fixed a stupid bug that could mess up medium rendering with analytic shapes

src/integrators/path/ptracer.cpp

@@ -32,7 +32,7 @@ MTS_NAMESPACE_BEGIN
  * For practical reasons, the integral is simultaneously computed
  * for every pixel on the image plane. This is done similarly to 
  * path tracing with next event estimation (PTNEE) by tracing a
- * shadow ray at every surface/volume interaction.
+ * shadow ray to the camera at every surface/volume interaction.
  * An independent accumulation buffer will be assigned to each
  * processor so that the rendering process can run in parallel.
  * When a perspective camera is used, the importance distribution 

src/libcore/spectrum.cpp

@@ -444,7 +444,7 @@ Float ProductSpectrum::eval(Float lambda) const {
 	return m_spec1.eval(lambda) * m_spec2.eval(lambda);
 }
 
-Float RayleighSpectrum::eval(Float lambda) {
+Float RayleighSpectrum::eval(Float lambda) const {
 	Float lambdaSqr = lambda*lambda;
 
 	return 1.0f / (lambdaSqr*lambdaSqr);

src/librender/scene.cpp

@@ -411,7 +411,8 @@ Spectrum Scene::getTransmittance(const Point &p1, const Point &p2,
 			bool surface = rayIntersect(ray, t, shape, n);
 
 			if (medium)
-				transmittance *= medium->getTransmittance(Ray(ray, 0, std::min(t, remaining)), sampler);
+				transmittance *= medium->getTransmittance(
+					Ray(ray, 0, std::min(t, remaining)), sampler);
 
 			if (!surface) 
 				break;

src/librender/skdtree.cpp

@@ -164,6 +164,7 @@ bool ShapeKDTree::rayIntersect(const Ray &ray, Float &t, ConstShapePtr &shape, N
 				} else {
 					/// Uh oh... -- much unnecessary work is done here
 					Intersection its;
+					its.t = t;
 					shape->fillIntersectionRecord(ray, 
 						reinterpret_cast<const uint8_t*>(temp) + 8, its);
 					n = its.geoFrame.n;