wavelength2rgb

// CIE 1931 color-matching functions (x,y,z)
// in 360..830 nm range, with 5 nm step.
// From http://cvrl.ioo.ucl.ac.uk/index.htm (CIE 1931 2-deg, XYZ CMFs).
static float CIE1931XYZtable[95+1][3]=
{
0.000129900000f,0.000003917000f,0.000606100000f,
0.000232100000f,0.000006965000f,0.001086000000f,
0.000414900000f,0.000012390000f,0.001946000000f,
0.000741600000f,0.000022020000f,0.003486000000f,
0.001368000000f,0.000039000000f,0.006450001000f,
0.002236000000f,0.000064000000f,0.010549990000f,
0.004243000000f,0.000120000000f,0.020050010000f,
0.007650000000f,0.000217000000f,0.036210000000f,
0.014310000000f,0.000396000000f,0.067850010000f,
0.023190000000f,0.000640000000f,0.110200000000f,
0.043510000000f,0.001210000000f,0.207400000000f,
0.077630000000f,0.002180000000f,0.371300000000f,
0.134380000000f,0.004000000000f,0.645600000000f,
0.214770000000f,0.007300000000f,1.039050100000f,
0.283900000000f,0.011600000000f,1.385600000000f,
0.328500000000f,0.016840000000f,1.622960000000f,
0.348280000000f,0.023000000000f,1.747060000000f,
0.348060000000f,0.029800000000f,1.782600000000f,
0.336200000000f,0.038000000000f,1.772110000000f,
0.318700000000f,0.048000000000f,1.744100000000f,
0.290800000000f,0.060000000000f,1.669200000000f,
0.251100000000f,0.073900000000f,1.528100000000f,
0.195360000000f,0.090980000000f,1.287640000000f,
0.142100000000f,0.112600000000f,1.041900000000f,
0.095640000000f,0.139020000000f,0.812950100000f,
0.057950010000f,0.169300000000f,0.616200000000f,
0.032010000000f,0.208020000000f,0.465180000000f,
0.014700000000f,0.258600000000f,0.353300000000f,
0.004900000000f,0.323000000000f,0.272000000000f,
0.002400000000f,0.407300000000f,0.212300000000f,
0.009300000000f,0.503000000000f,0.158200000000f,
0.029100000000f,0.608200000000f,0.111700000000f,
0.063270000000f,0.710000000000f,0.078249990000f,
0.109600000000f,0.793200000000f,0.057250010000f,
0.165500000000f,0.862000000000f,0.042160000000f,
0.225749900000f,0.914850100000f,0.029840000000f,
0.290400000000f,0.954000000000f,0.020300000000f,
0.359700000000f,0.980300000000f,0.013400000000f,
0.433449900000f,0.994950100000f,0.008749999000f,
0.512050100000f,1.000000000000f,0.005749999000f,
0.594500000000f,0.995000000000f,0.003900000000f,
0.678400000000f,0.978600000000f,0.002749999000f,
0.762100000000f,0.952000000000f,0.002100000000f,
0.842500000000f,0.915400000000f,0.001800000000f,
0.916300000000f,0.870000000000f,0.001650001000f,
0.978600000000f,0.816300000000f,0.001400000000f,
1.026300000000f,0.757000000000f,0.001100000000f,
1.056700000000f,0.694900000000f,0.001000000000f,
1.062200000000f,0.631000000000f,0.000800000000f,
1.045600000000f,0.566800000000f,0.000600000000f,
1.002600000000f,0.503000000000f,0.000340000000f,
0.938400000000f,0.441200000000f,0.000240000000f,
0.854449900000f,0.381000000000f,0.000190000000f,
0.751400000000f,0.321000000000f,0.000100000000f,
0.642400000000f,0.265000000000f,0.000049999990f,
0.541900000000f,0.217000000000f,0.000030000000f,
0.447900000000f,0.175000000000f,0.000020000000f,
0.360800000000f,0.138200000000f,0.000010000000f,
0.283500000000f,0.107000000000f,0.000000000000f,
0.218700000000f,0.081600000000f,0.000000000000f,
0.164900000000f,0.061000000000f,0.000000000000f,
0.121200000000f,0.044580000000f,0.000000000000f,
0.087400000000f,0.032000000000f,0.000000000000f,
0.063600000000f,0.023200000000f,0.000000000000f,
0.046770000000f,0.017000000000f,0.000000000000f,
0.032900000000f,0.011920000000f,0.000000000000f,
0.022700000000f,0.008210000000f,0.000000000000f,
0.015840000000f,0.005723000000f,0.000000000000f,
0.011359160000f,0.004102000000f,0.000000000000f,
0.008110916000f,0.002929000000f,0.000000000000f,
0.005790346000f,0.002091000000f,0.000000000000f,
0.004109457000f,0.001484000000f,0.000000000000f,
0.002899327000f,0.001047000000f,0.000000000000f,
0.002049190000f,0.000740000000f,0.000000000000f,
0.001439971000f,0.000520000000f,0.000000000000f,
0.000999949300f,0.000361100000f,0.000000000000f,
0.000690078600f,0.000249200000f,0.000000000000f,
0.000476021300f,0.000171900000f,0.000000000000f,
0.000332301100f,0.000120000000f,0.000000000000f,
0.000234826100f,0.000084800000f,0.000000000000f,
0.000166150500f,0.000060000000f,0.000000000000f,
0.000117413000f,0.000042400000f,0.000000000000f,
0.000083075270f,0.000030000000f,0.000000000000f,
0.000058706520f,0.000021200000f,0.000000000000f,
0.000041509940f,0.000014990000f,0.000000000000f,
0.000029353260f,0.000010600000f,0.000000000000f,
0.000020673830f,0.000007465700f,0.000000000000f,
0.000014559770f,0.000005257800f,0.000000000000f,
0.000010253980f,0.000003702900f,0.000000000000f,
0.000007221456f,0.000002607800f,0.000000000000f,
0.000005085868f,0.000001836600f,0.000000000000f,
0.000003581652f,0.000001293400f,0.000000000000f,
0.000002522525f,0.000000910930f,0.000000000000f,
0.000001776509f,0.000000641530f,0.000000000000f,
0.000001251141f,0.000000451810f,0.000000000000f,

0.000001251141f,0.000000451810f,0.000000000000f // Copy of the previous row, for lerp purposes.
};

// Converts wavelength (in nanometers) to
// RGB intensities (in linear sRGB space).
// Resulting R, G, B values can be negative.
// Clamping or blending with gray background may be necessary.
// Range of output values is:
//   R: -0.93 +2.52
//   G: -0.22 +1.51
//   B: -0.17 +1.90
void wavelength2rgb(float lambda,float rgb[3])
{
    // Wavelength value clamping.
    if(!(lambda>360.0f)) lambda=360.0f; // Also catches NaNs.
    if(lambda>830.0f) lambda=830.0f;

    // Interpolation.
    float t=(lambda-360.0f)/5.0f;
    int id=(int)t;
    t-=(float)id;
    float s=1.0f-t;

    // CIE 1931 color space tristimulus values, with D65  standard illuminant.
    float X=CIE1931XYZtable[id][0]*s+CIE1931XYZtable[id+1][0]*t;
    float Y=CIE1931XYZtable[id][1]*s+CIE1931XYZtable[id+1][1]*t;
    float Z=CIE1931XYZtable[id][2]*s+CIE1931XYZtable[id+1][2]*t;

    // Linear RGB values (in sRGB colorspace).
    float R=+3.2406f*X-1.5372f*Y-0.4986f*Z;
    float G=-0.9689f*X+1.8758f*Y+0.0415f*Z;
    float B=+0.0557f*X-0.2040f*Y+1.0570f*Z;

    // These are in linear colorspace.
    // You may want to use linear2srgb conversion.
    // You may use conversion function:
//float linear2srgb(float x)
//{
//    if(x<=0.0031308f) return 12.92f*x;
//    else return 1.055f*powf(x,1.0f/2.4f)-0.055f;
//}
    // Be careful of negative R, G, B values.

    rgb[0]=R;
    rgb[1]=G;
    rgb[2]=B;
}