Gst Pipeline demo with control source bound to 'alpha' property of videomixer sink

gistfile1.txt

#!/usr/bin/python
import sys
import threading
import time

import gi
gi.require_version('Gst', '1.0')
from gi.repository import GObject
from gi.repository import Gst, GstController, GLib

def get_alpha_controller(pad):
    cs = GstController.InterpolationControlSource()
    cs.set_property('mode', GstController.InterpolationMode.LINEAR)

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

GObject.threads_init()
Gst.init(sys.argv)

pipeline = Gst.parse_launch ("""
    videomixer name="mix" ! autovideosink
    videotestsrc pattern="green" ! video/x-raw,width=400,height=400 ! decodebin ! mix.sink_0
    videotestsrc pattern="black" ! video/x-raw,width=200,height=200 ! decodebin ! mix.sink_1
""")

print([ pad.name for pad in pipeline.get_by_name('mix').pads ])
green_sink = pipeline.get_by_name('mix').get_request_pad('sink_0')
black_sink = pipeline.get_by_name('mix').get_request_pad('sink_1')
print([ pad.name for pad in pipeline.get_by_name('mix').pads ])
black_sink.set_property('xpos', 50)
black_sink.set_property('ypos', 50)
cs = get_alpha_controller(black_sink)
cs.set(1*Gst.SECOND, 1)
cs.set(4*Gst.SECOND, 0)
pipeline.set_state (Gst.State.PLAYING)
GLib.MainLoop.new(None, False).run()
start()