title: Python OpenCV video resolution
body: <p>I try to change video resolution (with mp4!) (to 800x600) in this way : but its doesn't work, when I use cap.get(3) and (4), its return every time defualt 1280x720! </p>
  
  <pre><code>import cv2
  cap = cv2.VideoCapture('file')
  while(cap.isOpened()):
      cv2.waitKey(10)
  
      ret, frame = cap.read()
      cap.set(3, 800)
      cap.set(4, 600)
  
      gray = cv2.cvtColor(frame, cv2.COLOR_BGR2BGRA) 
      print cap.get(3) # return default 1280       
  
      cv2.imshow('frame',gray)
      if cv2.waitKey(1) &amp; 0xFF == ord('q'):
          break
  
  cap.release()
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>What I'm doing wrong? </p>
  
  <p>I tried - </p>
  
  <pre><code>cv2.resizeWindow("ssss", 300, 300), 
  </code></pre>
  
  <p>and </p>
  
  <pre><code>cap.set(cv2.CAP_PROP_FRAME_HEIGHT, 600)
  cap.set(cv2.CAP_PROP_FRAME_WIDTH, 800)
  </code></pre>
  
  <p>no effect !</p>