// https://seblagarde.wordpress.com/2013/04/29/memo-on-fresnel-equations/
public static RgbColor Evaluate(float cosThetaI, RgbColor etai, RgbColor etat, RgbColor k)
{
cosThetaI = Math.Clamp(cosThetaI, -1, 1);
RgbColor eta = etat / etai;
RgbColor etak = k / etai;
float cosThetaI2 = cosThetaI * cosThetaI;
float sinThetaI2 = 1 - cosThetaI2;
RgbColor eta2 = eta * eta;
RgbColor etak2 = etak * etak;
RgbColor t0 = eta2 - etak2 - sinThetaI2;
RgbColor a2plusb2 = RgbColor.Sqrt(t0 * t0 + 4 * eta2 * etak2);
RgbColor t1 = a2plusb2 + cosThetaI2;
RgbColor a = RgbColor.Sqrt(0.5f * (a2plusb2 + t0));
RgbColor t2 = 2 * cosThetaI * a;
RgbColor Rs = (t1 - t2) / (t1 + t2);
RgbColor t3 = cosThetaI2 * a2plusb2 + sinThetaI2 * sinThetaI2;
RgbColor t4 = t2 * sinThetaI2;
RgbColor Rp = Rs * (t3 - t4) / (t3 + t4);
return(0.5f * (Rp + Rs));
}