mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-26 02:00:33 +00:00
102 lines
5.1 KiB
XML
102 lines
5.1 KiB
XML
<chapter id="chapter-dparams">
|
|
<title>Dynamic Controllable Parameters</title>
|
|
|
|
<sect1 id="section-dparams-getting-started">
|
|
<title>Getting Started</title>
|
|
<para>
|
|
The controller subsystem offers a lightweight way to adjust gobject
|
|
properties over stream-time. Normally these properties are changed using
|
|
<function>g_object_set()</function>. 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.
|
|
</para>
|
|
<para>
|
|
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
|
|
<filename>gstcontroller</filename> library.
|
|
You need to include the header in your application's source file:
|
|
</para>
|
|
<programlisting>
|
|
...
|
|
#include <gst/gst.h>
|
|
#include <gst/controller/gstinterpolationcontrolsource.h>
|
|
#include <gst/controller/gstdirectcontrolbinding.h>
|
|
...
|
|
</programlisting>
|
|
<para>
|
|
Your application should link to the shared library
|
|
<filename>gstreamer-controller</filename>. One can get the required flag for
|
|
compiler and linker by using pkg-config for gstreamer-controller-1.0.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="section-dparams-parameters">
|
|
<title>Setting up parameter control</title>
|
|
<para>
|
|
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:
|
|
</para>
|
|
<programlisting>
|
|
csource = gst_interpolation_control_source_new ();
|
|
g_object_set (csource, "mode", GST_INTERPOLATION_MODE_LINEAR, NULL);
|
|
</programlisting>
|
|
<para>
|
|
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.
|
|
</para>
|
|
<programlisting>
|
|
gst_object_add_control_binding (object, gst_direct_control_binding_new (object, "prop1", csource));
|
|
</programlisting>
|
|
<para>
|
|
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 <function>sin()</function> 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.
|
|
</para>
|
|
<para>
|
|
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.
|
|
</para>
|
|
<programlisting>
|
|
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);
|
|
</programlisting>
|
|
<para>
|
|
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%!
|
|
</para>
|
|
<para>
|
|
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 <function>g_object_set()</function>.
|
|
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).
|
|
</para>
|
|
</sect1>
|
|
|
|
</chapter>
|