text: import yt
  import numpy as np
  
  ds = yt.load("IsolatedGalaxy/galaxy0030/galaxy0030")
  
  # We start by building a default volume rendering scene
  
  im, sc = yt.volume_render(ds, field=("gas","density"), fname="v0.png", sigma_clip=6.0)
  
  sc.camera.set_width(ds.arr(0.1,'code_length'))
  tf = sc.get_source().transfer_function
  tf.clear()
  tf.add_layers(4, 0.01, col_bounds = [-27.5,-25.5],
          alpha=np.logspace(-3,0,4), colormap = 'RdBu_r')
  sc.render()
  sc.save("v1.png", sigma_clip=6.0)
  
  # In this case, the default alphas used (np.logspace(-3,0,Nbins)) does not
  # accentuate the outer regions of the galaxy. Let's start by bringing up the
  # alpha values for each contour to go between 0.1 and 1.0
  
  tf = sc.get_source().transfer_function
  tf.clear()
  tf.add_layers(4, 0.01, col_bounds = [-27.5,-25.5],
          alpha=np.logspace(0,0,4), colormap = 'RdBu_r')
  sc.render()
  sc.save("v2.png", sigma_clip=6.0)
  
  # Now let's set the grey_opacity to True.  This should make the inner portions
  # start to be obscured
  
  tf.grey_opacity = True
  sc.render()
  sc.save("v3.png", sigma_clip=6.0)
  
  # That looks pretty good, but let's start bumping up the opacity.
  
  tf.clear()
  tf.add_layers(4, 0.01, col_bounds = [-27.5,-25.5],
          alpha=10.0*np.ones(4,dtype='float64'), colormap = 'RdBu_r')
  sc.render()
  sc.save("v4.png", sigma_clip=6.0)
  
  # Let's bump up again to see if we can obscure the inner contour.
  
  tf.clear()
  tf.add_layers(4, 0.01, col_bounds = [-27.5,-25.5],
          alpha=30.0*np.ones(4,dtype='float64'), colormap = 'RdBu_r')
  sc.render()
  sc.save("v5.png", sigma_clip=6.0)
  
  # Now we are losing sight of everything.  Let's see if we can obscure the next
  # layer
  
  tf.clear()
  tf.add_layers(4, 0.01, col_bounds = [-27.5,-25.5],
          alpha=100.0*np.ones(4,dtype='float64'), colormap = 'RdBu_r')
  sc.render()
  sc.save("v6.png", sigma_clip=6.0)
  
  # That is very opaque!  Now lets go back and see what it would look like with
  # grey_opacity = False
  
  tf.grey_opacity=False
  sc.render()
  sc.save("v7.png", sigma_clip=6.0)
  
  # That looks pretty different, but the main thing is that you can see that the
  # inner contours are somewhat visible again.
  