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Original commit message from CVS: 2004-07-15 Zaheer Abbas Merali <zaheerabbas at merali dot org> * docs/manual/elements.xml: Fixed image reference
118 lines
4.7 KiB
XML
118 lines
4.7 KiB
XML
<chapter id="chapter-elements">
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<title>Elements</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="section-elements-design">
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<title>What is an element ?</title>
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<para>
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An element is the basic building block for the media pipeline.
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All the different high-level 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" (see <xref linkend="chapter-pads"/>), or link 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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</sect1>
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<sect1 id="section-elements-types">
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<title>Types of elements</title>
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<sect2 id="section-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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<xref linkend="section-element-srcimg"/> shows how we will visualise
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a source 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="section-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.ℑ" format="&IMAGE;" />
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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="section-elements-filter">
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<title>Filters and codecs</title>
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<para>
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Filter elements have both 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="section-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.ℑ" format="&IMAGE;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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<xref linkend="section-element-filterimg"/> shows how we will visualise
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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="section-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.ℑ"
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format="&IMAGE;" />
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</imageobject>
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</mediaobject>
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</figure>
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<para>
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<xref linkend="section-element-multifilterimg"/> 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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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 link an arbitrary element to the newly created pad.
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</para>
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</sect2>
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<sect2 id="section-elements-sink">
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<title>Sink elements</title>
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<para>
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Sink elements are end 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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<xref linkend="section-element-sinkimg"/> shows a sink element.
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</para>
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<figure float="1" id="section-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.ℑ" format="&IMAGE;" />
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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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</chapter>
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