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105 lines
3.7 KiB
XML
105 lines
3.7 KiB
XML
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<!-- ############ chapter ############# -->
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<chapter id="chapter-dparams">
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<title>Supporting Dynamic Parameters</title>
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<para>
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Sometimes object properties are not powerful enough to control the
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parameters that affect the behaviour of your element.
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When this is the case you can mark these parameters as being Controllable.
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Aware applications can use the controller subsystem to dynamically adjust
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the property values over time.
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</para>
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<sect1 id="section-dparam-start">
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<title>Getting Started</title>
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<para>
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The controller subsystem is contained within the
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<filename>gstcontroller</filename> library. You need to include the header in
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your element's source file:
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</para>
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<programlisting>
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...
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#include <gst/gst.h>
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#include <gst/controller/gstcontroller.h>
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...
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</programlisting>
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<para>
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Even though the <filename>gstcontroller</filename> library may be linked into
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the host application, you should make sure it is initialized in your
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<filename>plugin_init</filename> function:
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</para>
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<programlisting>
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static gboolean
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plugin_init (GstPlugin *plugin)
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{
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...
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/* initialize library */
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gst_controller_init (NULL, NULL);
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...
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}
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</programlisting>
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<para>
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It makes not sense for all GObject parameter to be real-time controlled.
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Therefore the next step is to mark controllable parameters.
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This is done by using the special flag <constant>GST_PARAM_CONTROLLABLE</constant>.
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when setting up GObject params in the <function>_class_init</function> method.
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</para>
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<programlisting>
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g_object_class_install_property (gobject_class, PROP_FREQ,
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g_param_spec_double ("freq", "Frequency", "Frequency of test signal",
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0.0, 20000.0, 440.0,
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G_PARAM_READWRITE | GST_PARAM_CONTROLLABLE | G_PARAM_STATIC_STRINGS));
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</programlisting>
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</sect1>
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<sect1 id="chapter-dparam-loop">
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<title>The Data Processing Loop</title>
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<para>
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In the last section we learned how to mark GObject params as controllable.
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Application developers can then queue parameter changes for these parameters.
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The approach the controller subsystem takes is to make plugins responsible
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for pulling the changes in. This requires just one action:
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</para>
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<programlisting>
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gst_object_sync_values(element,timestamp);
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</programlisting>
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<para>
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This call makes all parameter-changes for the given timestamp active by
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adjusting the GObject properties of the element. Its up to the element to
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determine the synchronisation rate.
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</para>
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<sect2 id="chapter-dparam-loop-video">
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<title>The Data Processing Loop for Video Elements</title>
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<para>
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For video processing elements it is the best to synchronise for every frame.
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That means one would add the <function>gst_object_sync_values()</function>
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call described in the previous section to the data processing function of
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the element.
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</para>
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</sect2>
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<sect2 id="chapter-dparam-loop-audio">
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<title>The Data Processing Loop for Audio Elements</title>
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<para>
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For audio processing elements the case is not as easy as for video
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processing elements. The problem here is that audio has a much higher rate.
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For PAL video one will e.g. process 25 full frames per second, but for
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standard audio it will be 44100 samples.
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It is rarely useful to synchronise controllable parameters that often.
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The easiest solution is also to have just one synchronisation call per
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buffer processing. This makes the control-rate depend on the buffer
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size.
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</para>
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<para>
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Elements that need a specific control-rate need to break their data
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processing loop to synchronise every n-samples.
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</para>
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</sect2>
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</sect1>
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</chapter>
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