mirror of
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501 lines
20 KiB
XML
501 lines
20 KiB
XML
<!-- ############ chapter ############# -->
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<chapter id="cha-intro-basics" xreflabel="Basic Concepts">
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<title>Basic Concepts</title>
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<para>
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This chapter of the guide introduces the basic concepts of &GStreamer;.
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Understanding these concepts will help you grok the issues involved in
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extending &GStreamer;. Many of these concepts are explained in greater
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detail in the &GstAppDevMan;; the basic concepts presented here serve mainly
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to refresh your memory.
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</para>
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<!-- ############ sect1 ############# -->
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<sect1 id="sect1-basics-elements" xreflabel="Elements and Plugins">
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<title>Elements and Plugins</title>
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<para>
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Elements are at the core of &GStreamer;. In the context of plugin
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development, an <emphasis>element</emphasis> is an object derived from the
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<classname>GstElement</classname> class. Elements provide some sort of
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functionality when linked with other elements: For example, a source
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element provides data to a stream, and a filter element acts on the data
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in a stream. Without elements, &GStreamer; is just a bunch of conceptual
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pipe fittings with nothing to link. A large number of elements ship
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with &GStreamer;, but extra elements can also be written.
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</para>
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<para>
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Just writing a new element is not entirely enough, however: You will need
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to encapsulate your element in a <emphasis>plugin</emphasis> to enable
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&GStreamer; to use it. A plugin is essentially a loadable block of code,
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usually called a shared object file or a dynamically linked library. A
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single plugin may contain the implementation of several elements, or just
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a single one. For simplicity, this guide concentrates primarily on plugins
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containing one element.
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</para>
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<para>
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A <emphasis>filter</emphasis> is an important type of element that
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processes a stream of data. Producers and consumers of data are called
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<emphasis>source</emphasis> and <emphasis>sink</emphasis> elements,
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respectively. <emphasis>Bin</emphasis> elements contain other elements.
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One type of bin is responsible for scheduling the elements that they
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contain so that data flows smoothly. Another type of bin, called
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<emphasis>autoplugger</emphasis> elements, automatically add other
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elements to the bin and link them together so that they act as a
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filter between two arbitary stream types.
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</para>
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<para>
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The plugin mechanism is used everywhere in &GStreamer;, even if only the
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standard packages are being used. A few very basic functions reside in the
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core library, and all others are implemented in plugins. A plugin registry
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is used to store the details of the plugins in an XML file. This way, a
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program using &GStreamer; does not have to load all plugins to determine
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which are needed. Plugins are only loaded when their provided elements are
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requested.
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</para>
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<para>
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See the &GstLibRef; for the current implementation details of <ulink
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type="http"
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url="../gstreamer/gstelement.html"><classname>GstElement</classname></ulink>
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and <ulink type="http"
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url="../gstreamer/gstreamer-gstplugin.html"><classname>GstPlugin</classname></ulink>.
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</para>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="sect1-basics-pads" xreflabel="Pads">
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<title>Pads</title>
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<para>
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<emphasis>Pads</emphasis> are used to negotiate links and data flow
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between elements in &GStreamer;. A pad can be viewed as a
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<quote>place</quote> or <quote>port</quote> on an element where
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links may be made with other elements, and through which data can
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flow to or from those elements. Pads have specific data handling
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capabilities: A pad can restrict the type of data that flows
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through it. Links are only allowed between two pads when the
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allowed data types of the two pads are compatible.
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</para>
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<para>
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An analogy may be helpful here. A pad is similar to a plug or jack on a
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physical device. Consider, for example, a home theater system consisting
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of an amplifier, a DVD player, and a (silent) video projector. Linking
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the DVD player to the amplifier is allowed because both devices have audio
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jacks, and linking the projector to the DVD player is allowed because
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both devices have compatible video jacks. Links between the
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projector and the amplifier may not be made because the projector and
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amplifier have different types of jacks. Pads in &GStreamer; serve the
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same purpose as the jacks in the home theater system.
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</para>
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<para>
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For the most part, all data in &GStreamer; flows one way through a link
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between elements. Data flows out of one element through one or more
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<emphasis>source pads</emphasis>, and elements accept incoming data through
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one or more <emphasis>sink pads</emphasis>. Source and sink elements have
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only source and sink pads, respectively.
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</para>
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<para>
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See the &GstLibRef; for the current implementation details of a <ulink
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type="http"
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url="../gstreamer/gstreamer-gstpad.html"><classname>GstPad</classname></ulink>.
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</para>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="sect1-basics-buffers" xreflabel="Buffers">
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<title>Buffers</title>
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<para>
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All streams of data in &GStreamer; are chopped up into chunks that are
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passed from a source pad on one element to a sink pad on another element.
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<emphasis>Buffers</emphasis> are structures used to hold these chunks of
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data. Buffers can be of any size, theoretically, and they may contain any
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sort of data that the two linked pads know how to handle. Normally, a
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buffer contains a chunk of some sort of audio or video data that flows
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from one element to another.
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</para>
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<para>
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Buffers also contain metadata describing the buffer's contents. Some of
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the important types of metadata are:
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<itemizedlist>
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<listitem>
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<para>
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A pointer to the buffer's data.
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</para>
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</listitem>
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<listitem>
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<para>
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An integer indicating the size of the buffer's data.
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</para>
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</listitem>
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<listitem>
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<para>
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A <classname>GstData</classname> object describing the type of the
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buffer's data.
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</para>
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</listitem>
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<listitem>
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<para>
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A reference count indicating the number of elements currently
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holding a reference to the buffer. When the buffer reference count
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falls to zero, the buffer will be unlinked, and its memory will be
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freed in some sense (see below for more details).
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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See the &GstLibRef; for the current implementation details of a <ulink
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type="http"
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url="../gstreamer/gstreamer-gstbuffer.html"><classname>GstBuffer</classname></ulink>.
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</para>
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<sect2 id="sect2-buffers-bufferpools" xreflabel="Buffer Allocation and
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Buffer Pools">
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<title>Buffer Allocation and Buffer Pools</title>
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<para>
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Buffers can be allocated using various schemes, and they may either be
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passed on by an element or unreferenced, thus freeing the memory used by
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the buffer. Buffer allocation and unlinking are important concepts when
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dealing with real time media processing, since memory allocation is
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relatively slow on most systems.
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</para>
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<para>
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To improve the latency in a media pipeline, many &GStreamer; elements
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use a <emphasis>buffer pool</emphasis> to handle buffer allocation and
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unlinking. A buffer pool is a relatively large chunk of memory that is
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the &GStreamer; process requests early on from the operating system.
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Later, when elements request memory for a new buffer, the buffer pool
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can serve the request quickly by giving out a piece of the allocated
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memory. This saves a call to the operating system and lowers latency.
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[If it seems at this point like &GStreamer; is acting like an operating
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system (doing memory management, etc.), don't worry: &GStreamer;OS isn't
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due out for quite a few years!]
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</para>
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<para>
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Normally in a media pipeline, most filter elements in &GStreamer; deal
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with a buffer in place, meaning that they do not create or destroy
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buffers. Sometimes, however, elements might need to alter the reference
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count of a buffer, either by copying or destroying the buffer, or by
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creating a new buffer. These topics are generally reserved for
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non-filter elements, so they will be addressed at that point.
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</para>
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</sect2>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="sect1-basics-types" xreflabel="Types and Properties">
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<title>Types and Properties</title>
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<para>
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&GStreamer; uses a type system to ensure that the data passed between
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elements is in a recognized format. The type system is also important for
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ensuring that the parameters required to fully specify a format match up
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correctly when linking pads between elements. Each link that is
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made between elements has a specified type.
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</para>
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<!-- ############ sect2 ############# -->
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<sect2 id="sect2-types-basictypes" xreflabel="Basic Types">
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<title>The Basic Types</title>
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<para>
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&GStreamer; already supports many basic media types. Following is a
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table of the basic types used for buffers in &GStreamer;. The table
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contains the name ("mime type") and a description of the type, the
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properties associated with the type, and the meaning of each property.
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</para>
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<table frame="all" id="table-basictypes" xreflabel="Table of Basic Types">
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<title>Table of Basic Types</title>
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<tgroup cols="6" align="left" colsep="1" rowsep="1">
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<thead>
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<row>
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<entry>Mime Type</entry>
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<entry>Description</entry>
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<entry>Property</entry>
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<entry>Property Type</entry>
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<entry>Property Values</entry>
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<entry>Property Description</entry>
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</row>
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</thead>
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<tbody valign="top">
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<!-- ############ type ############# -->
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<row>
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<entry morerows="10">audio/raw</entry>
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<entry morerows="10">
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Unstructured and uncompressed raw audio data.
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</entry>
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<entry>rate</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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The sample rate of the data, in samples per second.
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</entry>
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</row>
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<row>
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<entry>channels</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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The number of channels of audio data.
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</entry>
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</row>
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<row>
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<entry>format</entry>
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<entry>string</entry>
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<entry><quote>int</quote> or <quote>float</quote></entry>
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<entry>
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The format in which the audio data is passed.
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</entry>
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</row>
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<row>
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<entry>law</entry>
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<entry>integer</entry>
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<entry>0, 1, or 2</entry>
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<entry>
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(Valid only if the data is in integer format.) The law used to
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describe the data. The value 0 indicates <quote>linear</quote>, 1
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indicates <quote>mu law</quote>, and 2 indicates
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<quote>A law</quote>.
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</entry>
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</row>
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<row>
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<entry>endianness</entry>
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<entry>boolean</entry>
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<entry>0 or 1</entry>
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<entry>
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(Valid only if the data is in integer format.) The order of bytes
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in a sample. The value 0 means <quote>little-endian</quote> (bytes
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are least significant first). The value 1 means
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<quote>big-endian</quote> (most significant byte first).
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</entry>
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</row>
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<row>
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<entry>signed</entry>
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<entry>boolean</entry>
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<entry>0 or 1</entry>
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<entry>
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(Valid only if the data is in integer format.) Whether the samples
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are signed or not.
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</entry>
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</row>
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<row>
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<entry>width</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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(Valid only if the data is in integer format.) The number of bits
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per sample.
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</entry>
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</row>
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<row>
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<entry>depth</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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(Valid only if the data is in integer format.) The number of bits
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used per sample. This must be less than or equal to the width: If
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the depth is less than the width, the low bits are assumed to be
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the ones used. For example, a width of 32 and a depth of 24 means
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that each sample is stored in a 32 bit word, but only the low 24
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bits are actually used.
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</entry>
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</row>
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<row>
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<entry>layout</entry>
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<entry>string</entry>
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<entry><quote>gfloat</quote></entry>
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<entry>
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(Valid only if the data is in float format.) A string representing
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the way in which the floating point data is represented.
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</entry>
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</row>
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<row>
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<entry>intercept</entry>
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<entry>float</entry>
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<entry>any, normally 0</entry>
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<entry>
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(Valid only if the data is in float format.) A floating point
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value representing the value that the signal
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<quote>centers</quote> on.
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</entry>
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</row>
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<row>
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<entry>slope</entry>
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<entry>float</entry>
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<entry>any, normally 1.0</entry>
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<entry>
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(Valid only if the data is in float format.) A floating point
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value representing how far the signal deviates from the intercept.
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A slope of 1.0 and an intercept of 0.0 would mean an audio signal
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with minimum and maximum values of -1.0 and 1.0. A slope of
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0.5 and intercept of 0.5 would represent values in the range 0.0
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to 1.0.
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</entry>
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</row>
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<!-- ############ type ############# -->
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<row>
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<entry morerows="4">audio/mp3</entry>
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<entry morerows="4">
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Audio data compressed using the mp3 encoding scheme.
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</entry>
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<entry>framed</entry>
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<entry>boolean</entry>
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<entry>0 or 1</entry>
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<entry>
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A true value indicates that each buffer contains exactly one
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frame. A false value indicates that frames and buffers do not
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necessarily match up.
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</entry>
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</row>
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<row>
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<entry>layer</entry>
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<entry>integer</entry>
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<entry>1, 2, or 3</entry>
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<entry>
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The compression scheme layer used to compress the data.
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</entry>
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</row>
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<row>
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<entry>bitrate</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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The bitrate, in kilobits per second. For VBR (variable bitrate)
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mp3 data, this is the average bitrate.
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</entry>
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</row>
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<row>
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<entry>channels</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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The number of channels of audio data present.
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</entry>
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</row>
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<row>
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<entry>joint-stereo</entry>
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<entry>boolean</entry>
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<entry>0 or 1</entry>
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<entry>
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If true, this implies that stereo data is stored as a combined
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signal and the difference between the signals, rather than as two
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entirely separate signals. If true, the <quote>channels</quote>
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attribute must not be zero.
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</entry>
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</row>
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<!-- ############ type ############# -->
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<row>
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<entry morerows="0">audio/x-ogg</entry>
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<entry morerows="0">
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Audio data compressed using the Ogg Vorbis encoding scheme.
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</entry>
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<entry></entry>
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<entry></entry>
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<entry></entry>
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<entry>
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FIXME: There are currently no parameters defined for this type.
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</entry>
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</row>
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<!-- ############ type ############# -->
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<row>
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<entry morerows="2">video/raw</entry>
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<entry morerows="2">
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Raw video data.
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</entry>
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<entry>fourcc</entry>
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<entry>FOURCC code</entry>
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<entry></entry>
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<entry>
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A FOURCC code identifying the format in which this data is stored.
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FOURCC (Four Character Code) is a simple system to allow
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unambiguous identification of a video datastream format. See
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<ulink url="http://www.webartz.com/fourcc/"
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type="http">http://www.webartz.com/fourcc/</ulink>
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</entry>
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</row>
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<row>
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<entry>width</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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The number of pixels wide that each video frame is.
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</entry>
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</row>
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<row>
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<entry>height</entry>
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<entry>integer</entry>
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<entry>greater than 0</entry>
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<entry>
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The number of pixels high that each video frame is.
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</entry>
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</row>
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<!-- ############ type ############# -->
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<row>
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<entry morerows="0">video/mpeg</entry>
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<entry morerows="0">
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Video data compressed using an MPEG encoding scheme.
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</entry>
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<entry></entry>
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<entry></entry>
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<entry></entry>
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<entry>
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FIXME: There are currently no parameters defined for this type.
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</entry>
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</row>
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<!-- ############ type ############# -->
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<row>
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<entry morerows="0">video/x-msvideo</entry>
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<entry morerows="0">
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Video data compressed using the AVI encoding scheme.
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</entry>
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<entry></entry>
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<entry></entry>
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<entry></entry>
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<entry>
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FIXME: There are currently no parameters defined for this type.
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</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</sect2>
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</sect1>
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<!-- ############ sect1 ############# -->
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<sect1 id="sect1-basics-events" xreflabel="Events">
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<title>Events</title>
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<para>
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Sometimes elements in a media processing pipeline need to know that
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something has happened. An <emphasis>event</emphasis> is a special type of
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data in &GStreamer; designed to serve this purpose. Events describe some
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sort of activity that has happened somewhere in an element's pipeline, for
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example, the end of the media stream or a clock discontinuity. Just like
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any other data type, an event comes to an element on a sink pad and is
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contained in a normal buffer. Unlike normal stream buffers, though, an
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event buffer contains only an event, not any media stream data.
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</para>
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<para>
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See the &GstLibRef; for the current implementation details of a <ulink
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type="http"
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url="../gstreamer/gstreamer-gstevent.html"><classname>GstEvent</classname></ulink>.
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</para>
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</sect1>
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</chapter>
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