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Chapter 6 ■ Thinning 235

each boundary pixel individually. Then each line would exert a single force
on each pixel, rather than many forces. In addition, the visibility calculation
can be simplified by using a simple distance threshold. Line segments further
than d units away would not contribute a force.
Figure 6.24 shows the forces computed for a hand-printed 8, and gives the
skeleton as determined by the force-based method. For comparison purposes
the skeleton found by the Zhang-Suen algorithm is shown also.

(b)

(c)

(a) (d)
Figure 6.24: Force-based thinning applied to a handprinted 8. (a) The forces at each
pixel, where force is proportional to the length of the line drawn from the pixels, and
direction is normal to them. (b) The original figure. (c) Thinned using Zhang-Suen. (d)
Force-based skeleton.

6.7 Source Code for Zhang-Suen/Stentiford/Holt
Combined Algorithm

/ * Zhang & Suen’s thinning algorithm * /
#define MAX
#include “lib.h“
#include <math.h>
#define TRUE 1