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100 lines
4.2 KiB
XML
100 lines
4.2 KiB
XML
<chapter id="chapter-dparams">
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<title>Dynamic Controllable Parameters</title>
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<sect1 id="section-dparams-getting-started">
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<title>Getting Started</title>
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<para>
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The controller subsystem offers a lightweight way to adjust gobject
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properties over stream-time. Normaly these properties are changed using
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<function>g_object_set()</function>. Timing those calls reliably so that
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the changes affect certain stream times is close to imposible. The
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controller takes time into account. It works by attaching control-sources
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to properties. Control-sources can provide new values for the properties
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for a given timestamp. At run-time the elements continously pull values
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changes for the current stream-time. GStreamer includes a few different
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control-sources, but applications can define their own by subclassing.
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</para>
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<para>
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This subsystem is contained within the
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<filename>gstcontroller</filename> library.
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You need to include the header in your application'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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Your application should link to the shared library <filename>gstreamer-controller</filename>.
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</para>
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<para>
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The <filename>gstreamer-controller</filename> library needs to be initialized
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when your application is run. This can be done after the the GStreamer
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library has been initialized.
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</para>
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<programlisting>
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...
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gst_init (&argc, &argv);
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gst_controller_init (&argc, &argv);
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...
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</programlisting>
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</sect1>
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<sect1 id="section-dparams-parameters">
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<title>Setting up parameter control</title>
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<para>
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The first step is to select the parameters that should be controlled.
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This returns a controller object that is needed to further adjust the
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behaviour.
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</para>
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<programlisting>
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controller = gst_object_control_properties(object, "prop1", "prop2",...);
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</programlisting>
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<para>
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Next we attach a control-source to each parameter. Lets use an
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interpolation control-source:
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</para>
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<programlisting>
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csource = gst_interpolation_control_source_new ();
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gst_interpolation_control_source_set_interpolation_mode(csource, GST_INTERPOLATE_LINEAR);
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</programlisting>
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<para>
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This control-source takes new property values from a list of time-stamped
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parameter changes. The source can e.g. fill gaps by smoothing parameter
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changes. This behaviour can be configured by setting the
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interpolation-mode.
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</para>
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<para>
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Now we can set some control points. These are time-stamped GValues.
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The values become active when the timestamp is reached. They still stay
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in the list. If e.g. the pipeline runs a loop (using a segmented seek),
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the control-curve gets repeated as well. Other control-sources have
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different functions to specify the control-changes over time.
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</para>
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<programlisting>
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gst_interpolation_control_source_set (csource, 0 * GST_SECOND, value1);
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gst_interpolation_control_source_set (csource, 1 * GST_SECOND, value2);
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</programlisting>
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<para>
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Finally we need to assign the control-source to the gobject property. One
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control source can only be assigned to one property.
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</para>
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<programlisting>
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gst_controller_set_control_source (controller, "prop1", csource);
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</programlisting>
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<para>
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Now everything is ready to play. One final note - the controller subsystem
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has a builtin live-mode. Even though a property has a control-source
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assigned one can change the GObject property through the
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<function>g_object_set()</function>.
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This is highly useful when binding the GObject properties to GUI widgets.
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When the user adjusts the value with the widget, one can set the GObject
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property and this remains active until the next programmed control-source
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value overrides it. This also works with smoothed parameters. It might not
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work for control-sources that constantly update the property (e.g. the lfo
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control-source).
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</para>
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</sect1>
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</chapter>
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