[vtkusers] Adding a transparency mask to an image

[David Doria]

Is there a way to set a transparency mask for a vtkImageData displayed
with a vtkImageActor? For example, I want to make a specific region
(in the example below, the top half) of the image transparent. This
would be used to overlay the result of a binary segmentation onto the
original image.

There is a "sandbox" here if you have any suggestions:
http://www.vtk.org/Wiki/VTK/Examples/Cxx/Broken/Images/Transparency

Thanks,

David