/***************************************************************************
 * Func: auto_contrast_stretch                                             *
 *                                                                         *
 * Desc: performs basic contrast stretching on an image                    *
 *                                                                         *
 * Params: source - pointer to source image                                *
 *         cols - number of columns in the image                           *
 *         rows - height of image                                          *
 ***************************************************************************/

void auto_contrast_stretch(image_ptr source, int cols, int rows)
    {
    long i;                     /* loop variable */
    long number_of_pixels;      /* total number of pixels in image */
    long histogram[256];        /* image histogram */
    unsigned char LUT[256];     /* Look-up table for point process */
    int lowthresh, highthresh;  /* lower and upper thresholds */
    float scale_factor;         /* scaling factor for contrast stretch */

    /* compute histogram */
    number_of_pixels = (long)cols * rows;

    for(i=0; i<256; i++)
	histogram[i]=0;

    for(i=0; i<number_of_pixels; i++)
	histogram[source[i]]++;

    /* compute low and high thresholds */
    for(i=0; i<256; i++)
	if(histogram[i])
	    {
	    lowthresh = i;
	    break;
	    }

    for(i=255; i>0; i--)
	if(histogram[i])
	  {
	  highthresh = i;
	  break;
	  }

    printf("Low threshold is %d   High threshold is %d\n",
	    lowthresh,highthresh);

    /* compute new LUT */
    for(i=0; i<lowthresh; i++)
       LUT[i]=0;

    for(i=255; i>highthresh; i--)
	LUT[i]=255;

    scale_factor = 255.0 / (highthresh-lowthresh);
    for(i=lowthresh; i<=highthresh; i++)
	LUT[i]=(unsigned char)((i - lowthresh) * scale_factor);

    /* transfer new image */
    for(i=0; i<number_of_pixels; i++)
	source[i] = LUT[source[i]];
    }