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78 Chapter 2 ■ Edge-Detection Techniques

void estimate_thresh (double *low, double *hi, int nr, int nc)
{
float vmax, vmin, scale, x;
int i,j,k, hist[256], count;
/* Build a histogram of the Laplacian image. */
vmin = vmax = fabs((float)(lap[20][20]));
for (i=0; i<nr; i++)
for (j=0; j<nc; j++)
{
if (i<OUTLINE || i >= nr-OUTLINE ||
j<OUTLINE || j >= nc-OUTLINE) continue;
x = lap[i][j];
if (vmin > x) vmin = x;
if (vmax < x) vmax = x;
}
for (k=0; k<256; k++) hist[k] = 0;

scale = 256.0/(vmax-vmin + 1);

for (i=0; i<nr; i++)
for (j=0; j<nc; j++)
{
if (i<OUTLINE || i >= nr-OUTLINE ||
j<OUTLINE || j >= nc-OUTLINE) continue;
x = lap[i][j];
k = (int)((x - vmin)*scale);
hist[k] += 1;
}

/* The high threshold should be > 80 or 90% of the pixels */
k = 255;
j = (int)(ratio*nr*nc);
count = hist[255];
while (count < j)
{
k--;
if (k<0) break;
count += hist[k];
}
*hi = (double)k/scale + vmin ;
*low = (*hi)/2;
}
void embed (IMAGE im, int width)
{
int i,j,I,J;
IMAGE new;
width += 2;