>     Yes, I use the cylinder/torus difference trick for other things as well.


Well, where would we be without it. Couldn't do much neoclassical 
architechture without it :)

It does have it's problems, however. If you combine it with an concave 
shape like on the attached image (code below), the shading looks 
strange. I don't know if it is a problem with pov's shading model, or I 
just have wrong expectations to how a shape like this should look, but I 
don't think it looks right.

/Ib

--- code start ---
camera {
    location  <0, 10, -100>
    angle 30
    look_at   <0, 10,  0 >
}

light_source { <0, 500, -250>, colour White*2}

union {
    sphere { <0, 10, 0>, 10 }
    difference {
       cylinder { <0, 0, 0>, <0, 10, 0>, 20 }
       torus { 20, 10 translate <0, 10, 0> }
    }
    texture { pigment { Gray50 } finish { Phong_Shiny } }
}
--- code end ---

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