mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
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472 lines
18 KiB
XML
472 lines
18 KiB
XML
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<!-- ############ 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 see 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. An element provides some sort of
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functionality when connected with other elements. Without elements,
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&GStreamer; is just a bunch of conceptual pipe fittings with nothing to
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connect. A large number of elements ship with &GStreamer;, but extra
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elements can also be written. The purpose of this guide is to help you
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learn to create new elements, which are encapsulated in plugins as
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described below.
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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, as opposed to producing or consuming streams
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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.
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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 plugin to enable &GStreamer; to use it. A
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<emphasis>plugin</emphasis> is essentially a loadable block of code,
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usually a shared object file or dynamically linked library. A single
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plugin may contain the implementation of several elements, or just a
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single one. For simplicity, this guide concentrates primarily on plugins
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containing one filter type element.
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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 package is 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="http://gstreamer.net/docs/0.4.0/gstreamer/gstelement.html"><classname>GstElement</classname></ulink>
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and <ulink type="http"
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url="http://gstreamer.net/docs/0.4.0/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 connections 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> on an element where connections may be made with
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other elements. Pads have specific data handling capabilities: That is, a
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pad only knows how to give or receive certain types of data. Connections
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are only allowed when the capabilities of 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 television. Connecting the DVD player
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to the amplifier is allowed only because both devices have audio jacks,
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and connecting the television to the DVD player is allowed because both
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devices have compatible video jacks. Pads in &GStreamer; serve the same
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purpose as the jacks in the home theater system.
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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="http://gstreamer.net/docs/0.4.0/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. Buffers may contain any
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sort of data that the two pads involved 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 the next part about <xref
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linkend="sect2-buffers-bufferpools"/> 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="http://gstreamer.net/docs/0.4.0/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.
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</para>
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<para>
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Normally, filter elements in &GStreamer; deal with a buffer in place,
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meaning that they do not create or destroy buffers. Sometimes, however,
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elements might need to alter the reference count of a buffer to copy or
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destroy the buffer, or to create a new buffer. For the most part, this
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guide does not deal with elements that alter a buffer's reference count,
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but buffer referencing is an important concept to know.
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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 connecting pads between elements. Each connection 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
|
||
|
|
entirely separate signals. If true, the <quote>channels</quote>
|
||
|
|
attribute must not be zero.
|
||
|
|
</entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
<!-- ############ type ############# -->
|
||
|
|
|
||
|
|
<row>
|
||
|
|
<entry morerows="0">audio/x-ogg</entry>
|
||
|
|
<entry morerows="0">
|
||
|
|
Audio data compressed using the Ogg Vorbis encoding scheme.
|
||
|
|
</entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry>
|
||
|
|
FIXME: There are currently no parameters defined for this type.
|
||
|
|
</entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
<!-- ############ type ############# -->
|
||
|
|
|
||
|
|
<row>
|
||
|
|
<entry morerows="2">video/raw</entry>
|
||
|
|
<entry morerows="2">
|
||
|
|
Raw video data.
|
||
|
|
</entry>
|
||
|
|
<entry>fourcc</entry>
|
||
|
|
<entry>FOURCC code</entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry>
|
||
|
|
A FOURCC code identifying the format in which this data is stored.
|
||
|
|
FOURCC (Four Character Code) is a simple system to allow
|
||
|
|
unambiguous identification of a video datastream format. See
|
||
|
|
<ulink url="http://www.webartz.com/fourcc/"
|
||
|
|
type="http">http://www.webartz.com/fourcc/</ulink>
|
||
|
|
</entry>
|
||
|
|
</row>
|
||
|
|
<row>
|
||
|
|
<entry>width</entry>
|
||
|
|
<entry>integer</entry>
|
||
|
|
<entry>greater than 0</entry>
|
||
|
|
<entry>
|
||
|
|
The number of pixels wide that each video frame is.
|
||
|
|
</entry>
|
||
|
|
</row>
|
||
|
|
<row>
|
||
|
|
<entry>height</entry>
|
||
|
|
<entry>integer</entry>
|
||
|
|
<entry>greater than 0</entry>
|
||
|
|
<entry>
|
||
|
|
The number of pixels high that each video frame is.
|
||
|
|
</entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
<!-- ############ type ############# -->
|
||
|
|
|
||
|
|
<row>
|
||
|
|
<entry morerows="0">video/mpeg</entry>
|
||
|
|
<entry morerows="0">
|
||
|
|
Video data compressed using an MPEG encoding scheme.
|
||
|
|
</entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry>
|
||
|
|
FIXME: There are currently no parameters defined for this type.
|
||
|
|
</entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
<!-- ############ type ############# -->
|
||
|
|
|
||
|
|
<row>
|
||
|
|
<entry morerows="0">video/avi</entry>
|
||
|
|
<entry morerows="0">
|
||
|
|
Video data compressed using the AVI encoding scheme.
|
||
|
|
</entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry></entry>
|
||
|
|
<entry>
|
||
|
|
FIXME: There are currently no parameters defined for this type.
|
||
|
|
</entry>
|
||
|
|
</row>
|
||
|
|
|
||
|
|
</tbody>
|
||
|
|
</tgroup>
|
||
|
|
</table>
|
||
|
|
</sect2>
|
||
|
|
</sect1>
|
||
|
|
|
||
|
|
<!-- ############ sect1 ############# -->
|
||
|
|
|
||
|
|
<sect1 id="sect1-basics-events" xreflabel="Events">
|
||
|
|
<title>Events</title>
|
||
|
|
<para>
|
||
|
|
Events are a special type of data in &GStreamer;.
|
||
|
|
<emphasis>Events</emphasis> indicate some sort of notable event that has
|
||
|
|
happened somewhere in an element's pipeline. Just like any other data
|
||
|
|
type, an event comes to an element through its pads. Events are contained
|
||
|
|
in a <classname>GstBuffer</classname>, so an event buffer will contain
|
||
|
|
only an event, not any other type of data.
|
||
|
|
</para>
|
||
|
|
<para>
|
||
|
|
See the &GstLibRef; for the current implementation details of a <ulink
|
||
|
|
type="http"
|
||
|
|
url="http://gstreamer.net/docs/0.4.0/gstreamer/gstreamer-gstevent.html"><classname>GstEvent</classname></ulink>.
|
||
|
|
</para>
|
||
|
|
</sect1>
|
||
|
|
</chapter>
|