[vtkusers] vtkLookupTable Transpacency == background color?

[Chris Want]

Hi all,

I would like to use a lookup table that is red and varies
in opacity. I am using code to generate the lookup table
that looks like this:

     lut = vtkLookupTable()
     lut.SetNumberOfColors(256)
     for i in range(256):
         lut.SetTableValue(i, 1.0, 0.0, 0.0, float(i)/255.0)
     lut.Build()

When I use this lookup table the rendered object varies in color
between red and the background color of the renderer instead of
varying between red and transparent. I get similar results when I try to
set the transparency using lut.SetAlphaRange(0.0, 1.0). When I set a 
solid transparency using actor.GetProperty().SetOpacity() I get the
transparency I would expect.

Is there something else that I have to set on the mapper or on the
vtkProperty of the actor to get this variation in opacity to work?

(platform linux+nvidia,
vtk version 4.x from the vtkNightlySrc.tar.gz on 2002-03-06)

Thanks,

Chris Want
Research Computing Support
Computing & Network Services
University of Alberta
Tel: 1-780-492-9418