gstreamer overlay alpha composite two webm VP9 files - gstreamer

Question

What command do I need to overlay or alpha composite two vp9 webm video files on top of each other in gstreamer ?
inputs
foreground.webm
background.webm
output
foreground_over_background.webm
from reading the NVidia GStreamer documentation, page 46:-
https://developer.download.nvidia.com/embedded/L4T/r32_Release_v1.0/Docs/Accelerated_GStreamer_User_Guide.pdf?bfPUdrIaI1j8RVvuoCD405aKTnLM7iySjOBRny2F8HQScIv521n_UOFV0GbrblQAyN0mkVbRu1bWt9iY4l6xR7Tk5aI3wR-D8_YSvpUhWq1xqi6IrBZhuuZElW_n7i5wfLaPvko4MoqeXpjNTdYEqXNvbedG9a_leDcWNl-WK9QlceGJB-Q
I was hoping I could do it like this:-
from threading import Thread
import gi
from gi.repository import Gst, GLib
gi.require_version("Gst", "1.0")
Gst.init()
main_loop = GLib.MainLoop()
thread = Thread(target=main_loop.run)
thread.start()
#pipeline = Gst.parse_launch("v4l2src ! decodebin ! videoconvert ! autovideosink")
pipeline = Gst.parse_launch("nvcompositor name=comp "
"sink_0::xpos=0 sink_0::ypos=0 sink_0::width=188 sink_0::height=188 "
"sink_1::xpos=0 sink_1::ypos=0 sink_1::width=188 sink_1::height=188 "
"! nvoverlaysink display-id=1 "
"filesrc location =<blend_text_188.webm> ! matroskademux | omxvp9dec ! comp. -e "
"filesrc location =<gradient_188.webm> ! matroskademux | omxvp9dec ! comp. -e")
pipeline.set_state(Gst.State.PLAYING)
try:
while True:
sleep(0.1)
except KeyboardInterrupt:
pass
pipeline.set_state(Gst.State.NULL)
main_loop.quit()
However, I get this error message:-
.../gstreamer.py:3: PyGIWarning: Gst was imported without specifying a version first. Use gi.require_version('Gst', '1.0') before import to ensure that the right version gets loaded.
from gi.repository import Gst, GLib
Traceback (most recent call last):
File ".../gstreamer.py", line 10, in <module>
pipeline = Gst.parse_launch("nvcompositor name=comp "
gi.repository.GLib.GError: gst_parse_error: no element "nvcompositor" (1)
any idea why I get this error, and what I need to do to get this to overlay ?
so, I tried it this way
from threading import Thread
import gi
gi.require_version('Gst', '1.0')
gi.require_version('GstRtspServer', '1.0')
import os
os.environ['GST_DEBUG'] = '3'
from gi.repository import Gst, GLib, GstRtspServer
Gst.init()
main_loop = GLib.MainLoop()
thread = Thread(target=main_loop.run)
thread.start()
Gst.init(None)
port = "8554"
mount_point = "/test"
server = GstRtspServer.RTSPServer.new()
server.set_service(port)
mounts = server.get_mount_points()
factory = GstRtspServer.RTSPMediaFactory.new()
#pipeline = Gst.parse_launch("v4l2src ! decodebin ! videoconvert ! autovideosink")
factory.set_launch("nvcompositor name=comp "
"sink_0::xpos=0 sink_0::ypos=0 sink_0::width=188 sink_0::height=188 "
"sink_1::xpos=0 sink_1::ypos=0 sink_1::width=188 sink_1::height=188 "
"! nvoverlaysink display-id=1 "
"filesrc location =<blend_text_188.webm> ! matroskademux | omxvp9dec ! comp. -e "
"filesrc location =<gradient_188.webm> ! matroskademux | omxvp9dec ! comp. -e")
#pipeline.set_state(Gst.State.PLAYING)
# try:
# while True:
# sleep(0.1)
# except KeyboardInterrupt:
# pass
mounts.add_factory(mount_point, factory)
server.attach()
# pipeline.set_state(Gst.State.NULL)
# main_loop.quit()
But this also didn't do anything, unfortunately.
I added this post onto the NVidia forum aswell here:-
https://forums.developer.nvidia.com/t/alpha-compositing-one-webm-file-directly-over-another-webm-file-using-gstreamer/228404