mtsgui: handle problematic scenes more gracefully where Mitsuba cannot generate any VPLs

src/librender/vpl.cpp

@@ -93,10 +93,16 @@ size_t generateVPLs(const Scene *scene, Random *random,
 		+ sensor->getShutterOpenTime() * sampler->next1D();
 
 	const Frame stdFrame(Vector(1,0,0), Vector(0,1,0), Vector(0,0,1));
+	int retries = 0;
 
 	while (vpls.size() < count) {
 		sampler->setSampleIndex(++offset);
 
+		if (vpls.empty() && ++retries > 10000) {
+			/* Unable to generate VPLs in this scene -- give up. */
+			return 0;
+		}
+
 		PositionSamplingRecord pRec(time);
 		DirectionSamplingRecord dRec;
 		Spectrum weight = scene->sampleEmitterPosition(pRec,

src/mtsgui/preview.cpp

@@ -397,6 +397,22 @@ void PreviewThread::run() {
 					m_backgroundScaleFactor = m_vplSampleOffset - oldOffset;
 				}
 
+				if (m_vpls.empty()) {
+					/* Unable to generate any VPLs for this scene. Give up. */
+					lock.lock();
+					target.buffer->activateTarget();
+					target.buffer->clear();
+					target.buffer->releaseTarget();
+					m_accumBuffer = target.buffer;
+					m_readyQueue.push_back(target);
+					if (m_useSync)
+						target.sync->init();
+					m_queueCV->signal();
+					lock.unlock();
+					sleep(100);
+					continue;
+				}
+
 				VPL vpl = m_vpls.front();
 				m_vpls.pop_front();