title: Scikit-image expanding active contour (snakes)
body: <p>I followed the example from this <a href="http://scikit-image.org/docs/dev/auto_examples/edges/plot_active_contours.html" rel="noreferrer">link</a>. However, the contour contracts from the init points. Is it possible to do contour that expands out? I want something like the image shown. The image on the left is what it looks like, and the image on the right is what I want it to look like - expand out instead of contracting. The red circle is the starting point, and the blue outline is after n iterations. Is there a parameter I can set - I looked at all of them, but none seems to be setting that. Also, when referring to 'active-contour', does it usually assume that the contour contracts? I read <a href="http://www.stat.wisc.edu/~mchung/teaching/MIA/reading/segmentation.active.contour.2007.pdf" rel="noreferrer">this paper</a> and thought that it can both contract and expand.</p>
  
  <pre><code>img = data.astronaut()
  img = rgb2gray(img)
  
  s = np.linspace(0, 2*np.pi, 400)
  x = 220 + 100*np.cos(s)
  y = 100 + 100*np.sin(s)
  init = np.array([x, y]).T
  
  if not new_scipy:
      print('You are using an old version of scipy. '
            'Active contours is implemented for scipy versions '
            '0.14.0 and above.')
  
  if new_scipy:
      snake = active_contour(gaussian(img, 3),
                             init, alpha=0.015, beta=10, gamma=0.001)
  
      fig = plt.figure(figsize=(7, 7))
      ax = fig.add_subplot(111)
      plt.gray()
      ax.imshow(img)
      ax.plot(init[:, 0], init[:, 1], '--r', lw=3)
      ax.plot(snake[:, 0], snake[:, 1], '-b', lw=3)
      ax.set_xticks([]), ax.set_yticks([])
      ax.axis([0, img.shape[1], img.shape[0], 0])
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/mKzGw.png" rel="noreferrer"><img src="https://i.stack.imgur.com/mKzGw.png" alt="enter image description here"></a></p>