Spectrum SeparableBSSRDF::Sample_Sp(const Scene &scene, Float u1,
const Point2f &u2, MemoryArena &arena,
SurfaceInteraction *pi, Float *pdf) const {
ProfilePhase pp(Prof::BSSRDFEvaluation);
// Choose projection axis for BSSRDF sampling
Vector3f vx, vy, vz;
if (u1 < .5f) {
vx = ss;
vy = ts;
vz = Vector3f(ns);
u1 *= 2;
} else if (u1 < .75f) {
// Prepare for sampling rays with respect to _ss_
vx = ts;
vy = Vector3f(ns);
vz = ss;
u1 = (u1 - .5f) * 4;
} else {
// Prepare for sampling rays with respect to _ts_
vx = Vector3f(ns);
vy = ss;
vz = ts;
u1 = (u1 - .75f) * 4;
}
// Choose spectral channel for BSSRDF sampling
int ch = Clamp((int)(u1 * Spectrum::nSamples), 0, Spectrum::nSamples - 1);
u1 = u1 * Spectrum::nSamples - ch;
// Sample BSSRDF profile in polar coordinates
Float r = Sample_Sr(ch, u2[0]);
if (r < 0) return Spectrum(0.f);
Float phi = 2 * Pi * u2[1];
// Compute BSSRDF profile bounds and intersection height
Float rMax = Sample_Sr(ch, 0.999f);
if (r > rMax) return Spectrum(0.f);
Float l = 2 * std::sqrt(rMax * rMax - r * r);
// Compute BSSRDF sampling ray segment
Interaction base;
base.p =
po.p + r * (vx * std::cos(phi) + vy * std::sin(phi)) - l * vz * 0.5f;
base.time = po.time;
Point3f pTarget = base.p + l * vz;
// Intersect BSSRDF sampling ray against the scene geometry
// Declare _IntersectionChain_ and linked list
struct IntersectionChain {
SurfaceInteraction si;
IntersectionChain *next = nullptr;
};
IntersectionChain *chain = ARENA_ALLOC(arena, IntersectionChain)();
// Accumulate chain of intersections along ray
IntersectionChain *ptr = chain;
int nFound = 0;
while (scene.Intersect(base.SpawnRayTo(pTarget), &ptr->si)) {
base = ptr->si;
// Append admissible intersection to _IntersectionChain_
if (ptr->si.primitive->GetMaterial() == this->material) {
IntersectionChain *next = ARENA_ALLOC(arena, IntersectionChain)();
ptr->next = next;
ptr = next;
nFound++;
}
}
// Randomly choose one of several intersections during BSSRDF sampling
if (nFound == 0) return Spectrum(0.0f);
int selected = Clamp((int)(u1 * nFound), 0, nFound - 1);
while (selected-- > 0) chain = chain->next;
*pi = chain->si;
// Compute sample PDF and return the spatial BSSRDF term $\Sp$
*pdf = this->Pdf_Sp(*pi) / nFound;
return this->Sp(*pi);
}
Spectrum SeparableBSSRDF::Sample_Sp(const Scene &scene, Float sample1,
const Point2f &sample2, MemoryArena &arena,
SurfaceInteraction *pi, Float *pdf) const {
ProfilePhase pp(Prof::BSSRDFEvaluation);
// Choose projection axis for BSSRDF sampling
Vector3f axis;
if (sample1 < .25f) {
axis = ss;
sample1 *= 4;
} else if (sample1 < .5f) {
axis = ts;
sample1 = (sample1 - .25f) * 4;
} else {
axis = Vector3f(ns);
sample1 = (sample1 - .5f) * 2;
}
// Choose spectral channel for BSSRDF sampling
int ch =
Clamp((int)(sample1 * Spectrum::nSamples), 0, Spectrum::nSamples - 1);
sample1 = sample1 * Spectrum::nSamples - ch;
// Sample BSSRDF profile in polar coordinates
Float r = Sample_Sr(ch, sample2.x);
if (r < 0) return Spectrum(0.f);
Float phi = 2 * Pi * sample2.y;
// Compute BSSRDF profile bounds and intersection height
Float rMax = Sample_Sr(ch, 0.999f);
if (r > rMax) return Spectrum(0.f);
Float l = 2 * std::sqrt(rMax * rMax - r * r);
// Compute BSSRDF sampling ray segment
Vector3f v0, v1;
CoordinateSystem(axis, &v0, &v1);
Interaction base;
base.p =
po.p + r * (v0 * std::cos(phi) + v1 * std::sin(phi)) - l * axis * 0.5f;
base.time = po.time;
Point3f target = base.p + l * axis;
// Intersect BSSRDF sampling ray against the scene geometry
struct IntersectionChain {
SurfaceInteraction si;
IntersectionChain *next;
};
IntersectionChain *chain = ARENA_ALLOC(arena, IntersectionChain)(),
*ptr = chain;
int nFound = 0;
while (true) {
if (!scene.Intersect(base.SpawnRayTo(target), &ptr->si)) break;
base = ptr->si;
// Append admissible intersection to _IntersectionChain_
if (ptr->si.primitive->GetMaterial() == material) {
IntersectionChain *next = ARENA_ALLOC(arena, IntersectionChain)();
ptr->next = next;
ptr = next;
nFound++;
}
}
// Randomly choose one of several intersections during BSSRDF sampling
if (nFound == 0) return Spectrum(0.0f);
int selected = Clamp((int)(sample1 * nFound), 0, nFound - 1);
while (selected-- > 0) chain = chain->next;
*pi = chain->si;
// Compute sample PDF and return the spatial BSSRDF term $\Sp$
*pdf = Pdf_Sp(*pi) / nFound;
return Sp(*pi);
}