Curvature is used to describe both curves and surfaces. For complex curves and surfaces the value is
constantly changing over the geometry. Its instantaneous value is simply the inverse of the radius of
the curve or surface at a contact point. For a line, the radius at any point along the line is infinite, so
the curvature at all points along the line is zero. For a circle or arc, the radius is constant at all points
along the curve, therefore the curvature is also constant and nonzero. For a spline, the radius, and thus
curvature, are constantly changing along the length of the curve. The curvature at any point on a
surface is a more complex calculation but describes the same information as the curvature of a curve.