This is more or less a direct translation of the examples found at gstreamer/tests/examples/controller/*.c to their equivalents using the gi bindings for Gstreamer under Python. The documentation can be found here. Reading the source also helps a lot.

The basic premise is that you can attach a controller to almost any property of an object, set an interpolation function and give it pairs of (time, value) so they are smoothly changed. I’m using a pad as a target instead of an element just because it fits my immediate needs but it really can be any Element.

First you need to import Gstreamer and initialize it:

#!/usr/bin/python
import gi
import sys
from gi.repository import GObject
gi.require_version('Gst', '1.0')
from gi.repository import Gst
from gi.repository import GstController
from gi.repository import Gtk
from gi.repository import GLib

Then create your elements. This is by no means the best way but lets me cut a bit on all the boilerplate.


p = Gst.parse_launch ("""videomixer name=mix ! videoconvert ! xvimagesink
videotestsrc pattern="snow" ! videoconvert ! mix.sink_0
videotestsrc ! videoconvert ! mix.sink_1
""")

m = p.get_by_name ("mix")
s0 = [pad for pad in m.pads if pad.name == 'sink_0'][0]
s0.set_property ("xpos", 100)


Here I created two test sources, one with bars and another with static that also has an horizontal offset. If we were to start the pipeline right now ( p.set_state (Gst.State.PLAYING) ) we would see something like this:

So far it works. Now I’d like to animate the alpha property of s0 (the sink pads of a videomixer have interesting properties like alpha, zorder, xpos and ypos). First we create a control source and set the interpolation mode:

cs = GstController.InterpolationControlSource()
cs.set_property('mode', GstController.InterpolationMode.LINEAR)

Then we create a control binding for the property we want to animate and add it to our element:

cb = GstController.DirectControlBinding.new(s0, 'alpha', cs)
s0.add_control_binding(cb)

It is worth noting that the same control source can be used with more than one control binding.

Now we just need to add a couple of points and play:

cs.set(0*Gst.SECOND, 1)
cs.set(4*Gst.SECOND, 0.5)
p.set_state (Gst.State.PLAYING)

If you are not running this from the interpreter remember to add GObject.MainLoop().run() , otherwise the script will end instead of keep playing. Here I’ve used absolute times, to animate in the middle of a playing state you need to get the current time and set the points accordingly, something like this will do most of the cases:


start = p.get_clock().get_time() # XXX: you better check for errors
end = start + endtime*Gst.SECOND


Avoiding too much bookkeeping

You can get the controller and control source of an element with:

control_binding = element.get_control_binding('property')
if control_binding:
    control_source = control_binding.get_property('control_source')