title: overflow warning as I try to do an add operation on the matrix cell
body: <p>I am seeing a weird warning , of which I have no clue what is going on. I am trying to do some operation on the image cell which looks like:</p>
  
  <pre><code>name = "./images/a.png" 
  ci = cv2.imread(name)
  img = cv2.cvtColor(ci, cv2.COLOR_BGR2GRAY)
  # In the following statement, i and j correspond to the row and column respectively
  op =  np.absolute(img[i, j+1] - img[i, j-1]) + np.absolute(img[i-1, j] - img[i, j-1])
  </code></pre>
  
  <p>As I run this statement, I get <code>RuntimeWarning: overflow encountered in ubyte_scalars</code>. I cannot understand the reason for it. Any idea what mistake am I making?</p>