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Chapter 4 ■ Grey-Level Segmentation 169

#define MAX
#include “lib.h“
void thrdd (IMAGE im);

void main (int argc, char *argv[])
{
IMAGE data;

if (argc < 3)
{
printf (“Usage: thrdd <input file> <output file>\n“);
exit (0);
}
data = Input_PBM (argv[1]);
if (data == NULL)
{
printf (“Bad input file '%s’\n“, argv[1]);
exit(1);
}
thrdd (data);
Output_PBM (data, argv[2]);
}

void thrdd (IMAGE im)
{
int NC, row, col, inc;
float mean, s, sum;
unsigned char *p;
long N, i;

N = (long)im->info->nc * (long)im->info->nr;
NC = im->info->nc; s = (int)(float)(NC/Navg);
sum = 127*s; row = col = 0; inc = 1;
p = &(im->data[0][0]);

for (i=0; i<N-1; i++) {
if (col >= NC) {
col = NC-1; row++; inc = -1;
p = &(im->data[row][col]);
} else if (col < 0)
{
col = 0; row++; inc = 1;
p = &(im->data[row][col]);
}
sum = sum - sum/s + *p;
mean = sum/s;
if (*p < mean*(100-pct)/100.0) *p = 0; else *p = 255;
p += inc; col += inc;
}
}

Figure 4.15: Source code for the program thrdd.c: adaptive thresholding using a moving
average.