// Delphi version
// Opracował Witold J.Janik; WJJ@CAD.PL
// ***
// Funkcja FnTecza umożliwia narysowanie
// tęczy przy zmianie [i] od [iMin] do [iMax]
// podziękowania dla Andrzeja Wąsika z [pl.comp.lang.delphi]


function FnTecza(iMin, iMax, i: Integer): TColor;
var 
  m: Double;
  r, g, b, mt: Byte;
begin
  m := (i - iMin)/(iMax - iMin + 1) * 6;
  mt := (round(frac(m)*$FF));
  case Trunc(m) of
  0: begin
      R := $FF;
      G := mt;
      B := 0;
    end;
  1: begin
      R := $FF - mt;
      G := $FF;
      B := 0;
    end;
  2: begin
      R := 0;
      G := $FF;
      B := mt;
    end;
  3: begin
      R := 0;
      G := $FF - mt;
      B := $FF;
    end;
  4: begin
      R := mt;
      G := 0;
      B := $FF;
    end;
  5: begin
      R := $ff;
      G := 0;
      B := $FF - mt;
    end;
end; // case

  Result := rgb(R,G,B);
end; 
/////


=================================

// C++ version
// here are some my modification but the main code is the same 
// as in Witold J.Janik code
//

Uint32 GiveRainbowColor(double position)

// this function gives 1D linear RGB color gradient 
// color is proportional to position 
// position  <0;1> 
// position means position of color in color gradient

{
  if (position>1)position=position-int(position);
  // if position > 1 then we have repetition of colors
  // it maybe useful
  Uint8 R, G, B;// byte
  int nmax=6;// number of color bars
  double m=nmax* position;
  int n=int(m); // integer of m
  double f=m-n;  // fraction of m
  Uint8 t=int(f*255);
  
  
switch( n){
   case 0: {
      R = 255;
      G = t;
      B = 0;
       break;
    };
   case 1: {
      R = 255 - t;
      G = 255;
      B = 0;
       break;
    };
   case 2: {
      R = 0;
      G = 255;
      B = t;
       break;
    };
   case 3: {
      R = 0;
      G = 255 - t;
      B = 255;
       break;
    };
   case 4: {
      R = t;
      G = 0;
      B = 255;
       break;
    };
   case 5: {
      R = 255;
      G = 0;
      B = 255 - t;
       break;
    };

}; // case


  return (R << 16) | (G << 8) | B;
}
//-------------------------


/* ============ C version ========================= */

void GiveRainbowColor(double position,unsigned char c[])
{
  /* if position > 1 then we have repetition of colors
   it maybe useful    */
      
  if (position>1.0){if (position-(int)position==0.0)position=1.0; else position=position-(int)position;}
  
  
 
  
  unsigned char nmax=6; /* number of color bars */
  double m=nmax* position;
  
  int n=(int)m; // integer of m
  
  double f=m-n;  // fraction of m
  unsigned char t=(int)(f*255);
  

  
switch( n){
   case 0: {
      c[0] = 255;
      c[1] = t;
      c[2] = 0;
       break;
    };
   case 1: {
      c[0] = 255 - t;
      c[1] = 255;
      c[2] = 0;
       break;
    };
   case 2: {
      c[0] = 0;
      c[1] = 255;
      c[2] = t;
       break;
    };
   case 3: {
      c[0] = 0;
      c[1] = 255 - t;
      c[2] = 255;
       break;
    };
   case 4: {
      c[0] = t;
      c[1] = 0;
      c[2] = 255;
       break;
    };
   case 5: {
      c[0] = 255;
      c[1] = 0;
      c[2] = 255 - t;
       break;
    };
    default: {
      c[0] = 255;
      c[1] = 0;
      c[2] = 0;
       break;
    };

}; // case


}


/* =====================================*/


















// fraktal.republika.pl
// Adam Majewski 2008.03