/***************************************************************************
 * Func: histogram_equalize                                                *
 *                                                                         *
 * Desc: histogram equalize an input image and write it out                *
 *                                                                         *
 * Params: buffer - pointer to image in memory                             *
 *         number_of_pixels - total number of pixels in image              *
 ***************************************************************************/

void histogram_equalize(image_ptr buffer, unsigned long number_of_pixels)
    {
    unsigned long histogram[256]; /* image histogram */
    unsigned long sum_hist[256];  /* sum of histogram elements */
    float scale_factor;           /* normalized scale factor */
    unsigned long i;              /* index variable */
    unsigned long sum;            /* variable used to increment sum of hist */

    /* clear histogram to 0 */
    for(i=0; i<256; i++)
	histogram[i]=0;

    /* calculate histogram */
    for(i=0; i<number_of_pixels; i++)
	histogram[buffer[i]]++;

    /* calculate normalized sum of hist */
    sum = 0;
    scale_factor = 255.0 / number_of_pixels;
    for(i=0; i<256; i++)
	{
	sum += histogram[i];
	sum_hist[i] = (sum * scale_factor) + 0.5;
	}

    /* transform image using new sum_hist as a LUT */
    for(i=0; i<number_of_pixels; i++)
	buffer[i] = sum_hist[buffer[i]];
    }