docs/manual/advanced-interfaces.xml

<chapter id="chapter-interfaces">
  <title>Interfaces</title>

  <para>
    In <xref linkend="section-elements-properties"/>, you have learned how
    to use <classname>GObject</classname> properties as a simple way to do
    interaction between applications and elements. This method suffices for
    the simple'n'straight settings, but fails for anything more complicated
    than a getter and setter. For the more complicated use cases, &GStreamer;
    uses interfaces based on the GObject <ulink type="http"
      url="http://library.gnome.org/devel/gobject/stable/gtype-non-instantiable-classed.html"><classname>GTypeInterface</classname></ulink>
    type.
  </para>

  <para>
    Most of the interfaces handled here will not contain any example code.
    See the API references for details. Here, we will just describe the
    scope and purpose of each interface.
  </para>

  <sect1 id="section-interfaces-uri">
    <title>The URI interface</title>

    <para>
      In all examples so far, we have only supported local files through the
      <quote>filesrc</quote> element. &GStreamer;, obviously, supports many
      more location sources. However, we don't want applications to need to
      know any particular element implementation details, such as element
      names for particular network source types and so on. Therefore, there
      is a URI interface, which can be used to get the source element that
      supports a particular URI type. There is no strict rule for URI naming,
      but in general we follow naming conventions that others use, too. For
      example, assuming you have the correct plugins installed, &GStreamer;
      supports <quote>file:///&lt;path&gt;/&lt;file&gt;</quote>,
      <quote>http://&lt;host&gt;/&lt;path&gt;/&lt;file&gt;</quote>,
      <quote>mms://&lt;host&gt;/&lt;path&gt;/&lt;file&gt;</quote>, and so on.
    </para>
    <para>
      In order to get the source or sink element supporting a particular URI,
      use <function>gst_element_make_from_uri ()</function>, with the URI
      type being either <classname>GST_URI_SRC</classname> for a source
      element, or <classname>GST_URI_SINK</classname> for a sink element.
    </para>
    <para>
      You can convert filenames to and from URIs using GLib's
      <function>g_filename_to_uri ()</function> and
      <function>g_uri_to_filename ()</function>.
    </para>
  </sect1>

  <sect1 id="section-interfaces-mixer">
    <title>The Mixer interface</title>

    <para>
      The mixer interface provides a uniform way to control the volume on a
      hardware (or software) mixer. The interface is primarily intended to
      be implemented by elements for audio inputs and outputs that talk
      directly to the hardware (e.g. OSS or ALSA plugins).
    </para>
    <para>
      Using this interface, it is possible to control a list of tracks
      (such as Line-in, Microphone, etc.) from a mixer element. They can
      be muted, their volume can be changed and, for input tracks, their
      record flag can be set as well.
    </para>
    <para>
      Example plugins implementing this interface include the OSS elements
      (osssrc, osssink, ossmixer) and the ALSA plugins (alsasrc, alsasink
      and alsamixer).
    </para>
    <para>
      You should not use this interface for volume control in a playback
      application. Either use a <classname>volume</classname> element or use
      <classname>playbin</classname>'s <quote>volume</quote> property, or use
      the <classname>audiosink</classname>'s <quote>volume</quote> property (if it has one).
    </para>
    <note>
      <para>
	In order for the <ulink type="http"
	  url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-gstmixer.html"><classname>GstMixer</classname></ulink>
	interface to be
        usable, the element implementing it needs to be in the right state,
        so that the underlying mixer device is open. This usually means the
        element needs to be at least in <classname>GST_STATE_READY</classname>
        before you can use this interface. You will get confusing warnings
        if the element is not in the right state when the interface is used.
      </para>
    </note>
  </sect1>

  <sect1 id="section-interfaces-tuner">
    <title>The Tuner interface</title>

    <para>
      The tuner interface is a uniform way to control inputs and outputs
      on a multi-input selection device. This is primarily used for input
      selection on elements for TV- and capture-cards.
    </para>
    <para>
      Using this interface, it is possible to select one track from a list
      of tracks supported by that tuner-element. The tuner will then select
      that track for media-processing internally. This can, for example, be
      used to switch inputs on a TV-card (e.g. from Composite to S-video).
    </para>
    <para>
      This interface is currently only implemented by the Video4linux and
      Video4linux2 elements.
    </para>
    <note>
      <para>
	In order for the <ulink type="http"
	  url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-gsttuner.html"><classname>GstTuner</classname></ulink>
	interface to be
        usable, the element implementing it needs to be in the right state,
        so that the underlying device is open. This usually means the
        element needs to be at least in <classname>GST_STATE_READY</classname>
        before you can use this interface. You will get confusing warnings
        if the element is not in the right state when the interface is used.
      </para>
    </note>
  </sect1>

  <sect1 id="section-interfaces-colorbalance">
    <title>The Color Balance interface</title>

    <para>
      The colorbalance interface is a way to control video-related properties
      on an element, such as brightness, contrast and so on. It's sole
      reason for existence is that, as far as its authors know, there's no
      way to dynamically register properties using
      <classname>GObject</classname>.
    </para>
    <para>
      The colorbalance interface is implemented by several plugins, including
      xvimagesink and the Video4linux and Video4linux2 elements.
    </para>
  </sect1>

  <sect1 id="section-interfaces-proprobe">
    <title>The Property Probe interface</title>

    <para>
      The property probe is a way to autodetect allowed values for a
      <classname>GObject</classname> property. It's primary use is to autodetect
      devices in several elements. For example, the OSS elements use this
      interface to detect all OSS devices on a system. Applications can then
      <quote>probe</quote> this property and get a list of detected devices.
    </para>
    <note>
      <para>
        Given the overlap between HAL and the practical implementations of this
        interface, this might in time be deprecated in favour of HAL.
      </para>
    </note>
    <para>
      This interface is currently implemented by many elements, including
      the ALSA, OSS, XVideo, Video4linux and Video4linux2 elements.
    </para>
  </sect1>

  <sect1 id="section-interfaces-xoverlay">
    <title>The X Overlay interface</title>

    <para>
      The X Overlay interface was created to solve the problem of embedding
      video streams in an application window. The application provides an
      X-window to the element implementing this interface to draw on, and
      the element will then use this X-window to draw on rather than creating
      a new toplevel window. This is useful to embed video in video players.
    </para>
    <para>
      This interface is implemented by, amongst others, the Video4linux and
      Video4linux2 elements and by ximagesink, xvimagesink and sdlvideosink.
    </para>
  </sect1>
</chapter>