2.5. Turn angle vs. normalized length
The reduced set of data points are used to describe the overall shape by polygon approximation or turn angle function, expressed as turn angle vs. normalized length. The turn angle is calculated with respect to the previous segment as shown in Fig. 4(a), so that a clockwise turn (h1 and h2) gives a negative angle and a counterclockwise turn (h3 and h4) gives a positive angle. In Fig. 4(a), solid line segments (l1–l5) are part of the original contour and dotted lines are the line segment directions before turning.