title: Unable to adjust resolution by opencv under python environment
body: <p>I am currently try to adjust the resolution of the video by <code>opencv</code>, according to my research, <code>cap.set</code> is the method to do so. thus, I write the code below:</p>
  
  <pre><code>cap = cv2.VideoCapture('IMG_2957.MOV')
  for i in range(19):
      print (""+str(i)+" "+str(cap.get(i)))
  cap.set(3,160) #Width
  cap.set(4,120) #Height
  for i in range(19):
      print (""+str(i)+" "+str(cap.get(i)))
  </code></pre>
  
  <p>very surprise, the value of "CV_CAP_PROP_FRAME_WIDTH" and "CV_CAP_PROP_FRAME_HEIGHT" remain unchanged</p>
  
  <pre><code>Before the cv.set 
  3 1920.0
  4 1080.0
  </code></pre>
  
  <p>And after the setting, the frame was not set as what I want</p>
  
  <pre><code>After the cv.set 
  3 1920.0
  4 1080.0
  </code></pre>
  
  <p>Is that we are not able to change the resolution by setting its property? Can anyone advice me the usage of <code>.set</code> of <code>cv2.videocapture</code>?</p>
  
  <pre><code>More information about the version:
  Python 3.6.0  [MSC v.1900 64 bit (AMD64)] on win32
  &gt;&gt;&gt; import cv2
  &gt;&gt;&gt; cv2.__version__
  '3.4.0'
  </code></pre>