title: Is there any problem with the OpenSlide.read_region function?
body: <p>I am using the python API of openslide packages to read some ndpi file.When I use the read_region function, sometimes it return a odd image. What problems could have happend?</p>
  
  <p>I have tried to read the full image, and it will be worked well. Therefore, I think there is no problem with the original file.</p>
  
  <pre class="lang-py prettyprint-override"><code>from openslide import OpenSlide
  import cv2
  import numpy as np
  
  slide = OpenSlide('/Users/xiaoying/django/ndpi-rest-api/slide/read/21814102D-PAS - 2018-05-28 17.18.24.ndpi')
  image = slide.read_region((1, 0),6, (780, 960))
  image.save('image1.png')
  </code></pre>
  
  <p>The output is strange <a href="https://i.stack.imgur.com/HauC9.png" rel="noreferrer">output</a></p>