Dynamic Controllable Parameters

Getting Started

The controller subsystem offers a lightweight way to adjust gobject properties over stream-time. Normally 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 using control-bindings. Control-sources provide values for a given time-stamp that are usually in the range of 0.0 to 1.0. Control-bindings map the control-value to a gobject property they are bound to - converting the type and scaling to the target property value range. At run-time the elements continuously pull values changes for the current stream-time to update the gobject properties. GStreamer includes a few different control-sources and control-bindings already, but applications can define their own by sub-classing from the respective base classes.

Most parts of the controller mechanism is implemented in GstObject. Also the base classes for control-sources and control-bindings are included in the core library. The existing implementations are contained within the gstcontroller library. You need to include the header in your application's source file:

#include <gst/gst.h>
#include <gst/controller/gstinterpolationcontrolsource.h>
#include <gst/controller/gstdirectcontrolbinding.h>

Your application should link to the shared library gstreamer-controller. One can get the required flag for compiler and linker by using pkg-config for gstreamer-controller-1.0.

Setting up parameter control

If we have our pipeline set up and want to control some parameters, we first need to create a control-source. Lets use an interpolation control-source:

  csource = gst_interpolation_control_source_new ();
  g_object_set (csource, "mode", GST_INTERPOLATION_MODE_LINEAR, NULL);

Now we need to attach the control-source to the gobject property. This is done with a control-binding. One control source can be attached to several object properties (even in different objects) using separate control-bindings.

      gst_object_add_control_binding (object, gst_direct_control_binding_new (object, "prop1", csource));

This type 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 behavior can be configured by setting the mode property of the control-source. Other control sources e.g. produce a stream of values by calling sin() function. They have parameters to control e.g. the frequency. As control-sources are GstObjects too, one can attach control-sources to these properties too.

Now we can set some control points. These are time-stamped gdouble values and are usually in the range of 0.0 to 1.0. A value of 1.0 is later mapped to the maximum value in the target properties value range. 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.

  GstTimedValueControlSource *tv_csource = (GstTimedValueControlSource *)csource;
  gst_timed_value_control_source_set (tv_csource, 0 * GST_SECOND, 0.0);
  gst_timed_value_control_source_set (tv_csource, 1 * GST_SECOND, 1.0);

Now everything is ready to play. If the control-source is e.g. bound to a volume property, we will head a fade-in over 1 second. One word of caution, the volume element that comes with gstreamer has a value range of 0.0 to 4.0 on its volume property. If the above control-source is attached to the property the volume will ramp up to 400%!

One final note - the controller subsystem has a built-in live-mode. Even though a property has a control-source assigned one can change the GObject property through the g_object_set(). 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 does not work for control-sources that constantly update the property (e.g. the lfo_control_source).

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