[vtkusers] rendering artifact when using opacity on volume contour

[Liam Kurmos]

I'm using vtkMarchingCubes/vtkContourFilter on some simple gaussian
volume data to extract a surface and then do GetProperty->SetOpacity()
on the actor.

When i view the results from certain angles in the interactor there
are some shading artifacts as show in the screen shot. This only
happens when i use opacity.

Does anyone know what is going on here?

cheers,

Liam
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