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68 Chapter 2 ■ Edge-Detection Techniques

{
if (g*MAG_SCALE <= 255)
mag->data[i][j] = (unsigned char)(g*MAG_SCALE);
else
mag->data[i][j] = 255;
ori->data[i][j] = (unsigned char) (atan2 (yc, xc) * ORI_SCALE);
} else
{
mag->data[i][j] = 0;
ori->data[i][j] = 0;
}

}
}
}
void estimate_thresh (IMAGE mag, int *hi, int *low)
{
int i,j,k, hist[256], count;

/* Build a histogram of the magnitude image. */
for (k=0; k<256; k++) hist[k] = 0;

for (i=WIDTH; i<mag->info->nr-WIDTH; i++)
for (j=WIDTH; j<mag->info->nc-WIDTH; j++)
hist[mag->data[i][j]]++;

/* The high threshold should be > 80 or 90% of the pixels
j = (int)(ratio*mag->info->nr*mag->info->nc);
*/
j = mag->info->nr;
if (j<mag->info->nc) j = mag->info->nc;
j = (int)(0.9*j);
k = 255;

count = hist[255];
while (count < j)
{
k--;
if (k<0) break;
count += hist[k];
}
*hi = k;

i=0;
while (hist[i]==0) i++;

*low = (*hi+i)/2.0f;
}