node_fresnel.h

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float fresnel_dielectric(vector Incoming, normal Normal, float eta)
{
    /* compute fresnel reflectance without explicitly computing
       the refracted direction */
    float c = fabs(dot(Incoming, Normal));
    float g = eta * eta - 1 + c * c;
    float result;

    if(g > 0) {
        g = sqrt(g);
        float A =(g - c)/(g + c);
        float B =(c *(g + c)- 1)/(c *(g - c)+ 1);
        result = 0.5 * A * A *(1 + B * B);
    }
    else
        result = 1.0;  /* TIR (no refracted component) */

    return result;
}