gstreamer/docs/fwg/dparams.xml

<chapter id="cha-dparam-start">
  <title>Getting Started</title>

  <para>
    The dparams subsystem is contained within the <filename>gstcontrol</filename> library.
    You need to include the header in your element's source file:
  </para>
  <programlisting>
  #include &lt;gst/control/control.h&gt;
  </programlisting>
  
  <para>
    Even though the <filename>gstcontrol</filename> library may be linked into the host
    application, you should make sure it is loaded in your <filename>plugin_init</filename> 
    function:
  </para>
  <programlisting>
  static gboolean
  plugin_init (GModule *module, GstPlugin *plugin)
  {
    ...
    
    /* load dparam support library */
    if (!gst_library_load ("gstcontrol"))
    {
      gst_info ("example: could not load support library: 'gstcontrol'\n");
      return FALSE;
    }
    
    ...
  }
  </programlisting>
  
  <para>
    You need to store an instance of <filename>GstDParamManager</filename> in your element's struct:
  </para>
  <programlisting>
  struct _GstExample {
    GstElement element;
    ...
    
    GstDParamManager *dpman;
    
    ...
  };
  </programlisting>
  
  <para>
    The <filename>GstDParamManager</filename> can be initialised in your element's
    init function:
  </para>
  <programlisting>
  static void
  gst_example_init (GstExample *example)
  {
    ...
    
    example-&gt;dpman = gst_dpman_new ("example_dpman", GST_ELEMENT(example));
    
    ...
  }
  </programlisting>
  
</chapter>

<chapter id="cha-dparam-define">
  <title>Defining Parameter Specificiations</title>
  <para>
    You can define the dparams you need anywhere within your element but will usually
    need to do so in only a couple of places:
    <itemizedlist>
      <listitem>
        In the element <filename>init</filename> function, 
        just after the call to <filename>gst_dpman_new</filename>
      </listitem>
      <listitem>
        Whenever a new pad is created so that parameters can affect data going into
        or out of a specific pad. An example of this would be a mixer element where 
        a seperate volume parameter is needed on every pad.
      </listitem>
    </itemizedlist>
  </para>
  <para>
    There are three different ways the dparams subsystem can pass parameters into your element.
    Which one you use will depend on how that parameter is used within your element.
    Each of these methods has its own function to define a required dparam:
    <itemizedlist>
      <listitem><filename>gst_dpman_add_required_dparam_direct</filename></listitem>
      <listitem><filename>gst_dpman_add_required_dparam_callback</filename></listitem>
      <listitem><filename>gst_dpman_add_required_dparam_array</filename></listitem>
    </itemizedlist>
    These functions will return TRUE if the required dparam was added successfully.  
    </para>
    <para>
    The following function will be used as an example.
    <programlisting>
  gboolean 
  gst_dpman_add_required_dparam_direct (GstDParamManager *dpman, 
                                        GParamSpec *param_spec,
                                        gboolean is_log,
                                        gboolean is_rate,
                                        gpointer update_data)
    </programlisting>
    The common parameters to these functions are:
    <itemizedlist>
      <listitem><filename>GstDParamManager *dpman</filename> the element's dparam manager</listitem>
      <listitem><filename>GParamSpec *param_spec</filename> the param spec which defines the required dparam</listitem>
      <listitem>
        <filename>gboolean is_log</filename> whether this dparam value should be 
        interpreted on a log scale (such as a frequency or a decibel value)
      </listitem>
      <listitem>
        <filename>gboolean is_rate</filename> whether this dparam 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>
  </para>
  <sect2 id="sect-dparam-direct">
    <title>Direct Method</title>
    <para>
      This method is the simplest and has the lowest overhead for parameters which change 
      less frequently than the sample rate.  First you need somewhere to store the parameter - 
      this will usually be in your element's stuct.
    </para>
    <programlisting>
  struct _GstExample {
    GstElement element;
    ...
    
    GstDParamManager *dpman;
    gfloat volume;
    ...
  };
    </programlisting>
    <para>
      Then to define the required dparam just call <filename>gst_dpman_add_required_dparam_direct</filename>
      and pass in the location of the parameter to change.  
      In this case the location is <filename>&amp;(example-&gt;volume)</filename>.
    </para>
    <programlisting>
  gst_dpman_add_required_dparam_direct (
    example-&gt;dpman, 
    g_param_spec_float("volume","Volume","Volume of the audio",
                       0.0, 1.0, 0.8, G_PARAM_READWRITE),
    FALSE,
    FALSE,
    &amp;(example-&gt;volume)
  );
    </programlisting>
    <para>
      You can now use <filename>example-&gt;volume</filename> anywhere in your element knowing
      that it will always contain the correct value to use.
    </para>
  </sect2>
  <sect2 id="sect-dparam-callback">
    <title>Callback Method</title>
    <para>
      This should be used if the you have other values to calculate whenever a parameter changes.
      If you used the direct method you wouldn't know if a parameter had changed so you would have to
      recalculate the other values every time you needed them. By using the callback method, other values
      only have to be recalculated when the dparam value actually changes.
    </para>
    <para>
      The following code illustrates an instance where you might want to use the callback method.
      If you had a volume dparam which was represented by a gfloat number, your element may only deal
      with integer arithmatic.  The callback could be used to calculate the integer scaler when the volume
      changes.  First you will need somewhere to store these values.
    </para>
    <programlisting>
  struct _GstExample {
    GstElement element;
    ...
    
    GstDParamManager *dpman;
    gfloat volume_f;
    gint   volume_i;
    ...
  };
    </programlisting>
    <para>
      When the required dparam is defined, the callback function <filename>gst_example_update_volume</filename>
      and some user data (which in this case is our element instance) is passed in to the call to 
      <filename>gst_dpman_add_required_dparam_callback</filename>.
    </para>
    <programlisting>
  gst_dpman_add_required_dparam_callback (
    example-&gt;dpman, 
    g_param_spec_float("volume","Volume","Volume of the audio",
                       0.0, 1.0, 0.8, G_PARAM_READWRITE),
    FALSE,
    FALSE,
    gst_example_update_volume, 
    example
  );
    </programlisting>
    <para>
      The callback function needs to conform to this signiture
    </para>
    <programlisting>
typedef void (*GstDPMUpdateFunction) (GValue *value, gpointer data);
    </programlisting>
    <para>
      In our example the callback function looks like this
    </para>
    <programlisting>
static void
gst_example_update_volume(GValue *value, gpointer data)
{
  GstExample *example = (GstExample*)data;
  g_return_if_fail(GST_IS_EXAMPLE(example));

  example-&gt;volume_f = g_value_get_float(value);
  example-&gt;volume_i = example-&gt;volume_f * 8192;
}
    </programlisting>
    <para>
      Now <filename>example-&gt;volume_i</filename> can be used elsewhere and it will always contain the correct value.
    </para>
  </sect2>
  <sect2 id="sect-dparam-array">
    <title>Array Method</title>
    <para>
      This method is quite different from the other two.  It could be thought of as
      a specialised method which should only be used if you need the advantages that it
      provides.  Instead of giving the element a single value it provides an array of values
      where each item in the array corresponds to a sample of audio in your buffer.
      There are a couple of reasons why this might be useful.
    </para>

    <itemizedlist>
      <listitem>
        Certain optimisations may be possible since you can iterate over your dparams array 
        and your buffer data together.
      </listitem>
      <listitem>
        Some dparams may be able to interpolate changing values at the sample rate. This would allow
        the array to contain very smoothly changing values which may be required for the stability
        and quality of some DSP algorithms.
      </listitem>
    </itemizedlist>
    <para>
      The array method is currently the least mature of the three methods and is not yet ready to be
      used in elements, but plugin writers should be aware of its existance for the future.
    </para>
  </sect2>
</chapter>

<chapter id="cha-dparam-loop">
  <title>The Data Processing Loop</title>
  <para>
    This is the most critical aspect of the dparams subsystem as it relates to elements.
    In a traditional audio processing loop, a <filename>for</filename> loop will usually iterate over each
    sample in the buffer, processing one sample at a time until the buffer is finished.
    A simplified loop with no error checking might look something like this.
  </para>
  <programlisting>
static void
example_chain (GstPad *pad, GstBuffer *buf)
{
  ...
  gfloat *float_data;
  int j;
  GstExample *example = GST_EXAMPLE(GST_OBJECT_PARENT (pad));
  int num_samples = GST_BUFFER_SIZE(buf)/sizeof(gfloat);
  float_data = (gfloat *)GST_BUFFER_DATA(buf);
  ...
  for (j = 0; j &lt; num_samples; j++) {
    float_data[j] *= example-&gt;volume;
  }
  ...
}
  </programlisting>
  <para>
    To make this dparams aware, a couple of changes are needed.
  </para>
  <programlisting>
static void
example_chain (GstPad *pad, GstBuffer *buf)
{
  ...
  int j = 0;
  GstExample *example = GST_EXAMPLE(GST_OBJECT_PARENT (pad));
  int num_samples = GST_BUFFER_SIZE(buf)/sizeof(gfloat);
  gfloat *float_data = (gfloat *)GST_BUFFER_DATA(buf);
  int frame_countdown = GST_DPMAN_PREPROCESS(example-&gt;dpman, num_samples, GST_BUFFER_TIMESTAMP(buf));
  ...
  while (GST_DPMAN_PROCESS_COUNTDOWN(example-&gt;dpman, frame_countdown, j)) {
    float_data[j++] *= example-&gt;volume;
  }
  ...
}
  </programlisting>
  <para>
    The biggest changes here are 2 new macros, <filename>GST_DPMAN_PREPROCESS</filename> 
    and <filename>GST_DPMAN_PROCESS_COUNTDOWN</filename>.
    You will also notice that the for loop has become a while loop.  <filename>GST_DPMAN_PROCESS_COUNTDOWN</filename>
    is called as the condition for the while loop so that any required dparams can be updated in the 
    middle of a buffer if required. This is because one of the required behaviours of dparams is that they
    can be <emphasis>sample accurate</emphasis>.  This means that parameters change at the exact timestamp
    that they are supposed to - not after the buffer has finished being processed.
  </para>
  <para>
    It may be alarming to see a macro as the condition for a while loop, but it is actually very efficient.
    The macro expands to the following.
  </para>
  <programlisting>
#define GST_DPMAN_PROCESS_COUNTDOWN(dpman, frame_countdown, frame_count) \
				(frame_countdown-- || \
				(frame_countdown = GST_DPMAN_PROCESS(dpman, frame_count)))
  </programlisting>
  <para>
    So as long as <filename>frame_countdown</filename> is greater than 0, <filename>GST_DPMAN_PROCESS</filename>
    will not be called at all.
    Also in many cases, <filename>GST_DPMAN_PROCESS</filename> will do nothing and simply 
    return 0, meaning that there is no more data in the buffer to process.
  </para>
  <para>
    The macro <filename>GST_DPMAN_PREPROCESS</filename> will do the following:
    <itemizedlist>
      <listitem>
        Update any dparams which are due to be updated.
      </listitem>
      <listitem>
        Calculate how many samples should be processed before the next required update
      </listitem>
      <listitem>
        Return the number of samples until next update, or the number of samples in the buffer - 
        whichever is less.
      </listitem>
    </itemizedlist>
    In fact <filename>GST_DPMAN_PROCESS</filename> may do the same things as <filename>GST_DPMAN_PREPROCESS</filename>
    depending on the mode that the dparam manager is running in (see below).
  </para>
  <sect2 id="sect-dparam-modes">
    <title>DParam Manager Modes</title>
    <para>
      A brief explanation of dparam manager modes might be useful here even though it doesn't generally affect
      the way your element is written. There are different ways media applications will be used which
      require that an element's parameters be updated in differently. These include:
      <itemizedlist>
        <listitem>
          <emphasis>Timelined</emphasis> - all parameter changes are known in advance before the pipeline is run.
        </listitem>
        <listitem>
          <emphasis>Realtime low-latency</emphasis> - Nothing is known ahead of time about when a parameter 
          might change.  Changes need to be propagated to the element as soon as possible.
        </listitem>
      </itemizedlist>
      When a dparam-aware application gets the dparam manager for an element, the first thing it will do
      is set the dparam manager mode.  Current modes are <filename>"synchronous"</filename>
      and <filename>"asynchronous"</filename>.  
    </para>
    <para>
      If you are in a realtime low-latency situation then the <filename>"synchronous"</filename> mode is appropriate.
      During <filename>GST_DPMAN_PREPROCESS</filename> this mode will poll all dparams for required updates
      and propagate them. <filename>GST_DPMAN_PROCESS</filename> will do nothing in this mode.
      To then achieve the desired latency, the size of the buffers needs to be reduced so that the dparams will be
      polled for updates at the desired frequency.
    </para>
    <para>
      In a timelined situation, the <filename>"asynchronous"</filename> mode will be required.  This mode
      hasn't actually been implemented yet but will be described anyway.  
      The <filename>GST_DPMAN_PREPROCESS</filename> call will precalculate when and how often each dparam needs
      to update for the duration of the current buffer.  From then on <filename>GST_DPMAN_PROCESS</filename> will
      propagate the calculated updates each time it is called until end of the buffer.  If the application is rendering
      to disk in non-realtime, the render could be sped up by increasing the buffer size. In the <filename>"asynchronous"</filename>
      mode this could be done without affecting the sample accuracy of the parameter updates
    </para>
  </sect2>
  <sect2 id="sect-dparam-audio-video">
    <title>DParam Manager Modes</title>
    <para>
      All of the explanation so far has presumed that the buffer contains audio data with many samples.
      Video should be regarded differently since a video buffer often contains only 1 frame.  In this case
      some of the complexity of dparams isn't required but the other benefits still make it useful for video
      parameters.  If a buffer only contains one frame of video, only a single call to <filename>GST_DPMAN_PREPROCESS</filename>
      should be required. For more than one frame per buffer, treat it the same as the audio case.
    </para>
  </sect2>
</chapter>