Create rtsp stream based on opencv images in python

Question

My goal is to read frames from an rtsp server, do some opencv manipulation of the frames and write the manipulated frames to a new rtsp server.

I tried the following based on Write in Gstreamer pipeline from opencv in python but i was unable to figure out what the appropriate gst-launch-1.0 arguments should be to create the rtsp server. Can anyone assist with appropriate arguments to gst-launch-1.0? The ones i tried got stuck in "Pipeline is PREROLLING"

import cv2

cap = cv2.VideoCapture("rtsp://....")

framerate = 25.0

out = cv2.VideoWriter('appsrc ! videoconvert ! '
  'x264enc noise-reduction=10000 speed-preset=ultrafast 
   tune=zerolatency ! '
  'rtph264pay config-interval=1 pt=96 !'
  'tcpserversink host=192.168.1.27 port=5000 sync=false',
  0, framerate, (640, 480))


counter = 0
while cap.isOpened():
  ret, frame = cap.read()
  if ret:
    out.write(frame)

    if cv2.waitKey(1) & 0xFF == ord('q'):
      break
  else:
    break

cap.release()
out.release()

I also tried another solution based on Write opencv frames into gstreamer rtsp server pipeline

import cv2
import gi 

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

class SensorFactory(GstRtspServer.RTSPMediaFactory):
  def __init__(self, **properties): 
    super(SensorFactory, self).__init__(**properties) 
    #self.cap = cv2.VideoCapture(0)
    self.cap = cv2.VideoCapture("rtsp://....")
    self.number_frames = 0 
    self.fps = 30
    self.duration = 1 / self.fps * Gst.SECOND  # duration of a frame in nanoseconds 
    self.launch_string = 'appsrc name=source is-live=true block=true format=GST_FORMAT_TIME ' \
                         'caps=video/x-raw,format=BGR,width=640,height=480,framerate={}/1 ' \
                         '! videoconvert ! video/x-raw,format=I420 ' \
                         '! x264enc speed-preset=ultrafast tune=zerolatency ' \
                         '! rtph264pay config-interval=1 name=pay0 pt=96'.format(self.fps)

  def on_need_data(self, src, lenght):
    if self.cap.isOpened():
      ret, frame = self.cap.read()
      if ret:
        data = frame.tostring() 
        buf = Gst.Buffer.new_allocate(None, len(data), None)
        buf.fill(0, data)
        buf.duration = self.duration
        timestamp = self.number_frames * self.duration
        buf.pts = buf.dts = int(timestamp)
        buf.offset = timestamp
        self.number_frames += 1
        retval = src.emit('push-buffer', buf) 

        print('pushed buffer, frame {}, duration {} ns, durations {} s'.format(self.number_frames, self.duration, self.duration / Gst.SECOND)) 

        if retval != Gst.FlowReturn.OK: 
          print(retval) 

  def do_create_element(self, url): 
    return Gst.parse_launch(self.launch_string) 

  def do_configure(self, rtsp_media): 
    self.number_frames = 0 
    appsrc = rtsp_media.get_element().get_child_by_name('source') 
    appsrc.connect('need-data', self.on_need_data) 


class GstServer(GstRtspServer.RTSPServer): 
  def __init__(self, **properties): 
    super(GstServer, self).__init__(**properties) 
    self.factory = SensorFactory() 
    self.factory.set_shared(True) 
    self.get_mount_points().add_factory("/test", self.factory) 
    self.attach(None) 


GObject.threads_init() 
Gst.init(None) 

server = GstServer() 

loop = GObject.MainLoop() 
loop.run()

This solution generates itself the rtsp server and streams to it. I am able to open the resulting rtsp stream in VLC, but it keeps displaying the first frame and does not update with new frames. Anyone who knows why?

I'm looking for any solution which will enable me with low latency to read frames from an rtsp server into an opencv format, manipulate the frames and output the frames into a new rtsp server (which i also need to create). The solution does not need to be based on gstreamer, if something better exists.

I am using Ubuntu 16.04 with python2.7 and opencv 3.4.1