Spectrum FresnelSpecular::Sample_f(const Vector3f &wo, Vector3f *wi,
const Point2f &u, Float *pdf,
BxDFType *sampledType) const {
Float F = FrDielectric(CosTheta(wo), etaA, etaB);
if (u[0] < F) {
// Compute specular reflection for _FresnelSpecular_
// Compute perfect specular reflection direction
*wi = Vector3f(-wo.x, -wo.y, wo.z);
if (sampledType)
*sampledType = BxDFType(BSDF_SPECULAR | BSDF_REFLECTION);
*pdf = F;
return F * R / AbsCosTheta(*wi);
} else {
// Compute specular transmission for _FresnelSpecular_
// Figure out which $\eta$ is incident and which is transmitted
bool entering = CosTheta(wo) > 0;
Float etaI = entering ? etaA : etaB;
Float etaT = entering ? etaB : etaA;
// Compute ray direction for specular transmission
if (!Refract(wo, Faceforward(Normal3f(0, 0, 1), wo), etaI / etaT, wi))
return 0;
Spectrum ft = T * (1 - F);
// Account for non-symmetry with transmission to different medium
if (mode == TransportMode::Radiance)
ft *= (etaI * etaI) / (etaT * etaT);
if (sampledType)
*sampledType = BxDFType(BSDF_SPECULAR | BSDF_TRANSMISSION);
*pdf = 1 - F;
return ft / AbsCosTheta(*wi);
}
}
Float BeamDiffusionSS(Float sigma_s, Float sigma_a, Float g, Float eta,
Float r) {
// Compute material parameters and minimum $t$ below the critical angle
Float sigma_t = sigma_a + sigma_s, rho = sigma_s / sigma_t;
Float tCrit = r * std::sqrt(eta * eta - 1);
Float Ess = 0;
const int nSamples = 100;
for (int i = 0; i < nSamples; ++i) {
// Evaluate single scattering integrand and add to _Ess_
Float ti = tCrit - std::log(1 - (i + .5f) / nSamples) / sigma_t;
// Determine length $d$ of connecting segment and $\cos\theta_\roman{o}$
Float d = std::sqrt(r * r + ti * ti);
Float cosThetaO = ti / d;
// Add contribution of single scattering at depth $t$
Ess += rho * std::exp(-sigma_t * (d + tCrit)) / (d * d) *
PhaseHG(cosThetaO, g) * (1 - FrDielectric(-cosThetaO, 1, eta)) *
std::abs(cosThetaO);
}
return Ess / nSamples;
}
Spectrum FresnelDielectric::Evaluate(Float cosThetaI) const {
return FrDielectric(cosThetaI, etaI, etaT);
}
