diff --git a/include/mitsuba/hw/vpl.h b/include/mitsuba/hw/vpl.h
index 1f3e348e..2bab4119 100644
--- a/include/mitsuba/hw/vpl.h
+++ b/include/mitsuba/hw/vpl.h
@@ -50,7 +50,7 @@ public:
 		const Luminaire *luminaire, const Point &camPos, bool faceNormals);
 
 	/// Draw the background if there is an environment luminaire
-	void drawBackground(const Transform &clipToWorld, const Point &camPos);
+	void drawBackground(const Transform &clipToWorld, const Point &camPos, Float scaleFactor);
 
 	/// Release bound resources
 	void unbind();
diff --git a/include/mitsuba/render/preview.h b/include/mitsuba/render/preview.h
index 9518efdf..579cb9eb 100644
--- a/include/mitsuba/render/preview.h
+++ b/include/mitsuba/render/preview.h
@@ -32,11 +32,11 @@ class MTS_EXPORT_RENDER PreviewWorker : public WorkProcessor {
 public:
 	inline PreviewWorker(int blockSize, Point cameraO, Vector cameraTL, 
 		Vector cameraDx, Vector cameraDy, const VPL &vpl, Float minDist, bool coherent,
-		bool diffuseSources, bool diffuseReceivers) 
+		bool diffuseSources, bool diffuseReceivers, Float backgroundScale) 
 		: m_blockSize(blockSize), m_cameraO(cameraO), m_cameraTL(cameraTL),
 		m_cameraDx(cameraDx), m_cameraDy(cameraDy), m_vpl(vpl), 
 		m_minDist(minDist), m_coherent(coherent), m_diffuseSources(diffuseSources),
-		m_diffuseReceivers(diffuseReceivers) {
+		m_diffuseReceivers(diffuseReceivers), m_backgroundScale(backgroundScale) {
 	}
 
 	void processIncoherent(const WorkUnit *workUnit, WorkResult *workResult, 
@@ -71,6 +71,7 @@ private:
 	const std::vector<const Shape *> *m_shapes;
 	bool m_coherent;
 	bool m_diffuseSources, m_diffuseReceivers;
+	Float m_backgroundScale;
 };
 
 MTS_NAMESPACE_END
diff --git a/include/mitsuba/render/vpl.h b/include/mitsuba/render/vpl.h
index da282a16..d9f58017 100644
--- a/include/mitsuba/render/vpl.h
+++ b/include/mitsuba/render/vpl.h
@@ -46,16 +46,20 @@ struct VPL {
 
 /**
  * Generate a series of point light sources by sampling from the Halton
- * sequence (as is done in Instant Radiosity). The parameter <code>offset</code> 
+ * sequence (as is done in Instant Radiosity). The parameter \c offset
  * allows setting the initial QMC sample index (should be set to 0 if no offset is
  * desired), and the last index is returned after the function finishes. This can 
  * be used to generate an arbitrary number of VPLs incrementally. Note that the 
- * parameter <code>count</code> is only a suggestion. Generally, the implementation 
- * will produce a few more VPLs. After VPL generation is done, their power must be scaled
- * by the inverse of the returned index.
+ * value supplied with the parameter \c count is only a suggestion to the implementation.
+ * Generally, it will produce a few more VPLs than the requsted amount. After VPL
+ * generation is done, their power must be scaled by the inverse of the returned index.
+ * The implementation here also needs an pseudorandom number generator, which
+ * is used to prune VPLs in an unbiased manner.
  */
-extern MTS_EXPORT_RENDER size_t generateVPLs(const Scene *scene, size_t offset, 
-	size_t count, int maxDepth, std::deque<VPL> &vpls);
+extern MTS_EXPORT_RENDER size_t generateVPLs(const Scene *scene,
+		Random *random, size_t offset, 
+		size_t count, int maxDepth, bool prune,
+		std::deque<VPL> &vpls);
 
 MTS_NAMESPACE_END
 
diff --git a/src/integrators/vpl/vpl.cpp b/src/integrators/vpl/vpl.cpp
index 568f1b77..69051dfd 100644
--- a/src/integrators/vpl/vpl.cpp
+++ b/src/integrators/vpl/vpl.cpp
@@ -17,6 +17,7 @@
 */
 
 #include <mitsuba/core/statistics.h>
+#include <mitsuba/core/plugin.h>
 #include <mitsuba/hw/vpl.h>
 #include <mitsuba/hw/session.h>
 #include <mitsuba/hw/device.h>
@@ -46,6 +47,7 @@ public:
 		m_session = Session::create();
 		m_device = Device::create(m_session);
 		m_renderer = Renderer::create(m_session);
+	
 		m_random = new Random();
 	}
 
@@ -73,7 +75,7 @@ public:
 			m_shaderManager->unbind();
 		}
 		m_renderer->endDrawingMeshes();
-		m_shaderManager->drawBackground(clipToWorld, camPos);
+		m_shaderManager->drawBackground(clipToWorld, camPos, 1.0f);
 	}
 
 	bool preprocess(const Scene *scene, RenderQueue *queue, const RenderJob *job,
@@ -81,7 +83,8 @@ public:
 		Integrator::preprocess(scene, queue, job, sceneResID, cameraResID, samplerResID);
 
 		if (m_vpls.size() == 0) {
-			Float normalization = (Float) 1 / generateVPLs(scene, 0, m_vplCount, m_maxDepth, m_vpls);
+			Float normalization = (Float) 1 / generateVPLs(scene, m_random,
+					0, m_vplCount, m_maxDepth, true, m_vpls);
 			for (size_t i=0; i<m_vpls.size(); ++i)
 				m_vpls[i].P *= normalization;
 			Log(EInfo, "Generated %i virtual point lights", m_vpls.size());
diff --git a/src/libhw/vpl.cpp b/src/libhw/vpl.cpp
index bfd67efa..7d6f375e 100644
--- a/src/libhw/vpl.cpp
+++ b/src/libhw/vpl.cpp
@@ -177,8 +177,9 @@ void VPLShaderManager::init() {
 
 		oss << "varying vec3 d;" << endl
 			<< "uniform vec3 camPos;" << endl
+			<< "uniform float scale;" << endl
 			<< "void main() {" << endl
-			<< "  gl_FragColor.rgb = " << evalName << "_background(normalize(d - camPos));" << endl
+			<< "  gl_FragColor.rgb = scale * " << evalName << "_background(normalize(d - camPos));" << endl
 			<< "  gl_FragColor.a = 1.0;" << endl
 			<< "}" << endl;
 
@@ -606,7 +607,7 @@ void VPLShaderManager::configure(const VPL &vpl, const BSDF *bsdf,
 	m_targetConfig.bind(program, config, textureUnitOffset);
 }
 
-void VPLShaderManager::drawBackground(const Transform &clipToWorld, const Point &camPos) {
+void VPLShaderManager::drawBackground(const Transform &clipToWorld, const Point &camPos, Float scaleFactor) {
 	if (m_backgroundProgram == NULL)
 		return;
 	int textureUnitOffset = 0;	
@@ -615,6 +616,7 @@ void VPLShaderManager::drawBackground(const Transform &clipToWorld, const Point
 		m_backgroundDependencies, textureUnitOffset);
 	m_backgroundProgram->setParameter("clipToWorld", clipToWorld, false);
 	m_backgroundProgram->setParameter("camPos", camPos, false);
+	m_backgroundProgram->setParameter("scale", scaleFactor);
 	m_renderer->blitQuad(false);
 	m_backgroundProgram->unbind();
 	m_backgroundDependencies.recursiveUnbind();
diff --git a/src/librender/preview.cpp b/src/librender/preview.cpp
index c47076c7..99ac72f3 100644
--- a/src/librender/preview.cpp
+++ b/src/librender/preview.cpp
@@ -79,7 +79,7 @@ void PreviewWorker::processIncoherent(const WorkUnit *workUnit, WorkResult *work
 
 			++numRays;
 			if (!m_kdtree->rayIntersect(primary, its)) {
-				block->setPixel(pos++, m_scene->LeBackground(primary));
+				block->setPixel(pos++, m_scene->LeBackground(primary)*m_backgroundScale);
 				continue;
 			}
 
@@ -283,7 +283,7 @@ void PreviewWorker::processCoherent(const WorkUnit *workUnit, WorkResult *workRe
 						Point(primRay4.o[0].f[idx], primRay4.o[1].f[idx], primRay4.o[2].f[idx]),
 						Vector(primRay4.d[0].f[idx], primRay4.d[1].f[idx], primRay4.d[2].f[idx]),
 						0.0f
-					));
+					)) * m_backgroundScale;
 					memset(&direct[idx], 0, sizeof(Spectrum));
 					continue;
 				}
@@ -457,7 +457,7 @@ void PreviewWorker::processCoherent(const WorkUnit *workUnit, WorkResult *workRe
 ref<WorkProcessor> PreviewWorker::clone() const {
 	return new PreviewWorker(m_blockSize, m_cameraO, m_cameraTL, 
 		m_cameraDx, m_cameraDy, m_vpl, m_minDist, m_coherent,
-		m_diffuseSources, m_diffuseReceivers);
+		m_diffuseSources, m_diffuseReceivers, m_backgroundScale);
 }
 
 MTS_IMPLEMENT_CLASS(PreviewWorker, false, WorkProcessor)
diff --git a/src/librender/vpl.cpp b/src/librender/vpl.cpp
index a2e90b8e..8513dbc9 100644
--- a/src/librender/vpl.cpp
+++ b/src/librender/vpl.cpp
@@ -18,11 +18,48 @@
 
 #include <mitsuba/render/vpl.h>
 #include <mitsuba/core/plugin.h>
+#include <mitsuba/core/statistics.h>
 
 MTS_NAMESPACE_BEGIN
 
-size_t generateVPLs(const Scene *scene, size_t offset, size_t count, int maxDepth, 
-		std::deque<VPL> &vpls) {
+static StatsCounter prunedVPLs("VPL generator", "Pruned VPLs", EPercentage);
+
+static void appendVPL(const Scene *scene, Random *random,
+	VPL &vpl, bool prune, std::deque<VPL> &vpls) {
+	prunedVPLs.incrementBase();
+	if (prune) {
+		/* Possibly reject VPLs if they are unlikely to be
+		visible from the camera */
+		int nSuccesses = 0, nSamples = 50;
+		const Shape *shape;
+		Normal n;
+		Ray ray;
+		Float t;
+		for (int i=0; i<nSamples; ++i) {
+			scene->getCamera()->generateRay(Point2(random->nextFloat(),
+				random->nextFloat()), Point2(0.0f), 0, ray);
+			if (scene->rayIntersect(ray, t, shape, n)) {
+				if (!scene->isOccluded(ray(t), vpl.its.p, 0))
+					++nSuccesses;
+			} else {
+				++nSuccesses; // be conservative
+			}
+		}
+		/// Have a small chance of acceptance in any case
+		Float acceptanceProb = (nSuccesses+1) / (Float) (nSamples+1);
+		if (random->nextFloat() < acceptanceProb) {
+			vpl.P /= acceptanceProb;
+			vpls.push_back(vpl);
+		} else {
+			++prunedVPLs;
+		}
+	} else {
+		vpls.push_back(vpl);
+	}
+}
+
+size_t generateVPLs(const Scene *scene, Random *random,
+		size_t offset, size_t count, int maxDepth, bool prune, std::deque<VPL> &vpls) {
 	ref<Sampler> sampler = static_cast<Sampler *> (PluginManager::getInstance()->
 		createObject(MTS_CLASS(Sampler), Properties("halton")));
 	EmissionRecord eRec;
@@ -50,7 +87,7 @@ size_t generateVPLs(const Scene *scene, size_t offset, size_t count, int maxDept
 		lumVPL.its.shFrame = (eRec.luminaire->getType() & Luminaire::EOnSurface)
 			? Frame(eRec.sRec.n) : stdFrame;
 		lumVPL.luminaire = eRec.luminaire;
-		vpls.push_back(lumVPL);
+		appendVPL(scene, random, lumVPL, prune, vpls);
 
 		weight *= eRec.luminaire->sampleEmissionDirection(eRec, dirSample);
 		Float cosTheta = (eRec.luminaire->getType() & Luminaire::EOnSurface)
@@ -87,7 +124,7 @@ size_t generateVPLs(const Scene *scene, size_t offset, size_t count, int maxDept
 
 			VPL vpl(ESurfaceVPL, weight);
 			vpl.its = its;
-			vpls.push_back(vpl);
+			appendVPL(scene, random, vpl, prune, vpls);
 	
 			weight *= bsdfVal;
 		
diff --git a/src/qtgui/preview.cpp b/src/qtgui/preview.cpp
index 024e4ae1..4a9e0507 100644
--- a/src/qtgui/preview.cpp
+++ b/src/qtgui/preview.cpp
@@ -29,8 +29,8 @@ PreviewThread::PreviewThread(Device *parentDevice, Renderer *parentRenderer)
 	m_renderer = Renderer::create(m_session);
 	m_mutex = new Mutex();
 	m_queueCV = new ConditionVariable(m_mutex);
-	m_random = new Random();
 	m_bufferCount = 3;
+	m_backgroundScaleFactor = 1.0f;
 	m_queueEntryIndex = 0;
 	m_session->init();
 	m_timer = new Timer();
@@ -57,6 +57,8 @@ PreviewThread::PreviewThread(Device *parentDevice, Renderer *parentRenderer)
 	m_framebuffer = m_renderer->createGPUTexture("Framebuffer");
 	for (int i=0; i<m_bufferCount; ++i) 
 		m_recycleQueue.push_back(PreviewQueueEntry(m_queueEntryIndex++));
+	
+	m_random = new Random();
 
 	MTS_AUTORELEASE_END()
 }
@@ -350,9 +352,12 @@ void PreviewThread::run() {
 					m_timer->reset();
 				}
 
-				if (m_vpls.empty())
-					m_vplSampleOffset = generateVPLs(m_context->scene, m_vplSampleOffset,
-						1, m_context->pathLength, m_vpls);
+				if (m_vpls.empty()) {
+					size_t oldOffset = m_vplSampleOffset;
+					m_vplSampleOffset = generateVPLs(m_context->scene, m_random,
+						m_vplSampleOffset, 1, m_context->pathLength, !m_motion, m_vpls);
+					m_backgroundScaleFactor = m_vplSampleOffset - oldOffset;
+				}
 
 				VPL vpl = m_vpls.front();
 				m_vpls.pop_front();
@@ -388,9 +393,12 @@ void PreviewThread::run() {
 					m_timer->reset();
 				}
 
-				if (m_vpls.empty())
-					m_vplSampleOffset = generateVPLs(m_context->scene, m_vplSampleOffset,
-						1, m_context->pathLength, m_vpls);
+				if (m_vpls.empty()) {
+					size_t oldOffset = m_vplSampleOffset;
+					m_vplSampleOffset = generateVPLs(m_context->scene, m_random,
+						m_vplSampleOffset, 1, m_context->pathLength, !m_motion, m_vpls);
+					m_backgroundScaleFactor = m_vplSampleOffset - oldOffset;
+				}
 
 				VPL vpl = m_vpls.front();
 				m_vpls.pop_front();
@@ -500,7 +508,8 @@ void PreviewThread::oglRenderVPL(PreviewQueueEntry &target, const VPL &vpl) {
 		m_shaderManager->unbind();
 	}
 	m_renderer->endDrawingMeshes();
-	m_shaderManager->drawBackground(clipToWorld, camPos);
+	m_shaderManager->drawBackground(clipToWorld, camPos,
+		m_backgroundScaleFactor);
 	m_framebuffer->releaseTarget();
 
 	target.buffer->activateTarget();
@@ -592,7 +601,8 @@ void PreviewThread::rtrtRenderVPL(PreviewQueueEntry &target, const VPL &vpl) {
 		target.buffer->getBitmap(), 
 		m_context->previewMethod == ERayTraceCoherent,
 		m_context->diffuseSources,
-		m_context->diffuseReceivers);
+		m_context->diffuseReceivers,
+		m_backgroundScaleFactor);
 	m_mutex->unlock();
 
 	ref<Scheduler> sched = Scheduler::getInstance();
diff --git a/src/qtgui/preview.h b/src/qtgui/preview.h
index 57b08e3e..5ac19808 100644
--- a/src/qtgui/preview.h
+++ b/src/qtgui/preview.h
@@ -84,11 +84,11 @@ private:
 	ref<GPUTexture> m_framebuffer;
 	ref<GPUProgram> m_accumProgram;
 	ref<PreviewProcess> m_previewProc;
+	ref<Random> m_random;
 	int m_accumProgramParam_source1;
 	int m_accumProgramParam_source2;
 	const GPUTexture *m_accumBuffer;
 	ref<Mutex> m_mutex;
-	ref<Random> m_random;
 	ref<ConditionVariable> m_queueCV;
 	ref<Timer> m_timer;
 	ref<WaitFlag> m_started;
@@ -101,6 +101,7 @@ private:
 	std::vector<GPUTexture *> m_releaseList;
 	Point m_camPos;
 	Transform m_camViewTransform;
+	Float m_backgroundScaleFactor;
 	bool m_quit, m_sleep, m_motion, m_useSync;
 	bool m_refreshScene;
 };
diff --git a/src/qtgui/preview_proc.cpp b/src/qtgui/preview_proc.cpp
index 92aaca5b..502fe29d 100644
--- a/src/qtgui/preview_proc.cpp
+++ b/src/qtgui/preview_proc.cpp
@@ -19,8 +19,8 @@
 #include <mitsuba/render/rectwu.h>
 #include "preview_proc.h"
 
-PreviewProcess::PreviewProcess(const Scene *scene, int sceneResID, int blockSize) 
-	: m_vpl(NULL) {
+PreviewProcess::PreviewProcess(const Scene *scene, int sceneResID, 
+		int blockSize) : m_vpl(NULL) {
 	m_blockSize = blockSize;
 	m_logLevel = ETrace;
 	m_mutex = new Mutex();
@@ -73,7 +73,7 @@ void PreviewProcess::processResult(const WorkResult *result, bool cancelled) {
 
 void PreviewProcess::configure(const VPL &vpl, Float minDist, const Point2 &jitter, 
 		const Bitmap *source, Bitmap *target, bool coherent, bool diffuseSources,
-		bool diffuseReceivers) {
+		bool diffuseReceivers, Float backgroundScale) {
 	BlockedImageProcess::init(m_film->getCropOffset(), m_film->getCropSize(), m_blockSize);
 	m_source = source;
 	m_target = target;
@@ -83,6 +83,7 @@ void PreviewProcess::configure(const VPL &vpl, Float minDist, const Point2 &jitt
 	m_coherent = coherent;
 	m_diffuseSources = diffuseSources;
 	m_diffuseReceivers = diffuseReceivers;
+	m_backgroundScale = backgroundScale;
 
 	/* It is not necessary to shoot normalized rays. Instead, interpolate: 
 	   here, we generate the upper left corner ray as well as the 
@@ -109,7 +110,7 @@ void PreviewProcess::configure(const VPL &vpl, Float minDist, const Point2 &jitt
 ref<WorkProcessor> PreviewProcess::createWorkProcessor() const {
 	return new PreviewWorker(m_blockSize, m_cameraO, m_cameraTL,
 		m_cameraDx, m_cameraDy, *m_vpl, m_minDist, m_coherent,
-		m_diffuseSources, m_diffuseReceivers);
+		m_diffuseSources, m_diffuseReceivers, m_backgroundScale);
 }
 
 
diff --git a/src/qtgui/preview_proc.h b/src/qtgui/preview_proc.h
index 818fcf38..dd05d4ff 100644
--- a/src/qtgui/preview_proc.h
+++ b/src/qtgui/preview_proc.h
@@ -36,7 +36,7 @@ public:
 
 	void configure(const VPL &vpl, Float minDist, const Point2 &jitter, 
 		const Bitmap *source, Bitmap *target, bool coherent,
-		bool diffuseSources, bool diffuseReceivers); 
+		bool diffuseSources, bool diffuseReceivers, Float backgroundScale); 
 	inline int getRayCount() const { return m_numRays; }
 	inline const Scene *getScene() const { return m_scene; }
 
@@ -63,6 +63,7 @@ private:
 	bool m_coherent;
 	bool m_diffuseSources;
 	bool m_diffuseReceivers;
+	Float m_backgroundScale;
 };
 
 #endif /* __PREVIEW_PROC_H */