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>Chapter 2. Basic Concepts</H1
><P
> This chapter of the guide introduces the basic concepts of <SPAN
CLASS="application"
>GStreamer</SPAN
>.
Understanding these concepts will help you grok the issues involved in
extending <SPAN
CLASS="application"
>GStreamer</SPAN
>. Many of these concepts are explained in greater
detail in the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>GStreamer Application Development Manual</I
></SPAN
>; the basic concepts presented here serve mainly
to refresh your memory.
</P
><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="section-basics-elements"
>2.1. Elements and Plugins</A
></H1
><P
> Elements are at the core of <SPAN
CLASS="application"
>GStreamer</SPAN
>. In the context of plugin
development, an <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>element</I
></SPAN
> is an object derived from the
<CODE
CLASS="classname"
>GstElement</CODE
> class. Elements provide some sort of
functionality when linked with other elements: For example, a source
element provides data to a stream, and a filter element acts on the data
in a stream. Without elements, <SPAN
CLASS="application"
>GStreamer</SPAN
> is just a bunch of conceptual
pipe fittings with nothing to link. A large number of elements ship
with <SPAN
CLASS="application"
>GStreamer</SPAN
>, but extra elements can also be written.
</P
><P
> Just writing a new element is not entirely enough, however: You will need
to encapsulate your element in a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>plugin</I
></SPAN
> to enable
<SPAN
CLASS="application"
>GStreamer</SPAN
> to use it. A plugin is essentially a loadable block of code,
usually called a shared object file or a dynamically linked library. A
single plugin may contain the implementation of several elements, or just
a single one. For simplicity, this guide concentrates primarily on plugins
containing one element.
</P
><P
> A <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>filter</I
></SPAN
> is an important type of element that
processes a stream of data. Producers and consumers of data are called
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>source</I
></SPAN
> and <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>sink</I
></SPAN
> elements,
respectively. <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Bin</I
></SPAN
> elements contain other elements.
One type of bin is responsible for scheduling the elements that they
contain so that data flows smoothly. Another type of bin, called
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>autoplugger</I
></SPAN
> elements, automatically add other
elements to the bin and link them together so that they act as a
filter between two arbitary stream types.
</P
><P
> The plugin mechanism is used everywhere in <SPAN
CLASS="application"
>GStreamer</SPAN
>, even if only the
standard packages are being used. A few very basic functions reside in the
core library, and all others are implemented in plugins. A plugin registry
is used to store the details of the plugins in an XML file. This way, a
program using <SPAN
CLASS="application"
>GStreamer</SPAN
> does not have to load all plugins to determine
which are needed. Plugins are only loaded when their provided elements are
requested.
</P
><P
> See the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>GStreamer Library Reference</I
></SPAN
> for the current implementation details of <A
HREF="../gstreamer/gstelement.html"
TARGET="_top"
><CODE
CLASS="classname"
>GstElement</CODE
></A
>
and <A
HREF="../gstreamer/gstreamer-gstplugin.html"
TARGET="_top"
><CODE
CLASS="classname"
>GstPlugin</CODE
></A
>.
</P
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