On 17/10/2012 4:31 PM, Tek wrote:
> I suggest you render a cross-section, preferably highlighting the cutaway
> surface (i.e. intersection { <hourglass> plane { -z, 0 pigment { rgb x } } } ).
>

Yes, I think that is the way to go. It has been ages since I have used 
SORs or Lathes and I am quite lazy. :-)

> But I think most of your problems come from using a spline type where you can't
> control the gradient where the mirrored bulbs meet. So either don't mirror them,
> and use a curve that describes both bulbs of the hourglass, or use a
> bezier_spline and place the control point tangentially to the desired curve to
> get a smooth transition.
>

I was using a cubic curve but my endpoint wasn't making making the joint 
take the right angle.

> Or you could download the source for my "Sands Of Time" IRTC entry:
>
http://exether.free.fr/irtc/index.php?sub=pg2004&lang=en&year=2004&month=8&typ=S&mrk=C
> ;)

Strange that you say that, Norbert suggested it too.
I found it interesting that you could use so few points and get such a 
good curve.

-- 
Regards
     Stephen

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