vMF distribution class improvements, python bindings for it

include/mitsuba/core/vmf.h

@@ -28,7 +28,9 @@ MTS_NAMESPACE_BEGIN
  * \brief Von Mises-Fisher distribution on the 2-sphere
  *
  * This is a basic implementation, which assumes that the
- * distribution is centered around the Z-axis.
+ * distribution is centered around the Z-axis. All provided
+ * functions are implemented in such a way that they avoid
+ * issues with numerical overflow.
  *
  * \author Wenzel Jakob
  * \ingroup libcore
@@ -39,13 +41,21 @@ public:
 	 * \brief Create a new von Mises-Fisher distribution
 	 * with the given concentration parameter
 	 */
-	VonMisesFisherDistr(Float kappa = 0);
+	explicit inline VonMisesFisherDistr(Float kappa = 0) : m_kappa(kappa) { }
+
+	/// Return the concentration parameter kappa
+	inline void setKappa(Float kappa) {
+		m_kappa = kappa;
+	}
 
 	/// Return the concentration parameter kappa
 	inline Float getKappa() const {
 		return m_kappa;
 	}
 
+	/// Return the mean cosine of the distribution
+	Float getMeanCosine() const;
+
 	/// Evaluate the distribution for a given value of cos(theta)
 	Float eval(Float cosTheta) const;
 
@@ -57,12 +67,21 @@ public:
 	 */
 	Vector sample(const Point2 &sample) const;
 
+	/// Return a string representation
+	std::string toString() const;
+
 	/**
 	 * \brief Compute an appropriate concentration parameter so that
 	 * the associated vMF distribution takes on the value \c x at its peak
 	 */
 	static Float forPeakValue(Float x);
 
+	/**
+	 * \brief Compute an appropriate concentration parameter so that
+	 * the associated vMF distribution has the mean cosine \c g.
+	 */
+	static Float forMeanCosine(Float g);
+
 	/**
 	 * \brief Compute an concentration parameter that approximately
 	 * corresponds to the spherical convolution of two vMF distributions.

src/libcore/vmf.cpp

@@ -18,11 +18,11 @@
 
 #include <mitsuba/core/vmf.h>
 #include <mitsuba/core/warp.h>
+#include <mitsuba/core/brent.h>
+#include <boost/bind.hpp>
 
 MTS_NAMESPACE_BEGIN
 
-VonMisesFisherDistr::VonMisesFisherDistr(Float kappa): m_kappa(kappa) { }
-
 Float VonMisesFisherDistr::eval(Float cosTheta) const {
 	if (m_kappa == 0.0f)
 		return INV_FOURPI;
@@ -63,22 +63,23 @@ Vector VonMisesFisherDistr::sample(const Point2 &sample) const {
 		sinPhi * sinTheta, cosTheta);
 }
 
-Float VonMisesFisherDistr::forPeakValue(Float x) {
-	if (x < INV_FOURPI) {
-		return 0.0f;
-	} else if (x > 0.795) {
-		return 2 * M_PI * x;
-	} else {
-		return std::max((Float) 0.0f,
-			(168.479f * x * x + 16.4585f * x - 2.39942f) /
-			(-1.12718f * x * x + 29.1433f * x + 1.0f));
-	}
+Float VonMisesFisherDistr::getMeanCosine() const {
+	if (m_kappa == 0)
+		return 0;
+	Float coth = m_kappa > 6 ? 1 : ((std::exp(2*m_kappa)+1)/(std::exp(2*m_kappa)-1));
+	return coth-1/m_kappa;
 }
 
 static Float A3(Float kappa) {
 	return 1/ std::tanh(kappa) - 1 / kappa;
 }
 
+std::string VonMisesFisherDistr::toString() const {
+	std::ostringstream oss;
+	oss << "VonMisesFisherDistr[kappa=" << m_kappa << "]";
+	return oss.str();
+}
+
 static Float dA3(Float kappa) {
 	Float csch = 2.0f /
 		(math::fastexp(kappa)-math::fastexp(-kappa));
@@ -110,4 +111,33 @@ Float VonMisesFisherDistr::convolve(Float kappa1, Float kappa2) {
 	return A3inv(A3(kappa1) * A3(kappa2), std::min(kappa1, kappa2));
 }
 
+Float VonMisesFisherDistr::forPeakValue(Float x) {
+	if (x < INV_FOURPI) {
+		return 0.0f;
+	} else if (x > 0.795) {
+		return 2 * M_PI * x;
+	} else {
+		return std::max((Float) 0.0f,
+			(168.479f * x * x + 16.4585f * x - 2.39942f) /
+			(-1.12718f * x * x + 29.1433f * x + 1.0f));
+	}
+}
+
+static Float meanCosineFunctor(Float kappa, Float g) {
+	return VonMisesFisherDistr(kappa).getMeanCosine()-g;
+}
+
+Float VonMisesFisherDistr::forMeanCosine(Float g) {
+	if (g == 0)
+		return 0;
+	else if (g < 0)
+		SLog(EError, "Error: vMF distribution cannot be created for g<0.");
+
+	BrentSolver brentSolver(100, 1e-6f);
+	BrentSolver::Result result = brentSolver.solve(
+		boost::bind(&meanCosineFunctor, _1, g), 0, 1000);
+	SAssert(result.success);
+	return result.x;
+}
+
 MTS_NAMESPACE_END

src/libpython/core.cpp

@@ -18,6 +18,7 @@
 #include <mitsuba/core/mstream.h>
 #include <mitsuba/core/cstream.h>
 #include <mitsuba/core/qmc.h>
+#include <mitsuba/core/vmf.h>
 #include <mitsuba/core/shvector.h>
 #include <mitsuba/core/sshstream.h>
 #include <mitsuba/render/scenehandler.h>
@@ -2251,6 +2252,20 @@ void export_core() {
 		.def("getMillisecondsSinceStart", &Timer::getMillisecondsSinceStart)
 		.def("getSecondsSinceStart", &Timer::getSecondsSinceStart);
 
+	BP_STRUCT(VonMisesFisherDistr, bp::init<Float>())
+		.def("getKappa", &VonMisesFisherDistr::getKappa)
+		.def("setKappa", &VonMisesFisherDistr::setKappa)
+		.def("eval", &VonMisesFisherDistr::eval)
+		.def("getMeanCosine", &VonMisesFisherDistr::getMeanCosine)
+		.def("sample", &VonMisesFisherDistr::sample, BP_RETURN_VALUE)
+		.def("forMeanCosine", &VonMisesFisherDistr::forMeanCosine)
+		.def("forPeakValue", &VonMisesFisherDistr::forPeakValue)
+		.def("convolve", &VonMisesFisherDistr::convolve)
+		.def("__repr__", &VonMisesFisherDistr::toString)
+		.staticmethod("forMeanCosine")
+		.staticmethod("forPeakValue")
+		.staticmethod("convolve");
+
 	bp::detail::current_scope = oldScope;
 }