From a721b1874073f10db56367f8979a163286e97ca9 Mon Sep 17 00:00:00 2001
From: Wenzel Jakob <wenzel@cs.cornell.edu>
Date: Thu, 24 Jan 2013 20:06:58 -0500
Subject: [PATCH] better computation of the alpha channel when rendering
 index-matched media

---
 include/mitsuba/render/records.inl | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/mitsuba/render/records.inl b/include/mitsuba/render/records.inl
index b6ee2894..2cc60831 100644
--- a/include/mitsuba/render/records.inl
+++ b/include/mitsuba/render/records.inl
@@ -101,12 +101,23 @@ inline bool RadianceQueryRecord::rayIntersect(const RayDifferential &ray) {
 	if (type & EIntersection) {
 		scene->rayIntersect(ray, its);
 		if (type & EOpacity) {
-			if (its.isValid())
-				alpha = 1.0f;
-			else if (medium == NULL)
+			int unused = INT_MAX;
+
+			if (its.isValid()) {
+				if (EXPECT_TAKEN(!its.isMediumTransition()))
+					alpha = 1.0f;
+				else
+					alpha = 1-scene->evalTransmittance(its.p, true,
+						ray(scene->getBSphere().radius*2), false,
+						ray.time, its.getTargetMedium(ray.d), unused).average();
+			} else if (medium) {
+				alpha = 1-scene->evalTransmittance(ray.o, false,
+					ray(scene->getBSphere().radius*2), false,
+					ray.time, medium, unused).average();
+			} else {
 				alpha = 0.0f;
-			else
-				alpha = 1-medium->evalTransmittance(ray).average();
+			}
+
 		}
 		if (type & EDistance)
 			dist = its.t;