code: def frames(self, flush=True):
          
          self.flush()
          ret_val, frame = self._sensor.read()
          if not ret_val:
              raise Exception("Unable to retrieve frame from OpenCVCameraSensor for id {0}".format(self._device_id))
          frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
          if self._upside_down:
              frame = np.flipud(frame).astype(np.uint8)
              frame = np.fliplr(frame).astype(np.uint8)
          return ColorImage(frame)
docs: Returns the latest color image from the stream
          Raises:
              Exception if opencv sensor gives ret_val of 0