/**************************************************************************
 * Func: rgb2hsi                                                          *
 *                                                                        *
 * Desc: converts a value from RGB to HSI color space                     *
 *                                                                        *
 * Params: r - normalized red value                                       *
 *         g - normalized green value                                     *
 *         b - normalized blue value                                      *
 *         h - hue in degrees                                             *
 *         s - normalized saturation                                      *
 *         i - normalized intensity                                       *
 **************************************************************************/
void rgb2hsi(float r,float g,float b,float *h,float *s,float *i)
    {
    float min, max;              /* minimum and maximum RGB values */
    float angle;                 /* temp variable used to compute Hue */

    if((r<=g) && (r<=b))
	min = r;
    else if((g<=r) && (g<=b))
	min = g;
    else
	min =b;

    /* compute intensity */
    *i=(r + g + b) / 3.0;

    /* compute hue and saturation */
    if((r==g) && (g==b))  /* gray-scale */
	{
	*s = 0.0;
	*h = 0.0;
	return;
	}
    else
	{
	*s= 1.0 - (3.0 / (r + g + b)) * min;
	angle = (r - 0.5 * g - 0.5 * b) / 
		 sqrt((r - g) * (r - g)+(r - b) * (g - b));
	*h = acos(angle);
	*h *= 57.29577951;          /* convert to degrees */
	}
    if(b>g)
	*h = 360.0 - *h;
    }