Volume rendering of a Gridded3DSet --

Hi:  Has anyone used a Gridded3DSet to do volume rendering?  The data that I
am dealing with is regular in X and Y, but irregular in Z (terrain following
data -- altitude varies within each 2D plane).  Since Gridded3DSet sets
allows for regular topologies & non-regular (non-degenerate) geometries, I
figured that this would be the best way to  represent my 3D-grid. 

The actual volume rendering doesn't look like a volume rendering at all.
There is no transparency or color blending between primitives.  The
resulting image looks like a solid plot of all data points.  You can  pretty
much see individual points and the spacing between them.  It looks somewhat
like the 3D output from example Test67.java in ~/visad/examples. 

So, I grabbed the Test61.java example which does volume rendering of a grid
represented by an Integer3DSet and modified it to use my data.  Lo and
behold, the image looks like a true volume rendering with blending, proper
interpolation, etc.  Problem is, even if I fix the Z for each plane using an
average, the delta Z between each plane is still non-uniform.  So the
IntegerSet won't suffice.  Even though a Linear3DSet would seem to work (as
illustrated in the examples), it won't suffice either.

So, any reason(s) why a Gridded3DSet would not be properly rendered as a
volume?  Any suggestions on how to proceed?