[vtkusers] How do I perform volume rendering offscreen?

[Alan]

Hi,
I'd like to perform volume rendering off-screen and then grab the 
rendered image for further processing/display in Java.

How can I set up my pipeline to render in a way that data is *not* 
displayed on screen;  for example, is there something equivalent to 
vtkRenderWindow which does not have to be displayed?

Thanks for all info.
   --Alan