Converting Colors Between RGB and HLS (HBS)

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

The code fragment below converts colors between RGB (Red, Green, Blue) and HLS/HBS (Hue, Lightness, Saturation/Hue, Brightness, Saturation).

MORE INFORMATION

/* Color Conversion Routines --

   1. RGBtoHLS (initialization)
   2. The scroll bar handlers

There are potential round-off errors throughout this sample. ((0.5 + x)/y) without floating point is phrased ((x + (y/2))/y), yielding a very small round-off error. This makes many of the following divisions look strange.

*/

#define  HLSMAX   RANGE /* H,L, and S vary over 0-HLSMAX */
#define  RGBMAX   255   /* R,G, and B vary over 0-RGBMAX */
                        /* HLSMAX BEST IF DIVISIBLE BY 6 */
                        /* RGBMAX, HLSMAX must each fit in a byte. */

/* Hue is undefined if Saturation is 0 (grey-scale) */
/* This value determines where the Hue scrollbar is */
/* initially set for achromatic colors */
#define UNDEFINED (HLSMAX*2/3)

void  RGBtoHLS(lRGBColor)

   WORD R,G,B;          /* input RGB values */
   BYTE cMax,cMin;      /* max and min RGB values */
   WORD  Rdelta,Gdelta,Bdelta; /* intermediate value: % of spread from max
*/

   /* get R, G, and B out of DWORD */
   R = GetRValue(lRGBColor);
   G = GetGValue(lRGBColor);
   B = GetBValue(lRGBColor);

   /* calculate lightness */
   cMax = max( max(R,G), B);
   cMin = min( min(R,G), B);
   L = ( ((cMax+cMin)*HLSMAX) + RGBMAX )/(2*RGBMAX);

   if (cMax == cMin) {           /* r=g=b --> achromatic case */
      S = 0;                     /* saturation */
      H = UNDEFINED;             /* hue */
   }
   else {                        /* chromatic case */
      /* saturation */
      if (L <= (HLSMAX/2))
         S = ( ((cMax-cMin)*HLSMAX) + ((cMax+cMin)/2) ) / (cMax+cMin);
      else
         S = ( ((cMax-cMin)*HLSMAX) + ((2*RGBMAX-cMax-cMin)/2) )
            / (2*RGBMAX-cMax-cMin);

      /* hue */
      Rdelta = ( ((cMax-R)*(HLSMAX/6)) + ((cMax-cMin)/2) ) / (cMax-cMin);
      Gdelta = ( ((cMax-G)*(HLSMAX/6)) + ((cMax-cMin)/2) ) / (cMax-cMin);
      Bdelta = ( ((cMax-B)*(HLSMAX/6)) + ((cMax-cMin)/2) ) / (cMax-cMin);

      if (R == cMax)
         H = Bdelta - Gdelta;
      else if (G == cMax)
         H = (HLSMAX/3) + Rdelta - Bdelta;
      else /* B == cMax */
         H = ((2*HLSMAX)/3) + Gdelta - Rdelta;

      if (H < 0)
         H += HLSMAX;
      if (H > HLSMAX)
         H -= HLSMAX;
   }
}

/* utility routine for HLStoRGB */

   /* range check: note values passed add/subtract thirds of range */
   if (hue < 0)
      hue += HLSMAX;

   if (hue > HLSMAX)
      hue -= HLSMAX;

   /* return r,g, or b value from this tridrant */
   if (hue < (HLSMAX/6))
      return ( n1 + (((n2-n1)*hue+(HLSMAX/12))/(HLSMAX/6)) );
   if (hue < (HLSMAX/2))
      return ( n2 );
   if (hue < ((HLSMAX*2)/3))
      return ( n1 + (((n2-n1)*(((HLSMAX*2)/3)-hue)+(HLSMAX/12))/(HLSMAX/6))

   else
      return ( n1 );

DWORD HLStoRGB(hue,lum,sat) WORD hue; WORD lum; WORD sat; {

   WORD R,G,B;                /* RGB component values */
   WORD  Magic1,Magic2;       /* calculated magic numbers (really!) */

   if (sat == 0) {            /* achromatic case */
      R=G=B=(lum*RGBMAX)/HLSMAX;
      if (hue != UNDEFINED) {
         /* ERROR */
      }
   }
   else  {                    /* chromatic case */
      /* set up magic numbers */
      if (lum <= (HLSMAX/2))
         Magic2 = (lum*(HLSMAX + sat) + (HLSMAX/2))/HLSMAX;
      else
         Magic2 = lum + sat - ((lum*sat) + (HLSMAX/2))/HLSMAX;
      Magic1 = 2*lum-Magic2;

      /* get RGB, change units from HLSMAX to RGBMAX */
      R = (HueToRGB(Magic1,Magic2,hue+(HLSMAX/3))*RGBMAX +

      G = (HueToRGB(Magic1,Magic2,hue)*RGBMAX + (HLSMAX/2)) / HLSMAX;
      B = (HueToRGB(Magic1,Magic2,hue-(HLSMAX/3))*RGBMAX +

   }

   return(RGB(R,G,B));