[vtkusers] Rendering too slow

[jiang]

Hi all,
In my program there are a volume ray cast rendering and a common line, which
are in a same vtkRenderer and vtkRenderWindow. I want to change the line's
postion. So I change LineActor->SetPosition(), then use renWin->Render().
However it works very slow. I consider change line's position alone is very
fast. The reason maybe is the volume rendering. How can I speed it up?
Thanks a lot!

Chunyan