[vtkusers] Bug? in Volume Rendering With Example Code.

[Kevin H. Hobbs]

I think there's a bug in the vtkFixedPointVolumeRayCastMapper. 

I used to get great resuts from this class but recently the images I get look 
like some of the data has been skipped.

In situations where I can threshold my data and use the 
vtkUnstructuredGridVolumeRayCastMapper I also get great results. 

I wrote a little example program to try and get to the problem by comparing 
the two mappers. 

The two images and the example program are attached.
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