[vtkusers] vtkPolyData and alpha values

[Aleksander Demko]

I can't seem to get simple alpha support to work for my polygon data set.

I have a polyData data set thats the result of a contour, decimatepro,
stripper and normals process. I have associated a lookup table via
SetLookupTable on its vtkPolyDataMapper. This table has alpha values,
that I have set to be 0 or 1.0 (and never in between). When I set the
alpha values to be 0 for certain colours in the lookup table, those
triangles that have that colour, rather than disappear seem to just go
black, blocking the triangles I want to see. Black is not in my
palette, either. It seems the alpha value is being used to just darken
the colour of the triangles, rather than make them be transparent.

Is there some kind of alpha test/blend function that I need to enable
to get basic z-buffer alpha testing? I don't need any fancy blending,
so I was hoping to avoid sorting the polys (which seems to be unusably
slow anyways) and other tricks.

I've been banging my head on this for a day or two now, so any help
would be really appreciated :)

Thanks