title: Indexing into channel data (numpy arrays) using OpenCV python bindings
body: <p>I am trying to do a channel compare on two numpy arrays generated from the OpenCV (2.3.1) Python bindings. So, I have a mask (array/image/channel) of shape (x, y) that I want to compare against each channel of an RGB array/image of shape (x,y,3).</p>
  
  <p>Having halfway wrapped my head around numpy's slicing, I can get closer to what I want:</p>
  
  <pre><code>channel = ndarr[...,i:i+1]     #where i is the channel I want
  </code></pre>
  
  <p>... but this returns an ndarray of shape (x,y,1) rather than the (x,y) I need. Is there an elegant way to do this in a single slicing operation. Failing that, what is the simplest way to do this?</p>