title: How to get index from itertools.product
body: <pre><code># loop through every NxN cell in the target image
      for (blk_row, blk_col) in itertools.product(xrange(0, height - 
        (cell_size - 1), cell_size), xrange(0, width - (cell_size - 1), cell_size)):
  </code></pre>
  
  <p>where N = cellsize, would it be valid to divide by cellsize to get an index?</p>