C++ Sampler::Get2D方法代码示例

// Integrator Utility Functions
Spectrum UniformSampleAllLights(const Interaction &it, const Scene &scene,
                                MemoryArena &arena, Sampler &sampler,
                                const std::vector<int> &nLightSamples,
                                bool handleMedia) {
    ProfilePhase p(Prof::DirectLighting);
    Spectrum L(0.f);
    for (size_t j = 0; j < scene.lights.size(); ++j) {
        // Accumulate contribution of _j_th light to _L_
        const std::shared_ptr<Light> &light = scene.lights[j];
        int nSamples = nLightSamples[j];
        const Point2f *uLightArray = sampler.Get2DArray(nSamples);
        const Point2f *uScatteringArray = sampler.Get2DArray(nSamples);
        if (!uLightArray || !uScatteringArray) {
            // Use a single sample for illumination from _light_
            Point2f uLight = sampler.Get2D();
            Point2f uScattering = sampler.Get2D();
            L += EstimateDirect(it, uScattering, *light, uLight, scene, sampler,
                                arena, handleMedia);
        } else {
            // Estimate direct lighting using sample arrays
            Spectrum Ld(0.f);
            for (int k = 0; k < nSamples; ++k)
                Ld += EstimateDirect(it, uScatteringArray[k], *light,
                                     uLightArray[k], scene, sampler, arena,
                                     handleMedia);
            L += Ld / nSamples;
        }
    }
    return L;
}

Spectrum UniformSampleOneLight(const Interaction &it, const Scene &scene,
                               MemoryArena &arena, Sampler &sampler,
                               bool handleMedia) {
    ProfilePhase p(Prof::DirectLighting);
    // Randomly choose a single light to sample, _light_
    int nLights = int(scene.lights.size());
    if (nLights == 0) return Spectrum(0.f);
    int lightNum = std::min((int)(sampler.Get1D() * nLights), nLights - 1);
    const std::shared_ptr<Light> &light = scene.lights[lightNum];
    Point2f uLight = sampler.Get2D();
    Point2f uScattering = sampler.Get2D();
    return (Float)nLights * EstimateDirect(it, uScattering, *light, uLight,
                                           scene, sampler, arena, handleMedia);
}

int GenerateLightSubpath(
    const Scene &scene, Sampler &sampler, MemoryArena &arena, int maxDepth,
    Float time, const Distribution1D &lightDistr,
    const std::unordered_map<const Light *, size_t> &lightToIndex,
    Vertex *path) {
    if (maxDepth == 0) return 0;
    // Sample initial ray for light subpath
    Float lightPdf;
    int lightNum = lightDistr.SampleDiscrete(sampler.Get1D(), &lightPdf);
    const std::shared_ptr<Light> &light = scene.lights[lightNum];
    RayDifferential ray;
    Normal3f nLight;
    Float pdfPos, pdfDir;
    Spectrum Le = light->Sample_Le(sampler.Get2D(), sampler.Get2D(), time, &ray,
                                   &nLight, &pdfPos, &pdfDir);
    if (pdfPos == 0 || pdfDir == 0 || Le.IsBlack()) return 0;

    // Generate first vertex on light subpath and start random walk
    path[0] =
        Vertex::CreateLight(light.get(), ray, nLight, Le, pdfPos * lightPdf);
    Spectrum beta = Le * AbsDot(nLight, ray.d) / (lightPdf * pdfPos * pdfDir);
    VLOG(2) << "Starting light subpath. Ray: " << ray << ", Le " << Le <<
        ", beta " << beta << ", pdfPos " << pdfPos << ", pdfDir " << pdfDir;
    int nVertices =
        RandomWalk(scene, ray, sampler, arena, beta, pdfDir, maxDepth - 1,
                   TransportMode::Importance, path + 1);

    // Correct subpath sampling densities for infinite area lights
    if (path[0].IsInfiniteLight()) {
        // Set spatial density of _path[1]_ for infinite area light
        if (nVertices > 0) {
            path[1].pdfFwd = pdfPos;
            if (path[1].IsOnSurface())
                path[1].pdfFwd *= AbsDot(ray.d, path[1].ng());
        }

        // Set spatial density of _path[0]_ for infinite area light
        path[0].pdfFwd =
            InfiniteLightDensity(scene, lightDistr, lightToIndex, ray.d);
    }
    return nVertices + 1;
}

int GenerateLightSubpath(const Scene &scene, Sampler &sampler,
                         MemoryArena &arena, int maxdepth, Float time,
                         const Distribution1D &lightDistr, Vertex *path) {
    if (maxdepth == 0) return 0;
    // Sample initial ray for light subpath
    Float lightPdf;
    int lightNum = lightDistr.SampleDiscrete(sampler.Get1D(), &lightPdf);
    const std::shared_ptr<Light> &light = scene.lights[lightNum];
    RayDifferential ray;
    Normal3f Nl;
    Float pdfPos, pdfDir;
    Spectrum Le = light->Sample_L(sampler.Get2D(), sampler.Get2D(), time, &ray,
                                  &Nl, &pdfPos, &pdfDir);
    if (pdfPos == 0.f || pdfDir == 0.f || Le.IsBlack()) return 0;

    // Generate first vertex on light subpath and start random walk
    Spectrum weight = Le * AbsDot(Nl, ray.d) / (lightPdf * pdfPos * pdfDir);
    path[0] = Vertex(VertexType::Light,
                     EndpointInteraction(light.get(), ray, Nl), Le);
    path[0].pdfFwd = pdfPos * lightPdf;
    int nvertices =
        RandomWalk(scene, ray, sampler, arena, weight, pdfDir, maxdepth - 1,
                   TransportMode::Importance, path + 1);

    // Correct sampling densities for infinite area lights
    if (path[0].IsInfiniteLight()) {
        // Set positional density of _path[1]_
        if (nvertices > 0) {
            path[1].pdfFwd = pdfPos;
            if (path[1].IsOnSurface())
                path[1].pdfFwd *= AbsDot(ray.d, path[1].GetGeoNormal());
        }

        // Set positional density of _path[0]_
        path[0].pdfFwd = InfiniteLightDensity(scene, lightDistr, ray.d);
    }
    return nvertices + 1;
}

Spectrum SamplerIntegrator::SpecularTransmit(
    const RayDifferential &ray, const SurfaceInteraction &isect,
    const Scene &scene, Sampler &sampler, MemoryArena &arena, int depth) const {
    Vector3f wo = isect.wo, wi;
    Float pdf;
    const Point3f &p = isect.p;
    const Normal3f &ns = isect.shading.n;
    const BSDF &bsdf = *isect.bsdf;
    Spectrum f = bsdf.Sample_f(wo, &wi, sampler.Get2D(), &pdf,
                               BxDFType(BSDF_TRANSMISSION | BSDF_SPECULAR));
    Spectrum L = Spectrum(0.f);
    if (pdf > 0.f && !f.IsBlack() && AbsDot(wi, ns) != 0.f) {
        // Compute ray differential _rd_ for specular transmission
        RayDifferential rd = isect.SpawnRay(wi);
        if (ray.hasDifferentials) {
            rd.hasDifferentials = true;
            rd.rxOrigin = p + isect.dpdx;
            rd.ryOrigin = p + isect.dpdy;

            Float eta = bsdf.eta;
            Vector3f w = -wo;
            if (Dot(wo, ns) < 0) eta = 1.f / eta;

            Normal3f dndx = isect.shading.dndu * isect.dudx +
                            isect.shading.dndv * isect.dvdx;
            Normal3f dndy = isect.shading.dndu * isect.dudy +
                            isect.shading.dndv * isect.dvdy;

            Vector3f dwodx = -ray.rxDirection - wo,
                     dwody = -ray.ryDirection - wo;
            Float dDNdx = Dot(dwodx, ns) + Dot(wo, dndx);
            Float dDNdy = Dot(dwody, ns) + Dot(wo, dndy);

            Float mu = eta * Dot(w, ns) - Dot(wi, ns);
            Float dmudx =
                (eta - (eta * eta * Dot(w, ns)) / Dot(wi, ns)) * dDNdx;
            Float dmudy =
                (eta - (eta * eta * Dot(w, ns)) / Dot(wi, ns)) * dDNdy;

            rd.rxDirection =
                wi + eta * dwodx - Vector3f(mu * dndx + dmudx * ns);
            rd.ryDirection =
                wi + eta * dwody - Vector3f(mu * dndy + dmudy * ns);
        }
        L = f * Li(rd, scene, sampler, arena, depth + 1) * AbsDot(wi, ns) / pdf;
    }
    return L;
}

// WhittedIntegrator Method Definitions
Spectrum WhittedIntegrator::Li(const RayDifferential &ray, const Scene &scene,
                               Sampler &sampler, MemoryArena &arena,
                               int depth) const {
    Spectrum L(0.);
    // Find closest ray intersection or return background radiance
    SurfaceInteraction isect;
    if (!scene.Intersect(ray, &isect)) {
        for (const auto &light : scene.lights) L += light->Le(ray);
        return L;
    }

    // Compute emitted and reflected light at ray intersection point

    // Initialize common variables for Whitted integrator
    const Normal3f &n = isect.shading.n;
    Vector3f wo = isect.wo;

    // Compute scattering functions for surface interaction
    isect.ComputeScatteringFunctions(ray, arena);
    if (!isect.bsdf)
        return Li(isect.SpawnRay(ray.d), scene, sampler, arena, depth);

    // Compute emitted light if ray hit an area light source
    L += isect.Le(wo);

    // Add contribution of each light source
    for (const auto &light : scene.lights) {
        Vector3f wi;
        Float pdf;
        VisibilityTester visibility;
        Spectrum Li =
            light->Sample_Li(isect, sampler.Get2D(), &wi, &pdf, &visibility);
        if (Li.IsBlack() || pdf == 0) continue;
        Spectrum f = isect.bsdf->f(wo, wi);
        if (!f.IsBlack() && visibility.Unoccluded(scene))
            L += f * Li * AbsDot(wi, n) / pdf;
    }
    if (depth + 1 < maxDepth) {
        // Trace rays for specular reflection and refraction
        L += SpecularReflect(ray, isect, scene, sampler, arena, depth);
        L += SpecularTransmit(ray, isect, scene, sampler, arena, depth);
    }
    return L;
}

int GenerateCameraSubpath(const Scene &scene, Sampler &sampler,
                          MemoryArena &arena, int maxDepth,
                          const Camera &camera, Point2f &pFilm, Vertex *path) {
    if (maxDepth == 0) return 0;
    // Sample initial ray for camera subpath
    CameraSample cameraSample;
    cameraSample.pFilm = pFilm;
    cameraSample.time = sampler.Get1D();
    cameraSample.pLens = sampler.Get2D();
    RayDifferential ray;
    Spectrum beta = camera.GenerateRayDifferential(cameraSample, &ray);
    ray.ScaleDifferentials(1 / std::sqrt(sampler.samplesPerPixel));

    // Generate first vertex on camera subpath and start random walk
    Float pdfPos, pdfDir;
    path[0] = Vertex::CreateCamera(&camera, ray, beta);
    camera.Pdf_We(ray, &pdfPos, &pdfDir);
    return RandomWalk(scene, ray, sampler, arena, beta, pdfDir, maxDepth - 1,
                      TransportMode::Radiance, path + 1) +
           1;
}

int GenerateCameraSubpath(const Scene &scene, Sampler &sampler,
                          MemoryArena &arena, int maxdepth,
                          const Camera &camera, Point2f &rasterPos,
                          Vertex *path) {
    if (maxdepth == 0) return 0;
    // Sample initial ray for camera subpath
    CameraSample cameraSample;
    cameraSample.pFilm = rasterPos;
    cameraSample.time = sampler.Get1D();
    cameraSample.pLens = sampler.Get2D();
    RayDifferential ray;
    Spectrum rayWeight(camera.GenerateRayDifferential(cameraSample, &ray));
    ray.ScaleDifferentials(1.f / std::sqrt(sampler.samplesPerPixel));

    // Generate first vertex on camera subpath and start random walk
    path[0] = Vertex(VertexType::Camera, EndpointInteraction(&camera, ray),
                     Spectrum(1.0f));
    return RandomWalk(scene, ray, sampler, arena, rayWeight,
                      camera.Pdf(path[0].ei, ray.d), maxdepth - 1,
                      TransportMode::Radiance, path + 1) +
           1;
}

Spectrum SamplerIntegrator::SpecularReflect(
    const RayDifferential &ray, const SurfaceInteraction &isect,
    const Scene &scene, Sampler &sampler, MemoryArena &arena, int depth) const {
    // Compute specular reflection direction _wi_ and BSDF value
    Vector3f wo = isect.wo, wi;
    Float pdf;
    BxDFType type = BxDFType(BSDF_REFLECTION | BSDF_SPECULAR);
    Spectrum f = isect.bsdf->Sample_f(wo, &wi, sampler.Get2D(), &pdf, type);

    // Return contribution of specular reflection
    const Normal3f &ns = isect.shading.n;
    if (pdf > 0.f && !f.IsBlack() && AbsDot(wi, ns) != 0.f) {
        // Compute ray differential _rd_ for specular reflection
        RayDifferential rd = isect.SpawnRay(wi);
        if (ray.hasDifferentials) {
            rd.hasDifferentials = true;
            rd.rxOrigin = isect.p + isect.dpdx;
            rd.ryOrigin = isect.p + isect.dpdy;
            // Compute differential reflected directions
            Normal3f dndx = isect.shading.dndu * isect.dudx +
                            isect.shading.dndv * isect.dvdx;
            Normal3f dndy = isect.shading.dndu * isect.dudy +
                            isect.shading.dndv * isect.dvdy;
            Vector3f dwodx = -ray.rxDirection - wo,
                     dwody = -ray.ryDirection - wo;
            Float dDNdx = Dot(dwodx, ns) + Dot(wo, dndx);
            Float dDNdy = Dot(dwody, ns) + Dot(wo, dndy);
            rd.rxDirection =
                wi - dwodx + 2.f * Vector3f(Dot(wo, ns) * dndx + dDNdx * ns);
            rd.ryDirection =
                wi - dwody + 2.f * Vector3f(Dot(wo, ns) * dndy + dDNdy * ns);
        }
        return f * Li(rd, scene, sampler, arena, depth + 1) * AbsDot(wi, ns) /
               pdf;
    } else
        return Spectrum(0.f);
}

// VolPathIntegrator Method Definitions
Spectrum VolPathIntegrator::Li(const RayDifferential &r, const Scene &scene,
                               Sampler &sampler, MemoryArena &arena,
                               int depth) const {
    ProfilePhase p(Prof::SamplerIntegratorLi);
    Spectrum L(0.f), alpha(1.f);
    RayDifferential ray(r);
    bool specularBounce = false;
    for (int bounces = 0;; ++bounces) {
        // Store intersection into _isect_
        SurfaceInteraction isect;
        bool foundIntersection = scene.Intersect(ray, &isect);

        // Sample the participating medium, if present
        MediumInteraction mi;
        if (ray.medium) alpha *= ray.medium->Sample(ray, sampler, arena, &mi);
        if (alpha.IsBlack()) break;

        // Handle an interaction with a medium or a surface
        if (mi.IsValid()) {
            // Handle medium scattering case
            Vector3f wo = -ray.d, wi;
            L += alpha * UniformSampleOneLight(mi, scene, sampler, arena, true);
            Point2f phaseSample = sampler.Get2D();
            mi.phase->Sample_p(wo, &wi, phaseSample);
            ray = mi.SpawnRay(wi);
        } else {
            // Handle surface scattering case

            // Possibly add emitted light and terminate
            if (bounces == 0 || specularBounce) {
                // Add emitted light at path vertex or from the environment
                if (foundIntersection)
                    L += alpha * isect.Le(-ray.d);
                else
                    for (const auto &light : scene.lights)
                        L += alpha * light->Le(ray);
            }
            if (!foundIntersection || bounces >= maxDepth) break;

            // Compute scattering functions and skip over medium boundaries
            isect.ComputeScatteringFunctions(ray, arena, true);
            if (!isect.bsdf) {
                ray = isect.SpawnRay(ray.d);
                bounces--;
                continue;
            }

            // Sample illumination from lights to find attenuated path
            // contribution
            L += alpha *
                 UniformSampleOneLight(isect, scene, sampler, arena, true);

            // Sample BSDF to get new path direction
            Vector3f wo = -ray.d, wi;
            Float pdf;
            BxDFType flags;
            Spectrum f = isect.bsdf->Sample_f(wo, &wi, sampler.Get2D(), &pdf,
                                              BSDF_ALL, &flags);
            if (f.IsBlack() || pdf == 0.f) break;
            alpha *= f * AbsDot(wi, isect.shading.n) / pdf;
            Assert(std::isinf(alpha.y()) == false);
            specularBounce = (flags & BSDF_SPECULAR) != 0;
            ray = isect.SpawnRay(wi);

            // Account for attenuated subsurface scattering, if applicable
            if (isect.bssrdf && (flags & BSDF_TRANSMISSION)) {
                // Importance sample the BSSRDF
                SurfaceInteraction pi;
                Spectrum S = isect.bssrdf->Sample_S(
                    scene, sampler.Get1D(), sampler.Get2D(), arena, &pi, &pdf);
#ifndef NDEBUG
                Assert(std::isinf(alpha.y()) == false);
#endif
                if (S.IsBlack() || pdf == 0) break;
                alpha *= S / pdf;

                // Account for the attenuated direct subsurface scattering
                // component
                L += alpha *
                     UniformSampleOneLight(pi, scene, sampler, arena, true);

                // Account for the indirect subsurface scattering component
                Spectrum f = pi.bsdf->Sample_f(pi.wo, &wi, sampler.Get2D(),
                                               &pdf, BSDF_ALL, &flags);
                if (f.IsBlack() || pdf == 0.f) break;
                alpha *= f * AbsDot(wi, pi.shading.n) / pdf;
#ifndef NDEBUG
                Assert(std::isinf(alpha.y()) == false);
#endif
                specularBounce = (flags & BSDF_SPECULAR) != 0;
                ray = pi.SpawnRay(wi);
            }
        }

        // Possibly terminate the path
        if (bounces > 3) {
            Float continueProbability = std::min((Float).5, alpha.y());
            if (sampler.Get1D() > continueProbability) break;
            alpha /= continueProbability;
//.........这里部分代码省略.........

// PathIntegrator Method Definitions
Spectrum PathIntegrator::Li(const RayDifferential &r, const Scene &scene,
                            Sampler &sampler, MemoryArena &arena) const {
    Spectrum L(0.f);
    // Declare common path integration variables
    RayDifferential ray(r);
    Spectrum pathThroughput = Spectrum(1.f);
    bool specularBounce = false;
    for (int bounces = 0;; ++bounces) {
        // Store intersection into _isect_
        SurfaceInteraction isect;
        bool foundIntersection = scene.Intersect(ray, &isect);

        // Possibly add emitted light and terminate
        if (bounces == 0 || specularBounce) {
            // Add emitted light at path vertex or from the environment
            if (foundIntersection)
                L += pathThroughput * isect.Le(-ray.d);
            else
                for (const auto &light : scene.lights)
                    L += pathThroughput * light->Le(ray);
        }
        if (!foundIntersection || bounces >= maxDepth) break;

        // Compute scattering functions and skip over medium boundaries
        isect.ComputeScatteringFunctions(ray, arena, true);
        if (!isect.bsdf) {
            ray = isect.SpawnRay(ray.d);
            bounces--;
            continue;
        }

        // Sample illumination from lights to find path contribution
        L += pathThroughput *
             UniformSampleOneLight(isect, scene, sampler, arena);

        // Sample BSDF to get new path direction
        Vector3f wo = -ray.d, wi;
        Float pdf;
        BxDFType flags;
        Spectrum f = isect.bsdf->Sample_f(wo, &wi, sampler.Get2D(), &pdf,
                                          BSDF_ALL, &flags);

        if (f.IsBlack() || pdf == 0.f) break;
        pathThroughput *= f * AbsDot(wi, isect.shading.n) / pdf;
#ifndef NDEBUG
        Assert(std::isinf(pathThroughput.y()) == false);
#endif
        specularBounce = (flags & BSDF_SPECULAR) != 0;
        ray = isect.SpawnRay(wi);

        // Account for subsurface scattering, if applicable
        if (isect.bssrdf && (flags & BSDF_TRANSMISSION)) {
            // Importance sample the BSSRDF
            BSSRDFSample bssrdfSample;
            bssrdfSample.uDiscrete = sampler.Get1D();
            bssrdfSample.pos = sampler.Get2D();
            SurfaceInteraction isect_out = isect;
            pathThroughput *= isect.bssrdf->Sample_f(
                isect_out, scene, ray.time, bssrdfSample, arena, &isect, &pdf);
#ifndef NDEBUG
            Assert(std::isinf(pathThroughput.y()) == false);
#endif
            if (pathThroughput.IsBlack()) break;

            // Account for the direct subsurface scattering component
            isect.wo = Vector3f(isect.shading.n);

            // Sample illumination from lights to find path contribution
            L += pathThroughput *
                 UniformSampleOneLight(isect, scene, sampler, arena);

            // Account for the indirect subsurface scattering component
            Spectrum f = isect.bsdf->Sample_f(isect.wo, &wi, sampler.Get2D(),
                                              &pdf, BSDF_ALL, &flags);
            if (f.IsBlack() || pdf == 0.f) break;
            pathThroughput *= f * AbsDot(wi, isect.shading.n) / pdf;
#ifndef NDEBUG
            Assert(std::isinf(pathThroughput.y()) == false);
#endif
            specularBounce = (flags & BSDF_SPECULAR) != 0;
            ray = isect.SpawnRay(wi);
        }

        // Possibly terminate the path
        if (bounces > 3) {
            Float continueProbability = std::min((Float).5, pathThroughput.y());
            if (sampler.Get1D() > continueProbability) break;
            pathThroughput /= continueProbability;
            Assert(std::isinf(pathThroughput.y()) == false);
        }
    }
    return L;
}

Spectrum VolPathIntegrator::Li(const RayDifferential &r, const Scene &scene,
                               Sampler &sampler, MemoryArena &arena,
                               int depth) const {
    ProfilePhase p(Prof::SamplerIntegratorLi);
    Spectrum L(0.f), beta(1.f);
    RayDifferential ray(r);
    bool specularBounce = false;
    int bounces;
    // Added after book publication: etaScale tracks the accumulated effect
    // of radiance scaling due to rays passing through refractive
    // boundaries (see the derivation on p. 527 of the third edition). We
    // track this value in order to remove it from beta when we apply
    // Russian roulette; this is worthwhile, since it lets us sometimes
    // avoid terminating refracted rays that are about to be refracted back
    // out of a medium and thus have their beta value increased.
    Float etaScale = 1;

    for (bounces = 0;; ++bounces) {
        // Intersect _ray_ with scene and store intersection in _isect_
        SurfaceInteraction isect;
        bool foundIntersection = scene.Intersect(ray, &isect);

        // Sample the participating medium, if present
        MediumInteraction mi;
        if (ray.medium) beta *= ray.medium->Sample(ray, sampler, arena, &mi);
        if (beta.IsBlack()) break;

        // Handle an interaction with a medium or a surface
        if (mi.IsValid()) {
            // Terminate path if ray escaped or _maxDepth_ was reached
            if (bounces >= maxDepth) break;

            ++volumeInteractions;
            // Handle scattering at point in medium for volumetric path tracer
            const Distribution1D *lightDistrib =
                lightDistribution->Lookup(mi.p);
            L += beta * UniformSampleOneLight(mi, scene, arena, sampler, true,
                                              lightDistrib);

            Vector3f wo = -ray.d, wi;
            mi.phase->Sample_p(wo, &wi, sampler.Get2D());
            ray = mi.SpawnRay(wi);
        } else {
            ++surfaceInteractions;
            // Handle scattering at point on surface for volumetric path tracer

            // Possibly add emitted light at intersection
            if (bounces == 0 || specularBounce) {
                // Add emitted light at path vertex or from the environment
                if (foundIntersection)
                    L += beta * isect.Le(-ray.d);
                else
                    for (const auto &light : scene.infiniteLights)
                        L += beta * light->Le(ray);
            }

            // Terminate path if ray escaped or _maxDepth_ was reached
            if (!foundIntersection || bounces >= maxDepth) break;

            // Compute scattering functions and skip over medium boundaries
            isect.ComputeScatteringFunctions(ray, arena, true);
            if (!isect.bsdf) {
                ray = isect.SpawnRay(ray.d);
                bounces--;
                continue;
            }

            // Sample illumination from lights to find attenuated path
            // contribution
            const Distribution1D *lightDistrib =
                lightDistribution->Lookup(isect.p);
            L += beta * UniformSampleOneLight(isect, scene, arena, sampler,
                                              true, lightDistrib);

            // Sample BSDF to get new path direction
            Vector3f wo = -ray.d, wi;
            Float pdf;
            BxDFType flags;
            Spectrum f = isect.bsdf->Sample_f(wo, &wi, sampler.Get2D(), &pdf,
                                              BSDF_ALL, &flags);
            if (f.IsBlack() || pdf == 0.f) break;
            beta *= f * AbsDot(wi, isect.shading.n) / pdf;
            DCHECK(std::isinf(beta.y()) == false);
            specularBounce = (flags & BSDF_SPECULAR) != 0;
            if ((flags & BSDF_SPECULAR) && (flags & BSDF_TRANSMISSION)) {
                Float eta = isect.bsdf->eta;
                // Update the term that tracks radiance scaling for refraction
                // depending on whether the ray is entering or leaving the
                // medium.
                etaScale *=
                    (Dot(wo, isect.n) > 0) ? (eta * eta) : 1 / (eta * eta);
            }
            ray = isect.SpawnRay(ray, wi, flags, isect.bsdf->eta);

            // Account for attenuated subsurface scattering, if applicable
            if (isect.bssrdf && (flags & BSDF_TRANSMISSION)) {
                // Importance sample the BSSRDF
                SurfaceInteraction pi;
                Spectrum S = isect.bssrdf->Sample_S(
                    scene, sampler.Get1D(), sampler.Get2D(), arena, &pi, &pdf);
//.........这里部分代码省略.........

Spectrum ConnectBDPT(const Scene &scene, Vertex *lightSubpath,
                     Vertex *cameraSubpath, int s, int t,
                     const Distribution1D &lightDistr, const Camera &camera,
                     Sampler &sampler, Point2f *rasterPos, Float *misWeight) {
    Spectrum weight(0.f);
    // Ignore invalid connections related to infinite area lights
    if (t > 1 && s != 0 && cameraSubpath[t - 1].type == VertexType::Light)
        return Spectrum(0.f);

    // Perform connection and write contribution to _weight_
    Vertex sampled;
    if (s == 0) {
        // Interpret the camera subpath as a complete path
        const Vertex &pt = cameraSubpath[t - 1];
        if (pt.IsLight()) {
            const Vertex &ptMinus = cameraSubpath[t - 2];
            weight = pt.Le(scene, ptMinus) * pt.weight;
        }
    } else if (t == 1) {
        // Sample a point on the camera and connect it to the light subpath
        const Vertex &qs = lightSubpath[s - 1];
        if (qs.IsConnectible()) {
            VisibilityTester vis;
            Vector3f wi;
            Float pdf;
            Spectrum cameraWeight =
                camera.Sample_We(qs.GetInteraction(), sampler.Get2D(), &wi,
                                 &pdf, rasterPos, &vis);
            if (pdf > 0 && !cameraWeight.IsBlack()) {
                // Initialize dynamically sampled vertex and _weight_
                sampled = Vertex(VertexType::Camera,
                                 EndpointInteraction(vis.P1(), &camera),
                                 cameraWeight);
                weight = qs.weight * qs.f(sampled) * vis.T(scene, sampler) *
                         sampled.weight;
                if (qs.IsOnSurface())
                    weight *= AbsDot(wi, qs.GetShadingNormal());
            }
        }
    } else if (s == 1) {
        // Sample a point on a light and connect it to the camera subpath
        const Vertex &pt = cameraSubpath[t - 1];
        if (pt.IsConnectible()) {
            Float lightPdf;
            VisibilityTester vis;
            Vector3f wi;
            Float pdf;
            int lightNum =
                lightDistr.SampleDiscrete(sampler.Get1D(), &lightPdf);
            const std::shared_ptr<Light> &light = scene.lights[lightNum];
            Spectrum lightWeight = light->Sample_L(
                pt.GetInteraction(), sampler.Get2D(), &wi, &pdf, &vis);
            if (pdf > 0 && !lightWeight.IsBlack()) {
                sampled = Vertex(VertexType::Light,
                                 EndpointInteraction(vis.P1(), light.get()),
                                 lightWeight / (pdf * lightPdf));
                sampled.pdfFwd = sampled.PdfLightOrigin(scene, pt, lightDistr);
                weight = pt.weight * pt.f(sampled) * vis.T(scene, sampler) *
                         sampled.weight;
                if (pt.IsOnSurface())
                    weight *= AbsDot(wi, pt.GetShadingNormal());
            }
        }
    } else {
        // Handle all other cases
        const Vertex &qs = lightSubpath[s - 1], &pt = cameraSubpath[t - 1];
        if (qs.IsConnectible() && pt.IsConnectible()) {
            weight = qs.weight * qs.f(pt) *
                     GeometryTerm(scene, sampler, qs, pt) * pt.f(qs) *
                     pt.weight;
        }
    }

    // Compute MIS weight for connection strategy
    *misWeight =
        weight.IsBlack() ? 0.f : MISWeight(scene, lightSubpath, cameraSubpath,
                                           sampled, s, t, lightDistr);
    return weight;
}

Spectrum ConnectBDPT(
    const Scene &scene, Vertex *lightVertices, Vertex *cameraVertices, int s,
    int t, const Distribution1D &lightDistr,
    const std::unordered_map<const Light *, size_t> &lightToIndex,
    const Camera &camera, Sampler &sampler, Point2f *pRaster,
    Float *misWeightPtr) {
    Spectrum L(0.f);
    // Ignore invalid connections related to infinite area lights
    if (t > 1 && s != 0 && cameraVertices[t - 1].type == VertexType::Light)
        return Spectrum(0.f);

    // Perform connection and write contribution to _L_
    Vertex sampled;
    if (s == 0) {
        // Interpret the camera subpath as a complete path
        const Vertex &pt = cameraVertices[t - 1];
        if (pt.IsLight()) L = pt.Le(scene, cameraVertices[t - 2]) * pt.beta;
        DCHECK(!L.HasNaNs());
    } else if (t == 1) {
        // Sample a point on the camera and connect it to the light subpath
        const Vertex &qs = lightVertices[s - 1];
        if (qs.IsConnectible()) {
            VisibilityTester vis;
            Vector3f wi;
            Float pdf;
            Spectrum Wi = camera.Sample_Wi(qs.GetInteraction(), sampler.Get2D(),
                                           &wi, &pdf, pRaster, &vis);
            if (pdf > 0 && !Wi.IsBlack()) {
                // Initialize dynamically sampled vertex and _L_ for $t=1$ case
                sampled = Vertex::CreateCamera(&camera, vis.P1(), Wi / pdf);
                L = qs.beta * qs.f(sampled, TransportMode::Importance) * sampled.beta;
                if (qs.IsOnSurface()) L *= AbsDot(wi, qs.ns());
                DCHECK(!L.HasNaNs());
                // Only check visibility after we know that the path would
                // make a non-zero contribution.
                if (!L.IsBlack()) L *= vis.Tr(scene, sampler);
            }
        }
    } else if (s == 1) {
        // Sample a point on a light and connect it to the camera subpath
        const Vertex &pt = cameraVertices[t - 1];
        if (pt.IsConnectible()) {
            Float lightPdf;
            VisibilityTester vis;
            Vector3f wi;
            Float pdf;
            int lightNum =
                lightDistr.SampleDiscrete(sampler.Get1D(), &lightPdf);
            const std::shared_ptr<Light> &light = scene.lights[lightNum];
            Spectrum lightWeight = light->Sample_Li(
                pt.GetInteraction(), sampler.Get2D(), &wi, &pdf, &vis);
            if (pdf > 0 && !lightWeight.IsBlack()) {
                EndpointInteraction ei(vis.P1(), light.get());
                sampled =
                    Vertex::CreateLight(ei, lightWeight / (pdf * lightPdf), 0);
                sampled.pdfFwd =
                    sampled.PdfLightOrigin(scene, pt, lightDistr, lightToIndex);
                L = pt.beta * pt.f(sampled, TransportMode::Radiance) * sampled.beta;
                if (pt.IsOnSurface()) L *= AbsDot(wi, pt.ns());
                // Only check visibility if the path would carry radiance.
                if (!L.IsBlack()) L *= vis.Tr(scene, sampler);
            }
        }
    } else {
        // Handle all other bidirectional connection cases
        const Vertex &qs = lightVertices[s - 1], &pt = cameraVertices[t - 1];
        if (qs.IsConnectible() && pt.IsConnectible()) {
            L = qs.beta * qs.f(pt, TransportMode::Importance) * pt.f(qs, TransportMode::Radiance) * pt.beta;
            VLOG(2) << "General connect s: " << s << ", t: " << t <<
                " qs: " << qs << ", pt: " << pt << ", qs.f(pt): " << qs.f(pt, TransportMode::Importance) <<
                ", pt.f(qs): " << pt.f(qs, TransportMode::Radiance) << ", G: " << G(scene, sampler, qs, pt) <<
                ", dist^2: " << DistanceSquared(qs.p(), pt.p());
            if (!L.IsBlack()) L *= G(scene, sampler, qs, pt);
        }
    }

    ++totalPaths;
    if (L.IsBlack()) ++zeroRadiancePaths;
    ReportValue(pathLength, s + t - 2);

    // Compute MIS weight for connection strategy
    Float misWeight =
        L.IsBlack() ? 0.f : MISWeight(scene, lightVertices, cameraVertices,
                                      sampled, s, t, lightDistr, lightToIndex);
    VLOG(2) << "MIS weight for (s,t) = (" << s << ", " << t << ") connection: "
            << misWeight;
    DCHECK(!std::isnan(misWeight));
    L *= misWeight;
    if (misWeightPtr) *misWeightPtr = misWeight;
    return L;
}

int RandomWalk(const Scene &scene, RayDifferential ray, Sampler &sampler,
               MemoryArena &arena, Spectrum beta, Float pdf, int maxDepth,
               TransportMode mode, Vertex *path) {
    if (maxDepth == 0) return 0;
    int bounces = 0;
    // Declare variables for forward and reverse probability densities
    Float pdfFwd = pdf, pdfRev = 0;
    while (true) {
        // Attempt to create the next subpath vertex in _path_
        MediumInteraction mi;

        VLOG(2) << "Random walk. Bounces " << bounces << ", beta " << beta <<
            ", pdfFwd " << pdfFwd << ", pdfRev " << pdfRev;
        // Trace a ray and sample the medium, if any
        SurfaceInteraction isect;
        bool foundIntersection = scene.Intersect(ray, &isect);
        if (ray.medium) beta *= ray.medium->Sample(ray, sampler, arena, &mi);
        if (beta.IsBlack()) break;
        Vertex &vertex = path[bounces], &prev = path[bounces - 1];
        if (mi.IsValid()) {
            // Record medium interaction in _path_ and compute forward density
            vertex = Vertex::CreateMedium(mi, beta, pdfFwd, prev);
            if (++bounces >= maxDepth) break;

            // Sample direction and compute reverse density at preceding vertex
            Vector3f wi;
            pdfFwd = pdfRev = mi.phase->Sample_p(-ray.d, &wi, sampler.Get2D());
            ray = mi.SpawnRay(wi);
        } else {
            // Handle surface interaction for path generation
            if (!foundIntersection) {
                // Capture escaped rays when tracing from the camera
                if (mode == TransportMode::Radiance) {
                    vertex = Vertex::CreateLight(EndpointInteraction(ray), beta,
                                                 pdfFwd);
                    ++bounces;
                }
                break;
            }

            // Compute scattering functions for _mode_ and skip over medium
            // boundaries
            isect.ComputeScatteringFunctions(ray, arena, true, mode);
            if (!isect.bsdf) {
                ray = isect.SpawnRay(ray.d);
                continue;
            }

            // Initialize _vertex_ with surface intersection information
            vertex = Vertex::CreateSurface(isect, beta, pdfFwd, prev);
            if (++bounces >= maxDepth) break;

            // Sample BSDF at current vertex and compute reverse probability
            Vector3f wi, wo = isect.wo;
            BxDFType type;
            Spectrum f = isect.bsdf->Sample_f(wo, &wi, sampler.Get2D(), &pdfFwd,
                                              BSDF_ALL, &type);
            VLOG(2) << "Random walk sampled dir " << wi << " f: " << f <<
                ", pdfFwd: " << pdfFwd;
            if (f.IsBlack() || pdfFwd == 0.f) break;
            beta *= f * AbsDot(wi, isect.shading.n) / pdfFwd;
            VLOG(2) << "Random walk beta now " << beta;
            pdfRev = isect.bsdf->Pdf(wi, wo, BSDF_ALL);
            if (type & BSDF_SPECULAR) {
                vertex.delta = true;
                pdfRev = pdfFwd = 0;
            }
            beta *= CorrectShadingNormal(isect, wo, wi, mode);
            VLOG(2) << "Random walk beta after shading normal correction " << beta;
            ray = isect.SpawnRay(wi);
        }

        // Compute reverse area density at preceding vertex
        prev.pdfRev = vertex.ConvertDensity(pdfRev, prev);
    }
    return bounces;
}