gstreamer/docs/pwg/intro-basics.xml

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<chapter id="cha-intro-basics" xreflabel="Basic Concepts">
  <title>Basic Concepts</title>
  <para>
    This chapter of the guide introduces the basic concepts of &GStreamer;.
    Understanding these concepts will help you grok the issues involved in
    extending &GStreamer;. Many of these concepts are explained in greater
    detail in the &GstAppDevMan;; the basic concepts presented here serve mainly
    to refresh your memory.
  </para>

  <!-- ############ sect1 ############# -->

  <sect1 id="sect1-basics-elements" xreflabel="Elements and Plugins">
    <title>Elements and Plugins</title>
    <para>
      Elements are at the core of &GStreamer;. In the context of plugin
      development, an <emphasis>element</emphasis> is an object derived from the
      <classname>GstElement</classname> class. Elements provide some sort of
      functionality when linked with other elements: For example, a source
      element provides data to a stream, and a filter element acts on the data
      in a stream. Without elements, &GStreamer; is just a bunch of conceptual
      pipe fittings with nothing to link. A large number of elements ship
      with &GStreamer;, but extra elements can also be written.
    </para>
    <para>
      Just writing a new element is not entirely enough, however: You will need
      to encapsulate your element in a <emphasis>plugin</emphasis> to enable
      &GStreamer; to use it. A plugin is essentially a loadable block of code,
      usually called a shared object file or a dynamically linked library. A
      single plugin may contain the implementation of several elements, or just
      a single one. For simplicity, this guide concentrates primarily on plugins
      containing one element.
    </para>
    <para>
      A <emphasis>filter</emphasis> is an important type of element that
      processes a stream of data. Producers and consumers of data are called
      <emphasis>source</emphasis> and <emphasis>sink</emphasis> elements,
      respectively. Elements that link other elements together are called
      <emphasis>autoplugger</emphasis> elements, and a <emphasis>bin</emphasis>
      element contains other elements. Bins are often responsible for scheduling
      the elements that they contain so that data flows smoothly.
    </para>
    <para>
      The plugin mechanism is used everywhere in &GStreamer;, even if only the
      standard package is being used. A few very basic functions reside in the
      core library, and all others are implemented in plugins. A plugin registry
      is used to store the details of the plugins in an XML file. This way, a
      program using &GStreamer; does not have to load all plugins to determine
      which are needed. Plugins are only loaded when their provided elements are
      requested.
    </para>
    <para>
      See the &GstLibRef; for the current implementation details of <ulink
      type="http"
      url="http://gstreamer.net/docs/0.4.0/gstreamer/gstelement.html"><classname>GstElement</classname></ulink>
      and <ulink type="http"
      url="http://gstreamer.net/docs/0.4.0/gstreamer/gstreamer-gstplugin.html"><classname>GstPlugin</classname></ulink>.
    </para>
  </sect1>

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  <sect1 id="sect1-basics-pads" xreflabel="Pads">
    <title>Pads</title>
    <para>
      <emphasis>Pads</emphasis> are used to negotiate links and data flow
      between elements in &GStreamer;. A pad can be viewed as a
      <quote>place</quote> or <quote>port</quote> on an element where
      links may be made with other elements. Pads have specific data
      handling capabilities: A pad only knows how to give or receive certain
      types of data. Links are only allowed when the capabilities of two
      pads are compatible.
    </para>
    <para>
      An analogy may be helpful here. A pad is similar to a plug or jack on a
      physical device. Consider, for example, a home theater system consisting
      of an amplifier, a DVD player, and a (silent) video projector. Linking
      the DVD player to the amplifier is allowed because both devices have audio
      jacks, and linking the projector to the DVD player is allowed because
      both devices have compatible video jacks. Links between the
      projector and the amplifier may not be made because the projector and
      amplifier have different types of jacks. Pads in &GStreamer; serve the
      same purpose as the jacks in the home theater system.
    </para>
    <para>
      For the moment, all data in &GStreamer; flows one way through a link
      between elements. Data flows out of one element through one or more
      <emphasis>source pads</emphasis>, and elements accept incoming data through
      one or more <emphasis>sink pads</emphasis>. Source and sink elements have
      only source and sink pads, respectively.
    </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-gstpad.html"><classname>GstPad</classname></ulink>.
    </para>
  </sect1>

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  <sect1 id="sect1-basics-buffers" xreflabel="Buffers">
    <title>Buffers</title>
    <para>
      All streams of data in &GStreamer; are chopped up into chunks that are
      passed from a source pad on one element to a sink pad on another element.
      <emphasis>Buffers</emphasis> are structures used to hold these chunks of
      data. Buffers can be of any size, theoretically, and they may contain any
      sort of data that the two linked pads know how to handle. Normally, a
      buffer contains a chunk of some sort of audio or video data that flows
      from one element to another.
    </para>
    <para>
      Buffers also contain metadata describing the buffer's contents. Some of
      the important types of metadata are:
      <itemizedlist>
        <listitem>
          <para>
            A pointer to the buffer's data.
          </para>
        </listitem>
        <listitem>
          <para>
            An integer indicating the size of the buffer's data.
          </para>
        </listitem>
        <listitem>
          <para>
            A <classname>GstData</classname> object describing the type of the
            buffer's data.
          </para>
        </listitem>
        <listitem>
          <para>
            A reference count indicating the number of elements currently
            holding a reference to the buffer. When the buffer reference count
            falls to zero, the buffer will be unlinked, and its memory will be
            freed in some sense (see below for more details).
          </para>
        </listitem>
      </itemizedlist>
    </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-gstbuffer.html"><classname>GstBuffer</classname></ulink>.
    </para>

    <sect2 id="sect2-buffers-bufferpools" xreflabel="Buffer Allocation and
    Buffer Pools">
      <title>Buffer Allocation and Buffer Pools</title>
      <para>
        Buffers can be allocated using various schemes, and they may either be
        passed on by an element or unreferenced, thus freeing the memory used by
        the buffer. Buffer allocation and unlinking are important concepts when
        dealing with real time media processing, since memory allocation is
        relatively slow on most systems.
      </para>
      <para>
        To improve the latency in a media pipeline, many &GStreamer; elements
        use a <emphasis>buffer pool</emphasis> to handle buffer allocation and
        unlinking. A buffer pool is a relatively large chunk of memory that is
        the &GStreamer; process requests early on from the operating system.
        Later, when elements request memory for a new buffer, the buffer pool
        can serve the request quickly by giving out a piece of the allocated
        memory. This saves a call to the operating system and lowers latency.
        [If it seems at this point like &GStreamer; is acting like an operating
        system (doing memory management, etc.), don't worry: &GStreamer;OS isn't
        due out for quite a few years!]
      </para>
      <para>
        Normally in a media pipeline, most filter elements in &GStreamer; deal
        with a buffer in place, meaning that they do not create or destroy
        buffers. Sometimes, however, elements might need to alter the reference
        count of a buffer, either by copying or destroying the buffer, or by
        creating a new buffer. These topics are generally reserved for
        non-filter elements, so they will be addressed at that point.
      </para>
    </sect2>
  </sect1>

  <!-- ############ sect1 ############# -->

  <sect1 id="sect1-basics-types" xreflabel="Types and Properties">
    <title>Types and Properties</title>
    <para>
      &GStreamer; uses a type system to ensure that the data passed between
      elements is in a recognized format. The type system is also important for
      ensuring that the parameters required to fully specify a format match up
      correctly when linking pads between elements. Each link that is
      made between elements has a specified type.
    </para>

    <!-- ############ sect2 ############# -->

    <sect2 id="sect2-types-basictypes" xreflabel="Basic Types">
      <title>The Basic Types</title>
      <para>
        &GStreamer; already supports many basic media types. Following is a
        table of the basic types used for buffers in &GStreamer;. The table
        contains the name ("mime type") and a description of the type, the
        properties associated with the type, and the meaning of each property.
      </para>

      <table frame="all" id="table-basictypes" xreflabel="Table of Basic Types">
        <title>Table of Basic Types</title>
        <tgroup cols="6" align="left" colsep="1" rowsep="1">

        <thead>
          <row>
            <entry>Mime Type</entry>
            <entry>Description</entry>
            <entry>Property</entry>
            <entry>Property Type</entry>
            <entry>Property Values</entry>
            <entry>Property Description</entry>
          </row>
        </thead>

        <tbody valign="top">

          <!-- ############ type ############# -->

          <row>
            <entry morerows="10">audio/raw</entry>
            <entry morerows="10">
              Unstructured and uncompressed raw audio data.
            </entry>
            <entry>rate</entry>
            <entry>integer</entry>
            <entry>greater than 0</entry>
            <entry>
              The sample rate of the data, in samples per second.
            </entry>
          </row>
          <row>
            <entry>channels</entry>
            <entry>integer</entry>
            <entry>greater than 0</entry>
            <entry>
              The number of channels of audio data.
            </entry>
          </row>
          <row>
            <entry>format</entry>
            <entry>string</entry>
            <entry><quote>int</quote> or <quote>float</quote></entry>
            <entry>
              The format in which the audio data is passed.
            </entry>
          </row>
          <row>
            <entry>law</entry>
            <entry>integer</entry>
            <entry>0, 1, or 2</entry>
            <entry>
              (Valid only if the data is in integer format.) The law used to
              describe the data. The value 0 indicates <quote>linear</quote>, 1
              indicates <quote>mu&nbsp;law</quote>, and 2 indicates
              <quote>A&nbsp;law</quote>.
            </entry>
          </row>
          <row>
            <entry>endianness</entry>
            <entry>boolean</entry>
            <entry>0 or 1</entry>
            <entry>
              (Valid only if the data is in integer format.) The order of bytes
              in a sample. The value 0 means <quote>little-endian</quote> (bytes
              are least significant first). The value 1 means
              <quote>big-endian</quote> (most significant byte first).
            </entry>
          </row>
          <row>
            <entry>signed</entry>
            <entry>boolean</entry>
            <entry>0 or 1</entry>
            <entry>
              (Valid only if the data is in integer format.) Whether the samples
              are signed or not.
            </entry>
          </row>
          <row>
            <entry>width</entry>
            <entry>integer</entry>
            <entry>greater than 0</entry>
            <entry>
              (Valid only if the data is in integer format.) The number of bits
              per sample.
            </entry>
          </row>
          <row>
            <entry>depth</entry>
            <entry>integer</entry>
            <entry>greater than 0</entry>
            <entry>
              (Valid only if the data is in integer format.) The number of bits
              used per sample. This must be less than or equal to the width: If
              the depth is less than the width, the low bits are assumed to be
              the ones used. For example, a width of 32 and a depth of 24 means
              that each sample is stored in a 32 bit word, but only the low 24
              bits are actually used.
            </entry>
          </row>
          <row>
            <entry>layout</entry>
            <entry>string</entry>
            <entry><quote>gfloat</quote></entry>
            <entry>
              (Valid only if the data is in float format.) A string representing
              the way in which the floating point data is represented.
            </entry>
          </row>
          <row>
            <entry>intercept</entry>
            <entry>float</entry>
            <entry>any, normally 0</entry>
            <entry>
              (Valid only if the data is in float format.) A floating point
              value representing the value that the signal
              <quote>centers</quote> on.
            </entry>
          </row>
          <row>
            <entry>slope</entry>
            <entry>float</entry>
            <entry>any, normally 1.0</entry>
            <entry>
              (Valid only if the data is in float format.) A floating point
              value representing how far the signal deviates from the intercept.
              A slope of 1.0 and an intercept of 0.0 would mean an audio signal
              with minimum and maximum values of -1.0 and 1.0. A slope of
              0.5 and intercept of 0.5 would represent values in the range 0.0
              to 1.0.
            </entry>
          </row>

          <!-- ############ type ############# -->

          <row>
            <entry morerows="4">audio/mp3</entry>
            <entry morerows="4">
              Audio data compressed using the mp3 encoding scheme.
            </entry>
            <entry>framed</entry>
            <entry>boolean</entry>
            <entry>0 or 1</entry>
            <entry>
              A true value indicates that each buffer contains exactly one
              frame. A false value indicates that frames and buffers do not
              necessarily match up.
            </entry>
          </row>
          <row>
            <entry>layer</entry>
            <entry>integer</entry>
            <entry>1, 2, or 3</entry>
            <entry>
              The compression scheme layer used to compress the data.
            </entry>
          </row>
          <row>
            <entry>bitrate</entry>
            <entry>integer</entry>
            <entry>greater than 0</entry>
            <entry>
              The bitrate, in kilobits per second. For VBR (variable bitrate)
              mp3 data, this is the average bitrate.
            </entry>
          </row>
          <row>
            <entry>channels</entry>
            <entry>integer</entry>
            <entry>greater than 0</entry>
            <entry>
              The number of channels of audio data present.
            </entry>
          </row>
          <row>
            <entry>joint-stereo</entry>
            <entry>boolean</entry>
            <entry>0 or 1</entry>
            <entry>
              If true, this implies that stereo data is stored as a combined
              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>

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  <sect1 id="sect1-basics-events" xreflabel="Events">
    <title>Events</title>
    <para>
      Sometimes elements in a media processing pipeline need to know that
      something has happened. An <emphasis>event</emphasis> is a special type of
      data in &GStreamer; designed to serve this purpose. Events describe some
      sort of activity that has happened somewhere in an element's pipeline, for
      example, the end of the media stream or a clock discontinuity. Just like
      any other data type, an event comes to an element on a sink pad and is
      contained in a normal buffer. Unlike normal stream buffers, though, an
      event buffer contains only an event, not any media stream 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>