[vtkusers] Rendering multiple volumes

[Weiguang guan]

There were a few posts in the past about this topic. The conclusion 
seemed to be that VTK didn't support multi-volume rendering in terms of 
proper visibility sorting/blending. I'm not sure it is still true.

I've done a test where two partially overlapped volumes are used. I've 
found that only the texture mapping mapper can render them correctly. 
All the other mappers (GPU based or ray casting) always have one volume 
occlude the other one no matter what direction the camera is viewing.

Is vtkImageOcclusionSpectrum designed for handling multiple volume 
rendering? Any comments will be appreciated. Thanks.

Best,
Weiguang