Using a biarc filter to compute curvature extremes of NURBS curves

A method to compute curvature minima and maxima of parametric curves (represented in NURBS format) is presented in this paper. Since the curvature changes vary rapidly along the path of (even smooth) curves, a biarc filter is employed to approximate the curvature function with a piecewise constant function. This allows the isolation of curvature extreme values that are found within-engineering tolerances via repeated biarc approximation followed by golden section search. Because the derivative of the curvature is numerically very unstable, only optimization without derivatives is feasible. However, given the excellent isolation property of biarc filters, curvature extremes are found within 10–20 steps even for high accuracy requirements ranging from 10⁻⁴ to 10⁻⁶.