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227 lines
8.4 KiB
XML
227 lines
8.4 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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The GstElement is the basic building block for the media pipeline. All the
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different components you are going to use are derived from this GstElement.
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This means that a lot of functions you are going to use operate on this object.
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</para>
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<para> Elements, from the perspective of GStreamer, are viewed as "black boxes" with a number of
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different aspects. One of these aspects is the presence of "pads", or connection points. This
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terminology arises from soldering; 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 reading from disk or from a
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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 src 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 see
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this in the figure because it only has a src pad. A src pad can only
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generate buffers.
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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 data they receive in their
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sink pads and produce data on their src pads. For example, MPEG decoders and volume filters
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would fall into 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 migh have; for example, a video
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mixer might have two input pads (the images of the two different video streams) and one
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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. This element has
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one sink (input) pad and one src (output) pad. Sink pads are drawn on the left
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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;" 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 more than one output pad.
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An example of such a filter is the AVI splitter (demuxer). This element will parse the input
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data and extracts the audio and video data. Most of these filters dynamically send out a
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signal when a new pad is created so that the application programmer can connect an arbitrary
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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 data but do not produce
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anything. Disk writing, soundcard playback, and video output woul all be implemented by sink
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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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GstElements are created from factories. To create an element, one has to get
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access the a <classname>GstElementFactory</classname> using a unique factoryname.
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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 to create the
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'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 elementfactory, you can create a real element with
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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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gst_element_factory_create () will use the elementfactory to create an element with the given
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name. The name of the element is something you can use later on to lookup the element in a
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bin, for example. You can pass NULL as the name 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 following example creates
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an element, named "decoder" from the elementfactory named "mad". This convenient function is
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most widely used to 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 GstElement can have several properties which are implemented using standard
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GObject properties. The usual GObject methods to query, set and get property values
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and GParamSpecs are therefore supported.
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</para>
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<para>
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Every GstElement inherits at least one property of its parent GstObject, the "name"
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property. This is the name you provide to gst_element_factory_make() or
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gst_element_factory_create(). You can get and set this property using the
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gst_object_set_name() and gst_object_get_name() or use the GObject property
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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 perticular 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 GObject properties we recommend to read the GObject
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manual.
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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 GstElement also provides various GObject 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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