title: Python array subtraction loops back to high number instead of giving negative value
body: <p>Here is my code:
  I'm using numpy and opencv</p>
  
  <pre><code>q = np.array(image)
  q = q.reshape(-1, q.shape[2])
  r = np.subtract(q,p)
  print r
  </code></pre>
  
  <p>Basically what happens is if the value in my q array is greater than p the subtraction loops back up to 256 and subtracts whats left from there.  I'd rather get a value of 0 if the subtraction goes negative.  Does anybody know a good way to do this?</p>