gstreamer/docs/manual/advanced-dparams.xml

<chapter id="cha-dparams">
  <title>Dynamic Parameters</title>
  
  <sect1>
    <title>Getting Started</title>
    <para>
      The dparams subsystem is contained within the <filename>gstcontrol</filename> library.
      
      You need to include the header in your applications's source file:
    </para>
    <programlisting>
...
#include &lt;gst/gst.h&gt;
#include &lt;gst/control/control.h&gt;
...
    </programlisting>
    <para>
      Your application should link to the shared library <filename>gstcontrol</filename>.
    </para>
    <para>
      The <filename>gstcontrol</filename> library needs to be initialised when your application is run.
      This can be done after the the GStreamer library has been initialised.
    </para>
    <programlisting>
  ...
  gst_init(&amp;argc,&amp;argv);
  gst_control_init(&amp;argc,&amp;argv);
  ...
    </programlisting>
  </sect1>

  <sect1>
    <title>Creating and Attaching Dynamic Parameters</title>
    <para>
      Once you have created your elements you can create and attach dparams to them.
      First you need to get the element's dparams manager. If you know exactly what kind of element
      you have, you may be able to get the dparams manager directly. However if this is not possible, 
      you can get the dparams manager by calling <filename>gst_dpman_get_manager</filename>.
    </para>
    <para>
      Once you have the dparams manager, you must set the mode that the manager will run in.
      There is currently only one mode implemented called <filename>"synchronous"</filename> - this is used for real-time
      applications where the dparam value cannot be known ahead of time (such as a slider in a GUI).
      The mode is called <filename>"synchronous"</filename> because the dparams are polled by the element for changes before
      each buffer is processed.  Another yet-to-be-implemented mode is <filename>"asynchronous"</filename>.  This is used when
      parameter changes are known ahead of time - such as with a timelined editor.  The mode is called 
      <filename>"asynchronous"</filename> because parameter changes may happen in the middle of a buffer being processed.
    </para>
    <programlisting>
  GstElement *sinesrc;
  GstDParamManager *dpman;
  ...
  sinesrc = gst_elementfactory_make("sinesrc","sine-source");
  ...
  dpman = gst_dpman_get_manager (sinesrc);
  gst_dpman_set_mode(dpman, "synchronous");
    </programlisting>
    <para>
      If you don't know the names of the required dparams for your element you can call 
      <filename>gst_dpman_list_dparam_specs(dpman)</filename> to get a NULL terminated array of param specs.
      This array should be freed after use.  You can find the name of the required dparam by calling
      <filename>g_param_spec_get_name</filename> on each param spec in the array. In our example, 
      <filename>"volume"</filename> will be the name of our required dparam.
    </para>
    <para>
      Each type of dparam currently has its own <filename>new</filename> function. This may eventually
      be replaced by a factory method for creating new instances.  A default dparam instance can be created
      with the <filename>gst_dparam_new</filename> function. Once it is created it can be attached to a 
      required dparam in the element.
    </para>
    <programlisting>
  GstDParam *volume;
  ...
  volume = gst_dparam_new(G_TYPE_FLOAT);
  if (gst_dpman_attach_dparam (dpman, "volume", volume)){
    /* the dparam was successfully attached */
    ...
  }
    </programlisting>
  </sect1>

  <sect1>
    <title>Changing Dynamic Parameter Values</title>
    <para>
      All interaction with dparams to actually set the dparam value is done through simple GObject properties.
      There is a property value for each type that dparams supports - these currently being 
      <filename>"value_float"</filename>, <filename>"value_int"</filename> and <filename>"value_int64"</filename>.
      To set the value of a dparam, simply set the property which matches the type of your dparam instance.
    </para>
    <programlisting>
#define ZERO(mem) memset(&amp;mem, 0, sizeof(mem))
...

  gfloat set_to_value;
  GstDParam *volume;
  GValue set_val;
  ZERO(set_val);
  g_value_init(&amp;set_val, G_TYPE_FLOAT);
  ...
  g_value_set_float(&amp;set_val, set_to_value);
  g_object_set_property(G_OBJECT(volume), "value_float", &amp;set_val);
    </programlisting>
    <para>Or if you create an actual GValue instance:</para>
    <programlisting>
  gfloat set_to_value;
  GstDParam *volume;
  GValue *set_val;
  set_val = g_new0(GValue,1);
  g_value_init(set_val, G_TYPE_FLOAT);
  ...
  g_value_set_float(set_val, set_to_value);
  g_object_set_property(G_OBJECT(volume), "value_float", set_val);
    </programlisting>

  </sect1>

  <sect1>
    <title>Different Types of Dynamic Parameter</title>
    <para>
      There are currently only two implementations of dparams so far.  They are both for real-time use so 
      should be run in the <filename>"synchronous"</filename> mode.
    </para>
    <sect2>
      <title>GstDParam - the base dparam type</title>
      <para>
        All dparam implementations will subclass from this type.  It provides a basic implementation which simply 
        propagates any value changes as soon as it can.  
        A new instance can be created with the function <filename>GstDParam* gst_dparam_new (GType type)</filename>.
        It has the following object properties:
      </para>
      <itemizedlist>
        <listitem><filename>"value_float"</filename>
          - the property to set and get if it is a float dparam
        </listitem>
        <listitem><filename>"value_int"</filename>
          - the property to set and get if it is an integer dparam
        </listitem>
        <listitem><filename>"value_int64"</filename>
          - the property to set and get if it is a 64 bit integer dparam
        </listitem>
        <listitem><filename>"is_log"</filename>
          - readonly boolean which is TRUE if the param should be displayed on a log scale
        </listitem>
        <listitem><filename>"is_rate"</filename>
          - readonly boolean which is TRUE if the value is a proportion of the sample rate.  
          For example with a sample rate of 44100, 0.5 would be 22050 Hz and 0.25 would be 11025 Hz.
        </listitem>
      </itemizedlist>
    </sect2>
    <sect2>
      <title>GstDParamSmooth - smoothing real-time dparam</title>
      <para>
        Some parameter changes can create audible artifacts if they change too rapidly.  The GstDParamSmooth
        implementation can greatly reduce these artifacts by limiting the rate at which the value can change.
        This is currently only supported for float dparams - the other types fall back to the default implementation.
        A new instance can be created with the function <filename>GstDParam* gst_dpsmooth_new (GType type)</filename>.
        It has the following object properties:
      </para>
      <itemizedlist>
        <listitem><filename>"update_period"</filename>
          - an int64 value specifying the number nanoseconds between updates. This will be ignored in 
          <filename>"synchronous"</filename> mode since the buffer size dictates the update period.
        </listitem>
        <listitem><filename>"slope_time"</filename>
          - an int64 value specifying the time period to use in the maximum slope calculation
        </listitem>
        <listitem><filename>"slope_delta_float"</filename>
          - a float specifying the amount a float value can change in the given slope_time.
        </listitem>
      </itemizedlist>
      <para>
        Audible artifacts may not be completely eliminated by using this dparam. The only way to eliminate
        artifacts such as "zipper noise" would be for the element to implement its required dparams using the 
        array method. This would allow dparams to change parameters at the sample rate which should eliminate
        any artifacts.
      </para>
      
    </sect2>
    <sect2>
      <title>Timelined dparams</title>
      <para>
        A yet-to-be-implemented subclass of GstDParam will add an API which allows the creation and manipulation
        of points on a timeline.  This subclass will also provide a dparam implementation which uses linear
        interpolation between these points to find the dparam value at any given time.  Further subclasses can 
        extend this functionality to implement more exotic interpolation algorithms such as splines.
      </para>
    </sect2>
  </sect1>
  
</chapter>