class_init ()</function> function
	should be set for this GObject in the function where you register
	the type with GLib.
	</para>
	<programlisting><!-- example-begin boilerplate.c c -->
	static void
	gst_my_filter_class_init (GstMyFilterClass * klass)
	{
	GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
	<!-- example-end boilerplate.c c -->
	[..]<!-- example-begin boilerplate.c d -->
	gst_element_class_set_static_metadata (element_klass,
	"An example plugin",
	"Example/FirstExample",
	"Shows the basic structure of a plugin",
	"your name <your.name@your.isp>");
	<!-- example-end boilerplate.c d -->
	}
	</programlisting>
	</sect1>

	<!-- ############ sect1 ############# -->

	<sect1 id="section-boiler-padtemplates">
	<title>GstStaticPadTemplate</title>
	<para>
	A GstStaticPadTemplate is a description of a pad that the element will
	(or might) create and use. It contains:
	</para>
	<itemizedlist>
	<listitem>
	<para>A short name for the pad.</para>
	</listitem>
	<listitem>
	<para>Pad direction.</para>
	</listitem>
	<listitem>
	<para>
	Existence property. This indicates whether the pad exists always (an
	<quote>always</quote> pad), only in some cases (a
	<quote>sometimes</quote> pad) or only if the application requested
	such a pad (a <quote>request</quote> pad).
	</para>
	</listitem>
	<listitem>
	<para>Supported types by this element (capabilities).</para>
	</listitem>
	</itemizedlist>
	<para>
	For example:
	</para>
	<programlisting><!-- example-begin boilerplate.c e -->
	static GstStaticPadTemplate sink_factory =
	GST_STATIC_PAD_TEMPLATE (
	"sink",
	GST_PAD_SINK,
	GST_PAD_ALWAYS,
	GST_STATIC_CAPS ("ANY")
	);
	<!-- example-end boilerplate.c e -->
	<!-- example-begin boilerplate.c f --><!--
	static GstStaticPadTemplate src_factory =
	GST_STATIC_PAD_TEMPLATE (
	"src",
	GST_PAD_SRC,
	GST_PAD_ALWAYS,
	GST_STATIC_CAPS ("ANY")
	);
	--><!-- example-end boilerplate.c f -->
	</programlisting>
	<para>
	Those pad templates are registered during the
	<function>_class_init ()</function> function with the
	<function>gst_element_class_add_pad_template ()</function>. For this
	function you need a handle the <classname>GstPadTemplate</classname>
	which you can create from the static pad template with
	<function>gst_static_pad_template_get ()</function>. See below for more
	details on this.
	</para>
	<para>
	Pads are created from these static templates in the element's
	<function>_init ()</function> function using
	<function>gst_pad_new_from_static_template ()</function>.
	In order to create a new pad from this
	template using <function>gst_pad_new_from_static_template ()</function>, you
	will need to declare the pad template as a global variable. More on
	this subject in <xref linkend="chapter-building-pads"/>.
	</para>
	<programlisting>
	static GstStaticPadTemplate sink_factory = [..],
	src_factory = [..];

	static void
	gst_my_filter_class_init (GstMyFilterClass * klass)
	{
	GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
	[..]
	<!-- example-begin boilerplate.c g -->
	gst_element_class_add_pad_template (element_class,
	gst_static_pad_template_get (&src_factory));
	gst_element_class_add_pad_template (element_class,
	gst_static_pad_template_get (&sink_factory));
	}
	<!-- example-end boilerplate.c g -->
	<!-- example-begin boilerplate.c h --><!--
	static void
	gst_my_filter_init (GstMyFilter * filter)
	{
	}

	#include "register.func"
	--><!-- example-end boilerplate.c h --></programlisting>
	<para>
	The last argument in a template is its type
	or list of supported types. In this example, we use 'ANY', which means
	that this element will accept all input. In real-life situations, you
	would set a media type and optionally a set of properties to make sure
	that only supported input will come in. This representation should be
	a string that starts with a media type, then a set of comma-separates
	properties with their supported values. In case of an audio filter that
	supports raw integer 16-bit audio, mono or stereo at any samplerate, the
	correct template would look like this:
	</para>
	<programlisting>
	<![CDATA[
	static GstStaticPadTemplate sink_factory =
	GST_STATIC_PAD_TEMPLATE (
	"sink",
	GST_PAD_SINK,
	GST_PAD_ALWAYS,
	GST_STATIC_CAPS (
	"audio/x-raw, "
	"format = (string) " GST_AUDIO_NE (S16) ", "
	"channels = (int) { 1, 2 }, "
	"rate = (int) [ 8000, 96000 ]"
	)
	);
	]]>
	</programlisting>
	<para>
	Values surrounded by curly brackets (<quote>{</quote> and
	<quote>}</quote>) are lists, values surrounded by square brackets
	(<quote>[</quote> and <quote>]</quote>) are ranges.
	Multiple sets of types are supported too, and should be separated by
	a semicolon (<quote>;</quote>). Later, in the chapter on pads, we will
	see how to use types to know the exact format of a stream:
	<xref linkend="chapter-building-pads"/>.
	</para>
	</sect1>

	<!-- ############ sect1 ############# -->

	<sect1 id="section-boiler-constructors">
	<title>Constructor Functions</title>
	<para>
	Each element has two functions which are used for construction of an
	element. The <function>_class_init()</function> function,
	which is used to initialise the class only once (specifying what signals,
	arguments and virtual functions the class has and setting up global
	state); and the <function>_init()</function> function, which is used to
	initialise a specific instance of this type.
	</para>
	</sect1>

	<!-- ############ sect1 ############# -->

	<sect1 id="section-boiler-plugininit">
	<title>The plugin_init function</title>
	<para>
	Once we have written code defining all the parts of the plugin, we need to
	write the plugin_init() function. This is a special function, which is
	called as soon as the plugin is loaded, and should return TRUE or FALSE
	depending on whether it loaded initialized any dependencies correctly.
	Also, in this function, any supported element type in the plugin should
	be registered.
	</para>
	<programlisting>
	<!-- example-begin register.func -->
	<![CDATA[
	static gboolean
	plugin_init (GstPlugin *plugin)
	{
	return gst_element_register (plugin, "my_filter",
	GST_RANK_NONE,
	GST_TYPE_MY_FILTER);
	}

	GST_PLUGIN_DEFINE (
	GST_VERSION_MAJOR,
	GST_VERSION_MINOR,
	my_filter,
	"My filter plugin",
	plugin_init,
	VERSION,
	"LGPL",
	"GStreamer",
	"http://gstreamer.net/"
	)
	]]>
	<!-- example-end register.func -->
	</programlisting>
	<para>
	Note that the information returned by the plugin_init() function will be
	cached in a central registry. For this reason, it is important that the
	same information is always returned by the function: for example, it
	must not make element factories available based on runtime conditions.
	If an element can only work in certain conditions (for example, if the
	soundcard is not being used by some other process) this must be reflected
	by the element being unable to enter the READY state if unavailable,
	rather than the plugin attempting to deny existence of the plugin.
	</para>
	</sect1>
	</chapter>