William F Pokorny <ano### [at] anonymousorg> wrote:
> On 9/24/25 15:46, Bald Eagle wrote:
> > What I meant, if you misunderstood, was to have one side be radius r, and the
> > other side be radius -r, so that it would make a "double-cone".
>
> Ah, yes, the double cone looks to work fine.

Well...  sorta.

I was actually wondering if the normal flipped, which apparently it used to:

https://news.povray.org/povray.general/thread/%3Cweb.58228e15512a2627116828db0%40news.povray.org%3E/?mtop=412640

"Note : a cone can take a negative radius. In this case, you'll get a
double cone. You can try this :
cone{2*x, 2, -2*x, -2 pigment{rgb 1} inside_texture{pigment{rgb<0,0,1>}}}
Yes, the left part IS blue."

And that's why I think that allowing the negative radius should be kept, and
perhaps even enhanced. *

I tried searching for documentation instances for "negative radius" but didn't
find much - but I seem to remember notes to the effect that the normal would be
flipped.  torus?  sphere?

* What I'm currently thinking about is whether there should be some mechanism
for "inside-out" shapes to be functionally equivalent to _inverse_.
So maybe in regular mode, the negative radius just flips the normal, but perhaps
with an additional keyword, any inside-out part would actually be inverted.

Which also had me wondering what other shapes we can cleverly play with using
negative values.

- bw

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