Let *r*
be the radius of the circle sweeping out the torus and *R*
the radius of the center circle. Assuming *r* is less
than *R-r*, the interval from the minimum to the supremum
curvature of the curve
is [*0, 1/r*). We map this to the interval
from 1 to infinity using *f(x) = 1/(1-rx)*.
The tubes are created by sweeping an ellipse in the normal
plane of the Frenet fram and varying its aspect-ratio with
*f* of the curvature. The ellipse does not rotate
within its frame and the major axis is aligned with the
binormal vector.