How To Converting Colors Between RGB and HLS (HBS) (29240)

MORE INFORMATION

RGBtoHLS() takes a DWORD RGB value, translates it to HLS, and stores the results in the global vars H, L, and S. HLStoRGB takes the current values of H, L, and S and returns the equivalent value in an RGB DWORD. The vars H, L, and S are only written to by:

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

A point of reference for the algorithms is Foley and Van Dam, "Fundamentals of Interactive Computer Graphics," Pages 618-19. Their algorithm is in floating point. CHART implements a less general (hardwired ranges) integral algorithm.

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)

   DWORD 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 */ 
   WORD HueToRGB(n1,n2,hue)
   WORD n1;
   WORD n2;
   WORD hue;
   {
      /* 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 +
   (HLSMAX/2))/HLSMAX;
         G = (HueToRGB(Magic1,Magic2,hue)*RGBMAX + (HLSMAX/2)) / HLSMAX;
         B = (HueToRGB(Magic1,Magic2,hue-(HLSMAX/3))*RGBMAX +
   (HLSMAX/2))/HLSMAX;
      }
      return(RGB(R,G,B));
    }