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247 lines
9.2 KiB
XML
247 lines
9.2 KiB
XML
<chapter id="cha-elements">
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<title>GstElement</title>
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<para>
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The most important object in <application>GStreamer</application> for the
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application programmer is the <classname>GstElement</classname> object.
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</para>
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<sect1 id="sec-elements-design">
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<title>What is a GstElement</title>
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<para>
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<classname>GstElement</classname> is the basic building block for the
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media pipeline. All the different components you are going to use are
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derived from <classname>GstElement</classname>. This means that a
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lot of functions you are going to use operate on objects of this class.
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</para>
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<para>
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Elements, from the perspective of GStreamer, are viewed as "black boxes"
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with a number of different aspects. One of these aspects is the presence
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of "pads", or connection points. This terminology arises from soldering;
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pads are where wires can be attached.
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</para>
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<sect2 id="sec-elements-src">
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<title>Source elements</title>
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<para>
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Source elements generate data for use by a pipeline, for example
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reading from disk or from a sound card.
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</para>
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<para>
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Below you see how we will visualize the element.
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We always draw a source pad to the right of the element.
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</para>
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<figure float="1" id="sec-element-srcimg">
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<title>Visualisation of a source element</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/src-element.&magic;" format="&MAGIC;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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Source elements do not accept data, they only generate data. You can
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see this in the figure because it only has a source pad. A source
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pad can only generate data.
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</para>
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</sect2>
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<sect2 id="sec-elements-filter">
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<title>Filters and codecs</title>
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<para>
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Filter elements both have input and output pads. They operate on
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data they receive in their sink pads and produce data on their source
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pads. For example, MPEG decoders and volume filters would fall into
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this category.
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</para>
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<para>
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Elements are not constrained as to the number of pads they might have;
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for example, a video mixer might have two input pads (the images of
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the two different video streams) and one output pad.
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</para>
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<figure float="1" id="sec-element-filterimg">
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<title>Visualisation of a filter element</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/filter-element.&magic;" format="&MAGIC;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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The above figure shows the visualisation of a filter element.
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This element has one sink (input) pad and one source (output) pad.
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Sink pads are drawn on the left of the element.
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</para>
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<figure float="1" id="sec-element-multifilterimg">
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<title>Visualisation of a filter element with
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more than one output pad</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/filter-element-multi.&magic;"
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format="&MAGIC;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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The above figure shows the visualisation of a filter element with
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more than one output pad. An example of such a filter is the AVI
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splitter (demultiplexer). This element will parse the input data and
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extract the audio and video data. Most of these filters dynamically
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send out a signal when a new pad is created so that the application
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programmer can connect an arbitrary element to the newly created pad.
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</para>
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</sect2>
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<sect2 id="sec-elements-sink">
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<title>Sink elements</title>
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<para>
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Sink elements are terminal points in a media pipeline. They accept
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data but do not produce anything. Disk writing, soundcard playback,
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and video output would all be implemented by sink elements.
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</para>
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<figure float="1" id="sec-element-sinkimg">
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<title>Visualisation of a sink element</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="images/sink-element.&magic;" format="&MAGIC;" />
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</imageobject>
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</mediaobject>
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</figure>
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</sect2>
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</sect1>
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<sect1 id="sec-elements-create">
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<title>Creating a GstElement</title>
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<para>
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A <classname>GstElement</classname> object is created from
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a factory. To create an element, you have to get access to a
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<classname>GstElementFactory</classname> object using a unique
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factory name.
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</para>
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<para>
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The following code example is used to get a factory that can be used
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to create the 'mad' element, an mp3 decoder.
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</para>
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<programlisting>
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GstElementFactory *factory;
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factory = gst_element_factory_find ("mad");
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</programlisting>
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<para>
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Once you have the handle to the element factory, you can create a
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real element with the following code fragment:
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</para>
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<programlisting>
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GstElement *element;
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element = gst_element_factory_create (factory, "decoder");
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</programlisting>
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<para>
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<function>gst_element_factory_create</function> will use the element
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factory to create an element with the given name. The name of the
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element is something you can use later on to look up the element in
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a bin, for example. You can pass <symbol>NULL</symbol> as the name
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argument to get a unique, default name.
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</para>
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<para>
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A simple shortcut exists for creating an element from a factory. The
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following example creates an element named "decoder" from the element
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factory named "mad". This convenience function is most widely used to
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create an element.
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</para>
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<programlisting>
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GstElement *element;
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element = gst_element_factory_make ("mad", "decoder");
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</programlisting>
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<para>
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When you don't need the element anymore, you need to unref it, as shown in the following
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example.
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</para>
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<programlisting>
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GstElement *element;
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...
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gst_element_unref (element);
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</programlisting>
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</sect1>
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<sect1 id="sec-elements-properties">
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<title>GstElement properties</title>
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<para>
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A <classname>GstElement</classname> can have several properties
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which are implemented using standard <classname>GObject</classname>
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properties. The usual <classname>GObject</classname> methods to query,
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set and get property values and <classname>GParamSpecs</classname>
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are therefore supported.
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</para>
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<para>
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Every <classname>GstElement</classname> inherits at least
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one property of its parent <classname>GstObject</classname>:
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the "name" property. This is the name you provide to the
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functions <function>gst_element_factory_make</function> or
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<function>gst_element_factory_create</function>. You can get and set
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this property using the functions
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<function>gst_object_set_name</function>
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and <function>gst_object_get_name</function> or use the
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<classname>GObject</classname> property mechanism as shown below.
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</para>
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<programlisting>
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GstElement *element;
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GValue value = { 0, }; /* initialize the GValue for g_object_get() */
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element = gst_element_factory_make ("mad", "decoder");
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g_object_set (G_OBJECT (element), "name", "mydecoder", NULL);
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...
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g_value_init (&value, G_TYPE_STRING);
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g_object_get_property (G_OBJECT (element), "name", &value);
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...
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</programlisting>
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<para>
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Most plugins provide additional properties to provide more information
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about their configuration or to configure the element.
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<command>gst-inspect</command> is a useful tool to query the properties
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of a particular element, it will also use property introspection to give
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a short explanation about the function of the property and about the
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parameter types and ranges it supports.
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</para>
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<para>
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For more information about <classname>GObject</classname>
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properties we recommend you read the <ulink
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url="http://developer.gnome.org/doc/API/2.0/gobject/index.html"
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type="http">GObject manual</ulink>.
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</para>
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</sect1>
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<sect1 id="sec-elements-signals">
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<title>GstElement signals</title>
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<para>
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A <classname>GstElement</classname> also provides various
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<classname>GObject</classname> signals that can be used as a flexible
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callback mechanism.
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</para>
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</sect1>
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<sect1 id="sec-elements-factories">
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<title>More about GstElementFactory</title>
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<para>
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We talk some more about the GstElementFactory object.
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</para>
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<sect2 id="sec-elements-factories-details">
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<title>Getting information about an element using the factory details</title>
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<para>
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</para>
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</sect2>
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<sect2 id="sec-elements-factories-padtemplates">
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<title>Finding out what pads an element can contain</title>
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<para>
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</para>
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</sect2>
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<sect2 id="sec-elements-factories-query">
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<title>Different ways of querying the factories</title>
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<para>
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</para>
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</sect2>
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</sect1>
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</chapter>
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