[vtkusers] Clipping bug

[Richard Whitehead]

This is in VTK 6.0.

Calling SetCroppingRegionPlanes on a vtkSmartVolumeMapper, I keep getting
exceptions in vtkGPUVolumeRayCastMapper.cxx.
The fundamental problem is that the volBounds obtained
from GetInput()->GetBounds() is slightly smaller than it should be, as if
it has suffered rounding error somewhere, and then there seems to be a
logical bug which means that an illegal clipping volume is calculated.

The extent of my data is 0..465 x 0..459 x 0..499, and I have set spacings
of 1.0 and origins of 0.0.

But the volBounds obtained is 0..463.155 x 0..458.009 x 0..496.6

I then try to set a cropping region in Z of 498 to 499.

The logic in vtkGPUVolumeRayCastMapper::ClipCroppingRegionPlanes()
calculates some bounds whose maximum Z is less than its minimum, and later
an assertion fails in vtkOpenGLGPUVolumeRayCastMapper::RenderSubVolume().

Please can anyone suggest what I might be doing wrong or if there is a
work-around?
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