/****************************************************************************
 * Func: difference                                                         *
 *                                                                          *
 * Desc: finds edges in an image window using the difference operator       *
 *                                                                          *
 * Param: window - 3x3 window of source image   |0 1 2|                     *
 *                                              |3 4 5|                     *
 *                                              |6 7 8|                     *
 *                                                                          *
 * Returns: the maximum value of the differences across the center pixel    *
 ****************************************************************************/

int difference(unsigned char *window)
    {
    int max;                 /* current maximum difference */
    int diff;                /* current difference */

    diff = abs(window[0] - window[8]);  /* upper left - lower right */
    max = diff;

    diff = abs(window[1] - window[7]);  /* upper - lower */
    if(diff > max)
	max = diff;

    diff = abs(window[2] - window[6]);  /* upper right - lower left */
    if(diff > max)
	max = diff;

    diff = abs(window[5] - window[3]);  /* left - right */
    if(diff > max)
	max = diff;

    return max;
    }