added MicrofacetDistribution::computeTransmissionProbability

src/bsdfs/microfacet.h

@@ -421,19 +421,21 @@ public:
 	}
 
 	/**
-	 * \brief Precompute the aggregate Fresnel transmittance through
-	 * a rough interface
+	 * \brief Compute a spline representation for the overall Fresnel
+	 * transmittance through a rough interface
 	 *
-	 * This function efficiently computes the integral of 
-	 *      \int_{S^2} D(m) * (1 - Fr(m<->wi)) dm
+	 * This function essentially computes the integral of 
+	 *      1 - \int_{S^2} f(w_i, w_o) *  dw_o
 	 * for incident directions 'wi' with a range of different inclinations
-	 * (where Fr denotes the fresnel reflectance). It returns a cubic spline 
-	 * interpolation parameterized by the cosine of the angle between 'wi' 
-	 * and the (macro-) surface normal.
+	 * (where f denotes a Cook-Torrance style reflectance model). It returns 
+	 * a cubic spline interpolation parameterized by the cosine of the angle
+	 * between 'wi' and the (macro-) surface normal.
+	 *
+	 * \remark This only works for isotropic microfacet distributions
 	 */
-	CubicSpline *getRoughTransmittance(Float extIOR, Float intIOR, Float alpha, size_t resolution) const {
+	CubicSpline *computeRoughTransmittance(Float extIOR, Float intIOR, Float alpha, size_t resolution) const {
 		if (isAnisotropic())
-			SLog(EError, "MicrofacetDistribution::getRoughTransmission(): only "
+			SLog(EError, "MicrofacetDistribution::computeRoughTransmission(): only "
 				"supports isotropic distributions!");
 
 		NDIntegrator integrator(1, 2, 5000, 0, 1e-5f);
@@ -449,7 +451,7 @@ public:
 				  integral = 0, error = 0;
 
 			integrator.integrateVectorized(
-				boost::bind(&MicrofacetDistribution::integrand, this,
+				boost::bind(&MicrofacetDistribution::integrand1, this,
 					wi, extIOR, intIOR, alpha, _1, _2, _3),
 				min, max, &integral, &error, &nEvals
 			);
@@ -465,6 +467,52 @@ public:
 		return spline;
 	}
 
+	/**
+	 * \brief Compute a spline representation that gives the probability
+	 * of sampling a transmission event e.g. in the plugin 'roughdielectric'.
+	 *
+	 * Like \ref computeRoughTransmittance, the spline is parameterized by the
+	 * cosine of the angle between the indident direction and the (macro-) 
+	 * surface normal.
+	 * 
+	 * \remark This function only works for isotropic microfacet distributions
+	 */
+	CubicSpline *computeTransmissionProbability(Float extIOR, Float intIOR, Float alpha, size_t resolution) const {
+		if (isAnisotropic())
+			SLog(EError, "MicrofacetDistribution::computeTransmissionProbability(): only "
+				"supports isotropic distributions!");
+
+		NDIntegrator integrator(1, 2, 5000, 0, 1e-5f);
+		CubicSpline *spline = new CubicSpline(resolution);
+		size_t nEvals, nEvalsTotal = 0;
+		ref<Timer> timer = new Timer();
+
+		Float stepSize = (1.0f-2*Epsilon)/(resolution-1);
+		for (size_t i=0; i<resolution; ++i) {
+			Float z = stepSize * i + Epsilon;
+			Vector wi(std::sqrt(std::max((Float) 0, 1-z*z)), 0, z);
+			Float min[2] = {0, 0}, max[2] = {1, 1},
+				  integral = 0, error = 0;
+
+			integrator.integrateVectorized(
+				boost::bind(&MicrofacetDistribution::integrand2, this,
+					wi, extIOR, intIOR, alpha, _1, _2, _3),
+				min, max, &integral, &error, &nEvals
+			);
+
+			spline->append(z, integral);
+
+			nEvalsTotal += nEvals;
+		}
+		SLog(EInfo, "Created a " SIZE_T_FMT "-node cubic spline approximation to the "
+				"transmission probability (integration took %i ms and " SIZE_T_FMT 
+				" function evaluations)", resolution, timer->getMilliseconds(), 
+				nEvalsTotal);
+
+		spline->build();
+		return spline;
+	}
+
 	std::string toString() const {
 		switch (m_type) {
 			case EBeckmann: return "beckmann"; break;
@@ -477,13 +525,13 @@ public:
 		}
 	}
 protected:
-	/// Integrand helper function called by \ref getRoughTransmission
-	void integrand(const Vector &wi, Float extIOR, Float intIOR, Float alpha,
+	/// Integrand helper function called by \ref computeRoughTransmission
+	void integrand1(const Vector &wi, Float extIOR, Float intIOR, Float alpha,
 			size_t nPts, const Float *in, Float *out) const {
 		for (int i=0; i<(int) nPts; ++i) {
 			Normal m = sample(Point2(in[2*i], in[2*i+1]), alpha);
 			Vector wo = 2 * dot(wi, m) * Vector(m) - wi;
-			if (Frame::cosTheta(wi) <= 0 || Frame::cosTheta(wo) <= 0) {
+			if (Frame::cosTheta(wo) <= 0) {
 				out[i] = 0;
 				continue;
 			}
@@ -495,6 +543,22 @@ protected:
 		}
 	}
 
+	/// Integrand helper function called by \ref computeTransmissionProbability
+	void integrand2(const Vector &wi, Float extIOR, Float intIOR, Float alpha,
+			size_t nPts, const Float *in, Float *out) const {
+		for (int i=0; i<(int) nPts; ++i) {
+			Normal m = sample(Point2(in[2*i], in[2*i+1]), alpha);
+			Vector wo = 2 * dot(wi, m) * Vector(m) - wi;
+			if (Frame::cosTheta(wo) <= 0) {
+				out[i] = 0;
+				continue;
+			}
+
+			/* Calculate the specular reflection component */
+			out[i] = 1 - fresnel(dot(wi, m), extIOR, intIOR);
+		}
+	}
+
 protected:
 	EType m_type;
 };

src/bsdfs/roughplastic.cpp

@@ -130,7 +130,7 @@ public:
 
 		if (m_roughTransmittance == NULL) {
 			Float alpha = m_distribution.transformRoughness(m_alpha->getValue(Intersection()).average());
-			m_roughTransmittance = m_distribution.getRoughTransmittance(m_extIOR, m_intIOR, alpha, 200);
+			m_roughTransmittance = m_distribution.computeRoughTransmittance(m_extIOR, m_intIOR, alpha, 200);
 		}
 
 		BSDF::configure();