hslt: always use a consistent edge direction in the bidirectional abstraction layer

include/mitsuba/bidir/edge.h

@@ -53,6 +53,8 @@ struct MTS_EXPORT_BIDIR PathEdge {
 
 	/**
 	 * \brief Normalized direction vector associated with this edge
+	 *
+	 * The direction always points along the light path (from the light)
 	 */
 	Vector d;
 

src/libbidir/edge.cpp

@@ -63,6 +63,9 @@ bool PathEdge::sampleNext(const Scene *scene, Sampler *sampler,
 		weight[1-mode] = mRec.transmittance / pdf[1-mode];
 	}
 	d = ray.d;
+	/* Direction always points along the light path (from the light source along the path) */
+	if(mode == ERadiance)
+		d = -d;
 
 	return true;
 }
@@ -105,6 +108,9 @@ bool PathEdge::perturbDirection(const Scene *scene,
 		return false;
 	}
 	d = ray.d;
+	/* Direction always points along the light path (from the light source along the path) */
+	if(mode == ERadiance)
+		d = -d;
 
 	if (length == 0)
 		return false;
@@ -274,6 +280,9 @@ bool PathEdge::connect(const Scene *scene,
 		}
 	}
 
+	/* Direction always points along the light path (from the light source along the path) */
+	d = -d;
+
 	return true;
 }
 
@@ -330,7 +339,8 @@ bool PathEdge::pathConnect(const Scene *scene, const PathEdge *predEdge,
 			result.append(edge);
 			edge->length = std::min(its.t, remaining);
 			edge->medium = medium;
-			edge->d = d;
+			/* Direction always points along the light path (from the light source along the path) */
+			edge->d = -d;
 
 			if (medium) {
 				MediumSamplingRecord mRec;
@@ -557,6 +567,9 @@ bool PathEdge::pathConnectAndCollapse(const Scene *scene, const PathEdge *predEd
 		}
 	}
 
+	/* Direction always points along the light path (from the light source along the path) */
+	d = -d;
+
 	return true;
 }