[vtkusers] volume rendering CAD model?

[Shang Mu]

How do I do volume rendering for .stl model or other surface model?

I think I can create a 3D image and clip it using the surface model
and then do the volume rendering, but is there a way to deal with
surface model directly?
I think it should be faster to do a volume rendering using a surface
model assuming homogeneous density.

Appreciate any suggestion.