Spectrum TabulatedBSSRDF::Sr(Float r) const {
Spectrum Sr(0.f);
for (int ch = 0; ch < Spectrum::nSamples; ++ch) {
// Convert $r$ into unitless optical radius $r_{\roman{optical}}$
Float rOptical = r * sigma_t[ch];
// Compute spline weights to interpolate BSSRDF on channel _ch_
int rhoOffset, radiusOffset;
Float rhoWeights[4], radiusWeights[4];
if (!CatmullRomWeights(table.nRhoSamples, table.rhoSamples.get(),
rho[ch], &rhoOffset, rhoWeights) ||
!CatmullRomWeights(table.nRadiusSamples, table.radiusSamples.get(),
rOptical, &radiusOffset, radiusWeights))
continue;
// Set BSSRDF value _Sr[ch]_ using tensor spline interpolation
Float sr = 0;
for (int i = 0; i < 4; ++i) {
for (int j = 0; j < 4; ++j) {
Float weight = rhoWeights[i] * radiusWeights[j];
if (weight != 0)
sr += weight *
table.EvalProfile(rhoOffset + i, radiusOffset + j);
}
}
// Cancel marginal PDF factor from tabulated BSSRDF profile
if (rOptical != 0) sr /= 2 * Pi * rOptical;
Sr[ch] = sr;
}
// Transform BSSRDF value into world space units
Sr *= sigma_t * sigma_t;
return Sr.Clamp();
}
Float TabulatedBSSRDF::Pdf_Sr(int ch, Float r) const {
// Convert $r$ into unitless optical radius $r_{\roman{optical}}$
Float rOptical = r * sigma_t[ch];
// Compute spline weights to interpolate BSSRDF density on channel _ch_
int rhoOffset, radiusOffset;
Float rhoWeights[4], radiusWeights[4];
if (!CatmullRomWeights(table.nRhoSamples, table.rhoSamples.get(), rho[ch],
&rhoOffset, rhoWeights) ||
!CatmullRomWeights(table.nRadiusSamples, table.radiusSamples.get(),
rOptical, &radiusOffset, radiusWeights))
return 0.f;
// Return BSSRDF profile density for channel _ch_
Float sr = 0, rhoEff = 0;
for (int i = 0; i < 4; ++i) {
if (rhoWeights[i] == 0) continue;
rhoEff += table.rhoEff[rhoOffset + i] * rhoWeights[i];
for (int j = 0; j < 4; ++j) {
if (radiusWeights[j] == 0) continue;
sr += table.EvalProfile(rhoOffset + i, radiusOffset + j) *
rhoWeights[i] * radiusWeights[j];
}
}
// Cancel marginal PDF factor from tabulated BSSRDF profile
if (rOptical != 0) sr /= 2 * Pi * rOptical;
return std::max((Float)0, sr * sigma_t[ch] * sigma_t[ch] / rhoEff);
}
bool FourierBSDFTable::GetWeightsAndOffset(Float cosTheta, int *offset,
Float weights[4]) const {
return CatmullRomWeights(nMu, mu, cosTheta, offset, weights);
}
