An envelope parameterized by its Gauss Map. The parametric equations for a hedgehog are
References
Langevin, R.; Levitt, G.; and Rosenberg, H. ``Hérissons et Multihérissons (Enveloppes paramétrées
par leur application de Gauss.'' Warsaw: Singularities, 245-253, 1985. Banach Center Pub. 20, PWN Warsaw, 1988.
Martinez-Maure, Y. ``A Note on the Tennis Ball Theorem.'' Amer. Math. Monthly 103, 338-340, 1996.