title: Python OpenCv, only read part of image
body: <p>I have thousands of large .png images (screenshots).  I'm using opencv to do image recognition on a small portion of each image.  I'm currently doing:</p>
  
  <pre><code>    image = cv2.imread(path)
      x,y,w,h = bounds
      image = image[y:y + h, x:x + w]
  </code></pre>
  
  <p>The profiler tells me <code>cv2.imread</code> is a bottleneck.  I'm wondering if I can make the script faster by only reading the part of each image I'm interested in rather than loading the entire image and then cropping to the bounds.  I can't find an OpenCV flag for that though.  Am I missing one?</p>