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using System;
using Colourful.Implementation;
namespace Colourful
{
/// <summary>
/// Can compute chromaticity from CCT (Correlated color temperature) and also
/// compute CCT of given chromaticity.
/// </summary>
public static class CCTConverter
{
/// <summary>
/// Returns chromaticity coordinates of given CCT (specified in K)
/// </summary>
public static xyChromaticityCoordinates GetChromaticityOfCCT(double temperature)
{
// approximation described here: http://en.wikipedia.org/wiki/Planckian_locus#Approximation
double x_c;
if (temperature <= 4000) // correctly 1667 <= T <= 4000
x_c = -0.2661239 * (1000000000 / MathUtils.Pow3(temperature)) - 0.2343580 * (1000000 / MathUtils.Pow2(temperature)) + 0.8776956 * (1000 / temperature) + 0.179910;
else // correctly 4000 <= T <= 25000
x_c = -3.0258469 * (1000000000 / MathUtils.Pow3(temperature)) + 2.1070379 * (1000000 / MathUtils.Pow2(temperature)) + 0.2226347 * (1000 / temperature) + 0.240390;
double y_c;
if (temperature <= 2222) // correctly 1667 <= T <= 2222
y_c = -1.1063814 * MathUtils.Pow3(x_c) - 1.34811020 * MathUtils.Pow2(x_c) + 2.18555832 * x_c - 0.20219683;
else if (temperature <= 4000) // correctly 2222 <= T <= 4000
y_c = -0.9549476 * MathUtils.Pow3(x_c) - 1.37418593 * MathUtils.Pow2(x_c) + 2.09137015 * x_c - 0.16748867;
else // correctly 4000 <= T <= 25000
y_c = +3.0817580 * MathUtils.Pow3(x_c) - 5.87338670 * MathUtils.Pow2(x_c) + 3.75112997 * x_c - 0.37001483;
return new xyChromaticityCoordinates(x_c, y_c);
}
/// <summary>
/// Returns CCT (specified in K) of given chromaticity coordinates
/// </summary>
/// <remarks>Ranges usually from around 0 to 25000</remarks>
public static double GetCCTOfChromaticity(in xyChromaticityCoordinates chromaticity)
{
// approximation described here: http://en.wikipedia.org/wiki/Color_temperature#Approximation
const double xe = 0.3366;
const double ye = 0.1735;
const double A0 = -949.86315;
const double A1 = 6253.80338;
const double t1 = 0.92159;
const double A2 = 28.70599;
const double t2 = 0.20039;
const double A3 = 0.00004;
const double t3 = 0.07125;
var n = (chromaticity.x - xe) / (chromaticity.y - ye);
var cct = A0 + A1 * Math.Exp(-n / t1) + A2 * Math.Exp(-n / t2) + A3 * Math.Exp(-n / t3);
return cct;
}
}
}
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