title: How can I subtract a scalar from every element in one axis of an ndarray
body: <p>I'm trying to normalize the background of an image read in with <code>imread()</code> which produces a NxMx3 ndarray.</p>
  
  <p>To do this I need to subtract the NxMx0 axis from 255, the NxMx1 axis from 250, and the NxMx2 axis from 225.</p>
  
  <p>How can I do this?</p>