Le 15/05/2017 à 21:17, Bald Eagle a écrit :
> To clarify,
> I'd just like to have a variable-minimum-radius torus.
> Small at "apogee", and wide at "perigee".
> 
> Here, the Dupin Cylide - it's exactly what I was thinking of.
> https://en.wikipedia.org/wiki/Dupin_cyclide

So, you even found a page with all that is needed:
* for a parametric
* for a polynomial (quartic)

Per your specifications, it's an elliptic cyclide and we have c < d
(ring cyclide).

Remains to be done: mapping your input (3 radii) into a,b,c & d.

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