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102 lines
5.1 KiB
XML
102 lines
5.1 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. Normally 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 impossible. The
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controller takes time into account. It works by attaching control-sources
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to properties using control-bindings. Control-sources provide values for a
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given time-stamp that are usually in the range of 0.0 to 1.0.
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Control-bindings map the control-value to a gobject property they are bound to
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- converting the type and scaling to the target property value range.
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At run-time the elements continuously pull values changes for the current
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stream-time to update the gobject properties. GStreamer includes a few
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different control-sources and control-bindings already, but applications can
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define their own by sub-classing from the respective base classes.
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</para>
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<para>
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Most parts of the controller mechanism is implemented in GstObject. Also the
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base classes for control-sources and control-bindings are included in the core
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library. The existing implementations are 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/gstinterpolationcontrolsource.h>
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#include <gst/controller/gstdirectcontrolbinding.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
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<filename>gstreamer-controller</filename>. One can get the required flag for
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compiler and linker by using pkg-config for gstreamer-controller-1.0.
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</para>
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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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If we have our pipeline set up and want to control some parameters, we first
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need to create a control-source. Lets use an 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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g_object_set (csource, "mode", GST_INTERPOLATION_MODE_LINEAR, NULL);
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</programlisting>
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<para>
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Now we need to attach the control-source to the gobject property. This is done
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with a control-binding. One control source can be attached to several object
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properties (even in different objects) using separate control-bindings.
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</para>
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<programlisting>
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gst_object_add_control_binding (object, gst_direct_control_binding_new (object, "prop1", csource));
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</programlisting>
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<para>
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This type 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 changes
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This behavior can be configured by setting the mode property of the
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control-source. Other control sources e.g. produce a stream of values by
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calling <function>sin()</function> function. They have parameters to control
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e.g. the frequency. As control-sources are GstObjects too, one can attach
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control-sources to these properties too.
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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 gdouble values and
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are usually in the range of 0.0 to 1.0. A value of 1.0 is later mapped to the
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maximum value in the target properties value range.
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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.
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</para>
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<programlisting>
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GstTimedValueControlSource *tv_csource = (GstTimedValueControlSource *)csource;
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gst_timed_value_control_source_set (tv_csource, 0 * GST_SECOND, 0.0);
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gst_timed_value_control_source_set (tv_csource, 1 * GST_SECOND, 1.0);
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</programlisting>
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<para>
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Now everything is ready to play. If the control-source is e.g. bound to a
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volume property, we will head a fade-in over 1 second. One word of caution,
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the volume element that comes with gstreamer has a value range of 0.0 to 4.0
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on its volume property. If the above control-source is attached to the property
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the volume will ramp up to 400%!
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</para>
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<para>
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One final note - the controller subsystem has a built-in live-mode. Even though
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a property has a control-source assigned one can change the GObject property
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through the <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 does not
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work for control-sources that constantly update the property (e.g. the
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lfo_control_source).
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</para>
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</sect1>
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</chapter>
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