> Can someone clarify the interior shape or suggest an alternate form of
> a hollow cylinder which has curves (coves and beads in woodturning
> lexicon) on the inside?
 
If you continue the spline, making it go inside itself, you get a lathe
with a hole in it (like a vase or drinking glass). To illustrate using
your example:

> /* set the control points to be used */
> #declare Red_Point    = <1.00, 1.00>;
> #declare Orange_Point = <0.00, 1.50>;
> #declare Green_Point  = <1.00, 3.50>;
> #declare Blue_Point   = <0.50, 4.00>;
> #declare Green_Point2 = <1.00, 4.50>;
> #declare Orange_Point2= <0.00, 6.50>;
> #declare Red_Point2   = <1.00, 7.00>;
/* all these points are closer to the axis than the ones above */
#declare   Orange_Point3= <-0.50, 6.50>;
#declare   Green_Point3 = <0.50, 4.50>;
#declare   Blue_Point2  = <-0.50, 4.00>;
#declare   Green_Point4 = <0.50, 3.50>;
#declare   Orange_Point4= <-0.50, 1.50>;
#declare   Red_Point3   = <0.50, 1.00>;
 

>   lathe {
>     bezier_spline
16,
>     Red_Point, Orange_Point, Green_Point, Blue_Point
>     Blue_Point, Green_Point2, Orange_Point2, Red_Point2
Red_Point2, Orange_Point3, Green_Point3, Blue_Point2
Blue_Point2, Green_Point4, Orange_Point4, Red_Point3
>     scale <6, 1, 1>
>     pigment { Cyan }
>     }

I don't think this example really shows what you mean, but hopefully it
should explain the technique.

Hope this is of help,
Daniel Hulme

-- 
The most important thing about a programming language is the name. A
language will not succeed without a name. I have recently invented a
very good name and I am looking for a suitable language.  -- Knuth
http://people.pwf.cam.ac.uk/dh286/publickey.asc  .:.  keyid 885b170d

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