gstreamer/docs/manual/intro-basics.xml

<chapter id="chapter-intro-basics">
  <title>Foundations</title>
  <para><!-- synchronize with PWG -->
    This chapter of the guide introduces the basic concepts of &GStreamer;.
    Understanding these concepts will be important in reading any of the
    rest of this guide, all of them assume understanding of these basic
    concepts.
  </para>

  <sect1 id="section-intro-basics-elements">
    <title>Elements</title>
    <para>
      An <emphasis>element</emphasis> is the most important class of objects
      in &GStreamer;. You will usually create a chain of elements linked
      together and let data flow through this chain of elements. An element
      has one specific function, which can be the reading of data from a
      file, decoding of this data or outputting this data to your sound
      card (or anything else). By chaining together several such elements,
      you create a <emphasis>pipeline</emphasis> that can do a specific task,
      for example media playback or capture. &GStreamer; ships with a large
      collection of elements by default, making the development of a large
      variety of media applications possible. If needed, you can also write
      new elements. That topic is explained in great deal in the &GstPWG;.
    </para>
  </sect1>

  <sect1 id="section-intro-basics-pads">
    <title>Pads</title>
    <para>
      <emphasis>Pads</emphasis> are element's input and output, where
      you can connect other elements. They are used to negotiate links and
      data flow
      between elements in &GStreamer;. A pad can be viewed as a
      <quote>plug</quote> or <quote>port</quote> on an element where
      links may be made with other elements, and through which data can
      flow to or from those elements. Pads have specific data handling
      capabilities: a pad can restrict the type of data that flows
      through it. Links are only allowed between two pads when the
      allowed data types of the two pads are compatible. Data types are
      negotiated between pads using a process called <emphasis>caps
      negotiation</emphasis>. Data types are described as a
      <classname>GstCaps</classname>.
    </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 most part, 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. Data usually
      means buffers (described by the <ulink type="http"
      url="&URLAPI;/gstreamer-GstBuffer.html"><classname>GstBuffer
      </classname></ulink> object) and events (described by the <ulink
      type="http" url="&URLAPI;/gstreamer-GstEvent.html"><classname>
      GstEvent</classname></ulink> object).
    </para>
  </sect1>

  <sect1 id="section-intro-basics-bins">
    <title>Bins and pipelines</title>

    <para>
      A <emphasis>bin</emphasis> is a container for a collection of elements.
      A <emphasis>pipeline</emphasis> is a special subtype of a bin that allows execution of all
      of its contained child elements. Since bins are subclasses of elements
      themselves, you can mostly control a bin as if it were an element,
      thereby abstracting away a lot of complexity for your application. You
      can, for example change state on all elements in a bin by changing the
      state of that bin itself. Bins also forward bus messages from their
      contained children (such as error messages, tag messages or EOS messages).
    </para>
    <para>
      A <emphasis>pipeline</emphasis> is a top-level bin. As you set it to PAUSED or PLAYING state,
      data flow will start and media processing will take place. Once started,
      pipelines will run in a separate thread until you stop them or the end
      of the data stream is reached.
    </para>

    <figure float="1" id="section-pipeline-img">
      <title>&GStreamer; pipeline for a simple ogg player</title>
      <mediaobject>
        <imageobject>
          <imagedata scale="75" fileref="images/simple-player.&image;" format="&IMAGE;" />
        </imageobject>
      </mediaobject>  
    </figure>

  </sect1>

  <sect1 id="section-intro-basics-communication">
    <title>Communication</title>

    <para>
      &GStreamer; provides several mechanisms for communication and data exchange
      between the <emphasis>application</emphasis> and the <emphasis>pipeline</emphasis>.
    </para>

    <itemizedlist>
      <listitem>
        <para>
          <emphasis>buffers</emphasis> are objects for passing streaming data
          between elements in the pipeline. Buffers always travel from sources
          to sinks (downstream).
        </para>
      </listitem>
      <listitem>
        <para>
          <emphasis>events</emphasis> are objects sent between elements or from
          the application to elements. Events can travel upstream and downstream.
          Downstream events can be synchronised to the data flow.
        </para>
      </listitem>
      <listitem>
        <para>
          <emphasis>messages</emphasis> are objects posted by elements on
          the pipeline's message bus, where they will be held for collection
          by the application. Messages can be intercepted synchronously from
          the streaming thread context of the element posting the message, but
          are usually handled asynchronously by the application from the
          application's main thread. Messages are used to transmit information
          such as errors, tags, state changes, buffering state, redirects etc.
          from elements to the application in a thread-safe way.
        </para>
      </listitem>
      <listitem>
        <para>
          <emphasis>queries</emphasis> allow applications to request information
          such as duration or current playback position from the pipeline.
          Queries are always answered synchronously. Elements can also use
          queries to request information from their peer elements (such as the
          file size or duration). They can be used both ways within a pipeline,
          but upstream queries are more common.
        </para>
      </listitem>
    </itemizedlist>

    <figure float="1" id="section-communication-img">
      <title>&GStreamer; pipeline with different communication flows</title>
      <mediaobject>
        <imageobject>
          <imagedata scale="75" fileref="images/communication.&image;" format="&IMAGE;" />
        </imageobject>
      </mediaobject>  
    </figure>

  </sect1>

</chapter>