Float TabulatedBSSRDF::Sample_Sr(int ch, Float u) const {
if (sigma_t[ch] == 0) return -1;
return SampleCatmullRom2D(table.nRhoSamples, table.nRadiusSamples,
table.rhoSamples.get(), table.radiusSamples.get(),
table.profile.get(), table.profileCDF.get(),
rho[ch], u) /
sigma_t[ch];
}
Spectrum FourierBSDF::Sample_f(const Vector3f &wo, Vector3f *wi,
const Point2f &u, Float *pdf,
BxDFType *sampledType) const {
// Sample zenith angle component for _FourierBSDF_
Float muO = CosTheta(wo);
Float pdfMu;
Float muI = SampleCatmullRom2D(bsdfTable.nMu, bsdfTable.nMu, bsdfTable.mu,
bsdfTable.mu, bsdfTable.avg, bsdfTable.cdf,
muO, u[1], nullptr, &pdfMu);
// Compute Fourier coefficients $a_k$ for $(\mui, \muo)$
// Determine offsets and weights for $\mui$ and $\muo$
int offsetI, offsetO;
Float weightsI[4], weightsO[4];
if (!bsdfTable.GetWeightsAndOffset(muI, &offsetI, weightsI) ||
!bsdfTable.GetWeightsAndOffset(muO, &offsetO, weightsO))
return Spectrum(0.f);
// Allocate storage to accumulate _ak_ coefficients
Float *ak = ALLOCA(Float, bsdfTable.mMax * bsdfTable.nChannels);
memset(ak, 0, bsdfTable.mMax * bsdfTable.nChannels * sizeof(Float));
// Accumulate weighted sums of nearby $a_k$ coefficients
int mMax = 0;
for (int b = 0; b < 4; ++b) {
for (int a = 0; a < 4; ++a) {
// Add contribution of _(a, b)_ to $a_k$ values
Float weight = weightsI[a] * weightsO[b];
if (weight != 0) {
int m;
const Float *ap = bsdfTable.GetAk(offsetI + a, offsetO + b, &m);
mMax = std::max(mMax, m);
for (int c = 0; c < bsdfTable.nChannels; ++c)
for (int k = 0; k < m; ++k)
ak[c * bsdfTable.mMax + k] += weight * ap[c * m + k];
}
}
}
// Importance sample the luminance Fourier expansion
Float phi, pdfPhi;
Float Y = SampleFourier(ak, bsdfTable.recip, mMax, u[0], &pdfPhi, &phi);
*pdf = std::max((Float)0, pdfPhi * pdfMu);
// Compute the scattered direction for _FourierBSDF_
Float sin2ThetaI = 1 - muI * muI;
Float norm = std::sqrt(sin2ThetaI / Sin2Theta(wo));
Float sinPhi = std::sin(phi), cosPhi = std::cos(phi);
*wi = -Vector3f(norm * (cosPhi * wo.x - sinPhi * wo.y),
norm * (sinPhi * wo.x + cosPhi * wo.y), muI);
// Evaluate remaining Fourier expansions for angle $\phi$
Float scale = muI != 0 ? (1 / std::abs(muI)) : (Float)0;
if (mode == TransportMode::Radiance && muI * muO > 0) {
float eta = muI > 0 ? 1 / bsdfTable.eta : bsdfTable.eta;
scale *= eta * eta;
}
if (bsdfTable.nChannels == 1) return Spectrum(Y * scale);
Float R = Fourier(ak + 1 * bsdfTable.mMax, mMax, cosPhi);
Float B = Fourier(ak + 2 * bsdfTable.mMax, mMax, cosPhi);
Float G = 1.39829f * Y - 0.100913f * B - 0.297375f * R;
Float rgb[3] = {R * scale, G * scale, B * scale};
return Spectrum::FromRGB(rgb).Clamp();
}
