procedure TInsertCodeForm.DrawColorCircle(const Size, HueLevel, SaturationLevel, ValueLevel : integer;
                                              const BackgroundColor : TColor;
                                              var BMP : TBitmap);
      function RGBtoRGBTriple(const red, green, blue : byte) : TRGBTriple;
        begin
          with Result
          do begin
               rgbtRed := red;
               rgbtGreen := green;
               rgbtBlue := blue
             end;
        end;
      function HSVtoRGBTriple (const H, S, V : integer ) : TRGBTriple;
        const
          divisor : integer = 255*60;
        var
          f,
          hTemp,
          p, q, t,
          VS : integer;
        begin
          if (S = 0)
          then Result := RGBtoRGBTriple(V, V, V) // achromatic: shades of gray
          else begin // chromatic color
                 if (H = 360)
                 then hTemp := 0
                 else hTemp := H;

                 f := hTemp mod 60; // f is IN [0, 59]
                 hTemp := hTemp div 60; // h is now IN [0,6)

                 VS := V*S;
                 p := V - VS div 255; // p = v * (1 - s)
                 q := V - (VS*f) div divisor; // q = v * (1 - s*f)
                 t := V - (VS*(60 - f)) div divisor; // t = v * (1 - s * (1 - f))

                 case hTemp of
                   0: Result := RGBtoRGBTriple(V, t, p);
                   1: Result := RGBtoRGBTriple(q, V, p);
                   2: Result := RGBtoRGBTriple(p, V, t);
                   3: Result := RGBtoRGBTriple(p, q, V);
                   4: Result := RGBtoRGBTriple(t, p, V);
                   5: Result := RGBtoRGBTriple(V, p, q);
                 else Result := RGBtoRGBTriple(0, 0, 0);
                 end;
               end;
        end;

      var
        dSquared,
        H, S, V,
        i, j,
        Radius,
        RadiusSquared,
        X, Y : integer;
        row : pRGBTripleArray;
      begin
        BMP.PixelFormat := pf24bit;
        BMP.Width := Size;
        BMP.Height := Size;

        // Fill with background color
        BMP.Canvas.Brush.Color := BackGroundColor;
        BMP.Canvas.FillRect(bmp.Canvas.ClipRect);

        Radius := size div 2;
        RadiusSquared := Radius*Radius;

        V := ValueLevel;
        for j := 0 to bmp.Height-1
        do begin
             Y := Size - 1 - j - Radius; {Center is Radius offset}
             row := BMP.Scanline[Size - 1 - j];

             for i := 0 to BMP.Width - 1
             do begin
                  X := i - Radius;
                  dSquared := (X * X) + (Y * Y);

                  if dSquared <= RadiusSquared
                  then begin
                         S := Round((255 * Sqrt(dSquared)) / Radius);
                         H := Round(180 * (1 + ArcTan2(X, Y) / PI)); // 0..360 degrees

                         // Shift 90 degrees so H=0 (red) occurs along "X" axis
                         H := H + 90;
                         if (H > 360)
                         then H := H - 360;

                         row[i] := HSVtoRGBTriple(H, S, V);
                       end;
                end;
           end;
      end;