Convert frequency to RGB

[Jim Hurley]

>
>Message: 14
>Date: Mon, 11 Aug 2003 12:20:03 -0600
>Subject: Re: Convert frequency to RGB
>From: Dar Scott <dsc at swcp.com>
>To: use-revolution at lists.runrev.com
>Reply-To: use-revolution at lists.runrev.com
>
>
>On Sunday, August 10, 2003, at 10:03 AM, Jim Hurley wrote:
>
>>  I am trying to create the effect of gradually running through all 
>>  colors of the visible spectrum.
>
>Maybe you can scan in a picture you like and then use the imageData as 
>your table.
>
>There are several factors:  Sun color, eye response tristimulus values 
>or chramaticity coordinates (see CIE), phosphor colors and the effect
>of faking colors outside what the phosphors can do.  I don't know much 
>about phosphors, but I can provide some info on the sun and on CIE data.
>
>For your effect, you might want to see if a simplification works.
>
>You should be fine with straight-line approximations between RGB 
>colors.  Do this by making a straight-line interpolation of each 
>component.  Create a table from wavelength (or lightspeed number or 
>whatever) to color.  Maybe three arrays will work.
>
>If you need help in getting even rough data for your table, let me know.
>
>Dar

Dar,

I think I have found what I need. I had hoped I could find an 
algebraic formula for the translation from wavelength to rgb values. 
I did a Google search and found the following program; I will see if 
I can translate it into Transcript.

Jim

PROCEDURE WavelengthToRGB(CONST Wavelength:  Nanometers;
                           VAR R,G,B:  BYTE);
   CONST
     Gamma        =   0.80;
     IntensityMax = 255;
   VAR
     Blue   :  DOUBLE;
     factor :  DOUBLE;
     Green  :  DOUBLE;
     Red    :  DOUBLE;
   FUNCTION Adjust(CONST Color, Factor:  DOUBLE):  INTEGER;
   BEGIN
     IF   Color = 0.0
     THEN RESULT := 0     // Don't want 0^x = 1 for x <> 0
     ELSE RESULT := ROUND(IntensityMax * Power(Color * Factor, Gamma))
   END {Adjust};
BEGIN
   CASE TRUNC(Wavelength) OF
     380..439:
       BEGIN
         Red   := -(Wavelength - 440) / (440 - 380);
         Green := 0.0;
         Blue  := 1.0
       END;
     440..489:
       BEGIN
         Red   := 0.0;
         Green := (Wavelength - 440) / (490 - 440);
         Blue  := 1.0
       END;
     490..509:
       BEGIN
         Red   := 0.0;
         Green := 1.0;
         Blue  := -(Wavelength - 510) / (510 - 490)
       END;
     510..579:
       BEGIN
         Red   := (Wavelength - 510) / (580 - 510);
         Green := 1.0;
         Blue  := 0.0
       END;
     580..644:
       BEGIN
         Red   := 1.0;
         Green := -(Wavelength - 645) / (645 - 580);
         Blue  := 0.0
       END;
     645..780:
       BEGIN
         Red   := 1.0;
         Green := 0.0;
         Blue  := 0.0
       END;
     ELSE
       Red   := 0.0;
       Green := 0.0;
       Blue  := 0.0
   END;
   // Let the intensity fall off near the vision limits
   CASE TRUNC(Wavelength) OF
     380..419:  factor := 0.3 + 0.7*(Wavelength - 380) / (420 - 380);
     420..700:  factor := 1.0;
     701..780:  factor := 0.3 + 0.7*(780 - Wavelength) / (780 - 700)
     ELSE       factor := 0.0
   END;

   R := Adjust(Red,   Factor);
   G := Adjust(Green, Factor);
   B := Adjust(Blue,  Factor)
END {WavelengthToRGB};                               
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