[vtkusers] Rendering multiple actors with opacity (alpha)

Michael Rice marice at knology.net
Mon Aug 14 22:08:51 EDT 2006

This sounds like a problem we had. If you look in the  
vtkOpenGLActor::Render() method, you see that the glDepthMask is set  
to false if the opacity != 0 and the renderer is not picking. Our  
simple hack was to comment out the if/else check that disables the  
depth check. It doesn't produce the exact transparency we want, but  
it is very close.

As an experiment, you may want to try and modify vtkOpenGLActor.cxx  
and see if that gets you closer to what you want.

When I have some spare time, I'd like to craft a "real" solution and  
submit it to Kitware for consideration.


On Aug 14, 2006, at 7:05 PM, Eugene Kim wrote:

> Hi everyone,
> 	I've had an annoying little problem which should have an easy  
> solution.  For any given actor that I add to an assembly, modifying  
> it's alpha values in a lookup table makes its opaque to itself, but  
> only somewhat so to other objects.  Depending on the order in which  
> objects are added to the scene (via assembly), the first object  
> will be opaque to itself but no other object, the second to only  
> itself and the first, the 3rd to itself the second and the first,  
> and so on.  It's very peculiar. Anyone know how to get this  
> properly working?
> Thanks,
> Eugene
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