title: change frame rate in opencv 3.4.2
body: <p>I want to reduce the number of frames acquired per second in a webcam, this is the code that I'm using</p>
  
  <pre><code>#!/usr/bin/env python
  
  import cv2
  
  cap = cv2.VideoCapture(0)
  cap.set(cv2.CAP_PROP_FPS, 10)
  fps = int(cap.get(5))
  print("fps:", fps)
  
  while(cap.isOpened()):
  
      ret,frame = cap.read()
      if not ret:
          break
  
      cv2.imshow('frame', frame)
  
      k = cv2.waitKey(1)
      if k == 27:
          break
  </code></pre>
  
  <p>But it doesn't take effect, I still have 30 fps by default instead of 10 set up by <code>cap.set(cv2.CAP_PROP_FPS, 10)</code> . I want to reduce the frame rate because I have a hand detector which take quite a lot of time to process each frame, I can not store frames in buffer since it would detect the hand in previous positions. I could run the detector using a timer or something else but I thought changing the fps was an easier way, but it didn't work and I don't know why.</p>
  
  <p>Im using Opencv 3.4.2 with Python 3.6.3 in Windows 8.1</p>