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>Chapter 16. Supporting Dynamic Parameters</TD
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CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="section-dparam-define"
>16.3. Defining Parameter Specificiations</A
></H1
><P
> You can define the dparams you need anywhere within your element but will
usually need to do so in only a couple of places:
<P
></P
><UL
><LI
><P
> In the element <TT
CLASS="filename"
>init</TT
> function, just after the call
to <TT
CLASS="filename"
>gst_dpman_new</TT
>
</P
></LI
><LI
><P
> 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.
</P
></LI
></UL
>
</P
><P
> 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:
<P
></P
><UL
><LI
><P
><TT
CLASS="filename"
>gst_dpman_add_required_dparam_direct</TT
></P
></LI
><LI
><P
><TT
CLASS="filename"
>gst_dpman_add_required_dparam_callback</TT
></P
></LI
><LI
><P
><TT
CLASS="filename"
>gst_dpman_add_required_dparam_array</TT
></P
></LI
></UL
>
These functions will return TRUE if the required dparam was added
successfully.
</P
><P
> The following function will be used as an example.
<PRE
CLASS="programlisting"
> gboolean
gst_dpman_add_required_dparam_direct (GstDParamManager *dpman,
GParamSpec *param_spec,
gboolean is_log,
gboolean is_rate,
gpointer update_data)
</PRE
>
The common parameters to these functions are:
<P
></P
><UL
><LI
><P
> <TT
CLASS="filename"
>GstDParamManager *dpman</TT
> the element's dparam
manager
</P
></LI
><LI
><P
> <TT
CLASS="filename"
>GParamSpec *param_spec</TT
> the param spec which defines
the required dparam
</P
></LI
><LI
><P
> <TT
CLASS="filename"
>gboolean is_log</TT
> whether this dparam value should be
interpreted on a log scale (such as a frequency or a decibel value)
</P
></LI
><LI
><P
> <TT
CLASS="filename"
>gboolean is_rate</TT
> 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.
</P
></LI
></UL
>
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-dparam-direct"
>16.3.1. Direct Method</A
></H2
><P
> 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.
</P
><PRE
CLASS="programlisting"
> struct _GstExample {
GstElement element;
...
GstDParamManager *dpman;
gfloat volume;
...
};
</PRE
><P
> Then to define the required dparam just call
<TT
CLASS="filename"
>gst_dpman_add_required_dparam_direct</TT
> and pass in the
location of the parameter to change. In this case the location is
<TT
CLASS="filename"
>&(example->volume)</TT
>.
</P
><PRE
CLASS="programlisting"
> gst_dpman_add_required_dparam_direct (
example->dpman,
g_param_spec_float("volume","Volume","Volume of the audio",
0.0, 1.0, 0.8, G_PARAM_READWRITE),
FALSE,
FALSE,
&(example->volume)
);
</PRE
><P
> You can now use <TT
CLASS="filename"
>example->volume</TT
> anywhere in your
element knowing that it will always contain the correct value to use.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-dparam-callback"
>16.3.2. Callback Method</A
></H2
><P
> 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.
</P
><P
> 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.
</P
><PRE
CLASS="programlisting"
> struct _GstExample {
GstElement element;
...
GstDParamManager *dpman;
gfloat volume_f;
gint volume_i;
...
};
</PRE
><P
> When the required dparam is defined, the callback function
<TT
CLASS="filename"
>gst_example_update_volume</TT
> and some user data (which
in this case is our element instance) is passed in to the call to
<TT
CLASS="filename"
>gst_dpman_add_required_dparam_callback</TT
>.
</P
><PRE
CLASS="programlisting"
> gst_dpman_add_required_dparam_callback (
example->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
);
</PRE
><P
> The callback function needs to conform to this signiture
</P
><PRE
CLASS="programlisting"
> typedef void (*GstDPMUpdateFunction) (GValue *value, gpointer data);
</PRE
><P
> In our example the callback function looks like this
</P
><PRE
CLASS="programlisting"
> static void
gst_example_update_volume(GValue *value, gpointer data)
{
GstExample *example = (GstExample*)data;
g_return_if_fail(GST_IS_EXAMPLE(example));
example->volume_f = g_value_get_float(value);
example->volume_i = example->volume_f * 8192;
}
</PRE
><P
> Now <TT
CLASS="filename"
>example->volume_i</TT
> can be used elsewhere and it
will always contain the correct value.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="section-dparam-array"
>16.3.3. Array Method</A
></H2
><P
> 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.
</P
><P
></P
><UL
><LI
><P
> Certain optimisations may be possible since you can iterate over your
dparams array and your buffer data together.
</P
></LI
><LI
><P
> 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.
</P
></LI
></UL
><P
> 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.
</P
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