envelope.m download [For Mathematica]

Starting with the paraboloid z = x^2 + y^2, imagine the set of spheres tangent to the xy-plane with center lying on the paraboloid. The boundary of this set of spheres (the envelope) is a sphere tangent to the xy-plane with center along the z-axis.

Below is the envelope of a set of spheres lying along a 3-2 torus knot. Each sphere's radius depends upon its position on the knot.