Hi,
Please refer to the python samples:
Doesn't work nvv4l2decoder for decoding RTSP in gstreamer + opencv - #3 by DaneLLL
Displaying to the screen with OpenCV and GStreamer - #9 by DaneLLL
Stream processed video with OpenCV on Jetson TX2 - #5 by DaneLLL
OpenvCV, Gstreamer, Python camera capture/access and streaming to RTP
These are similar to this use-case. Please refer to the samples for developing your use-case.