/*********************************************************/
/*   program 3: convert RGB from/to HSV                  */
/*                                                       */
/*   note: no main program is supplied for this example, */
/*    only the routines to perform the conversions.      */
/*********************************************************/

#define UNDEFINED 999.0

void rgb2hsv();
void hsv2rgb();

/***********************************************/
/*       rgb2hsv: RGB-to-HSV conversion        */
/***********************************************/

void rgb2hsv(r,g,b,h,s,v)
int r,g,b;
float *h,*s,*v;
  {
  float min,max;
  float rc,gc,bc;

  /* determine maximum of R,G,B */
  if ((r >= g) && (r >= b)) max = (float)r;
  if ((g >= r) && (g >= b)) max = (float)g;
  if ((b >= r) && (b >= g)) max = (float)b;

  /* determine minimum of R,G,B */
  if ((r <= g) && (r <= b)) min = (float)r;
  if ((g <= r) && (g <= b)) min = (float)g;
  if ((b <= r) && (b <= g)) min = (float)b;

  /* calculate V (simply maximum of R,G,B) */
  *v = max;

  /* calculate S (if RGB is black, saturation is 0) */
  if (max != 0)
    *s = (max-min) / max;
  else
    *s = 0.0;

  /* calculate H (if no saturation, hue is undefined) */
  if (*s == 0)
    {
    *h = UNDEFINED;
    }
  else
    {
    rc = (max - ((float)r / 255.0)) / (max - min);
    gc = (max - ((float)g / 255.0)) / (max - min);
    bc = (max - ((float)b / 255.0)) / (max - min);

    if (r == (unsigned char)max)
      *h = bc - gc;                 /* hue between yellow and magenta */
    else
      if (g == (unsigned char)max)
        *h = 2.0 + rc - bc;         /* hue between cyan and yellow */
      else
        *h = 4.0 + gc - rc;         /* hue between magenta and cyan */
    *h = *h * 60.0;                 /* convert to degrees */
    if (*h < 0.0) *h = *h + 360.0;  /* make non-negative */
    }

  return;
  }


/***********************************************/
/*       hsv2rgb: HSV-to-RGB conversion        */
/***********************************************/

void hsv2rgb(h,s,v,r,g,b)
float h,s,v;
int *r,*g,*b;
  {
  long int i;
  float hh;
  float f,p,q,t;

  if (s == 0)
    {
    if (h == UNDEFINED)
      {
      *r = (unsigned char)v;  /* achromatic, RGB is grey  scale */
      *g = (unsigned char)v;
      *b = (unsigned char)v;
      }
    else
      {
      *r = 0;                 /* ERROR! this should never occur */
      *g = 0;
      *b = 0;
      }
    }

  else
    {
    hh = h;
    if (hh == 360) hh = 0;
    hh = hh / 60.0;
    i = (long)hh;
    f = hh - (float)i;
    p = v * (1.0 - s);
    q = v * (1.0 - (s * f));
    t = v * (1.0 - (s * (1.0 - f)));

    switch(i)
      {
      case 0:
        *r = (unsigned char) (v * 255.0);
        *g = (unsigned char) (t * 255.0);
        *b = (unsigned char) (p * 255.0);
        break;
      case 1:
        *r = (unsigned char) (q * 255.0);
        *g = (unsigned char) (v * 255.0);
        *b = (unsigned char) (p * 255.0);
        break;
      case 2:
        *r = (unsigned char) (p * 255.0);
        *g = (unsigned char) (v * 255.0);
        *b = (unsigned char) (t * 255.0);
        break;
      case 3:
        *r = (unsigned char) (p * 255.0);
        *g = (unsigned char) (q * 255.0);
        *b = (unsigned char) (v * 255.0);
        break;
      case 4:
        *r = (unsigned char) (t * 255.0);
        *g = (unsigned char) (p * 255.0);
        *b = (unsigned char) (v * 255.0);
        break;
      case 5:
        *r = (unsigned char) (v * 255.0);
        *g = (unsigned char) (p * 255.0);
        *b = (unsigned char) (q * 255.0);
        break;
      default:
        break;
      }
    }

  return;
}