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