title: Is there a Numpy array broadcasting expression simpler than this list comprehension?
body: <p>In this code snippet:</p>
  <pre><code>triangles = np.float32([[[0, -2], [-2, 3], [1, 1]], [[0, -1], [-1, 3], [1, 1]]])
  centers = np.average(triangles, axis=1)
  samples = np.float32([t-centers[i] for i, t in enumerate(triangles)])
  </code></pre>
  <p>I would like to express <code>samples</code> as an array broadcast subtraction, i.e. something similar to <code>triangles-centers</code>, which doesn't work due to:</p>
  <pre><code>ValueError: operands could not be broadcast together with shapes (2,3,2) (2,2) 
  </code></pre>
  <p>Is there a simpler way to define <code>samples</code> than a list comprehension?</p>