I'm new to yt, and am having some trouble volume rendering my Castro simulation data (3D).
The simulation is of two stars orbiting the center of the domain, the domain being defined by defined by [0.0, 0.0, 0.0], [1.e10, 1.e10, 1.e10], in cgs. 
When I run the following script, I get an image that is either all black or all white, depending on which field I use.
Does anyone see anything wrong with my script?:

  from yt.mods import *
  pf = load("plt_derr_perr_grav_00020")
  field = "density"
  dd = pf.h.all_data()
  mi, ma = dd.quantities["Extrema"](field)[0]
  tf = ColorTransferFunction((mi, ma))
  c = [5.0e9, 5.0e9, 5.0e9]
  L = [1., 1., 1.]
  W = pf.domain_right_edge - pf.domain_left_edge
  Nvec = 512
  cam = pf.h.camera(c, L, W, (Nvec,Nvec), transfer_function = tf, fields=[field], pf=pf)
  tf.add_layers(4, colormap="hsv")

Any advice would be greatly appreciated.

Noel Scudder