[Radiance-general] Help with transfunc

[Mark Stock]

I am confused about the use of the transfunc. Function files took
me a little while to understand anyways, but what I can't figure out
is how to specify only a change in transmittance in a material.

        mod transfunc id
        2+ brtd funcfile transform
        0
        6+ red green blue rspec trans tspec A7 ..

I'd like to render a mesh object in "x-ray" form, but visualizing
its surface and not its volume. I'd like a material that diffuses
and reflects no light, and transmits light based on the "thickness"
of the surface (assuming that the object is a thin shell of fixed
thickness). I'd like the transmittance to be something like
1.0-(const/Rdot), but when I try the following:

        void transfunc meshcolor
        2 1.0-A5/Rdot .
        0
        6 0.5 0.5 0.5  1 0.66667 1

I've tried several different values for the function, and everything
gives me a totally opaque object, or an "undefined function" error.
I'd like to get something like this:

http://mark.technolope.org/vort3d/bd10/run168_first_sphere/burn1/run05/save_0053.jpg

Mark