void BVH4BuilderTopLevel::build (size_t threadIndex, size_t threadCount, size_t objectID)
    {
      /* ignore if no triangle mesh or not enabled */
      TriangleMesh* mesh = scene->getTriangleMeshSafe(objectID);
      if (mesh == NULL || !mesh->isEnabled() || mesh->numTimeSteps != 1) 
        return;
      
      BVH4*    object  = objects [objectID]; assert(object);
      Builder* builder = builders[objectID]; assert(builder);
            
      /* build object if it got modified */
      if (mesh->isModified()) {
        builder->build(threadIndex,threadCount);
        mesh->state = Geometry::ENABLED;
      }
      
      /* create build primitive */
      if (!object->bounds.empty())
	refs[nextRef++] = BuildRef(object->bounds,object->root);
    }
    void BVH4BuilderTwoLevel::build(size_t threadIndex, size_t threadCount) 
    {
      /* delete some objects */
      size_t N = scene->size();
      if (N < objects.size()) {
        parallel_for(N, objects.size(), [&] (const range<size_t>& r) {
            for (size_t i=r.begin(); i<r.end(); i++) {
              delete builders[i]; builders[i] = nullptr;
              delete objects[i]; objects[i] = nullptr;
            }
          });
      }

      /* reset memory allocator */
      bvh->alloc.reset();
      
      /* skip build for empty scene */
      const size_t numPrimitives = scene->getNumPrimitives<TriangleMesh,1>();
      if (numPrimitives == 0) {
        prims.resize(0);
        bvh->set(BVH4::emptyNode,empty,0);
        return;
      }

      double t0 = bvh->preBuild(TOSTRING(isa) "::BVH4BuilderTwoLevel");

#if PROFILE
	profile(2,20,numPrimitives,[&] (ProfileTimer& timer)
        {
#endif
          
      /* resize object array if scene got larger */
      if (objects.size()  < N) objects.resize(N);
      if (builders.size() < N) builders.resize(N);
      if (refs.size()     < N) refs.resize(N);
      nextRef = 0;
      
      /* create of acceleration structures */
      parallel_for(size_t(0), N, [&] (const range<size_t>& r) 
      {
        for (size_t objectID=r.begin(); objectID<r.end(); objectID++)
        {
          TriangleMesh* mesh = scene->getTriangleMeshSafe(objectID);
          
          /* verify meshes got deleted properly */
          if (mesh == nullptr || mesh->numTimeSteps != 1) {
            assert(objectID < objects.size () && objects[objectID] == nullptr);
            assert(objectID < builders.size() && builders[objectID] == nullptr);
            continue;
          }
          
          /* create BVH and builder for new meshes */
          if (objects[objectID] == nullptr)
            createTriangleMeshAccel(mesh,(AccelData*&)objects[objectID],builders[objectID]);
        }
      });

      /* parallel build of acceleration structures */
      parallel_for(size_t(0), N, [&] (const range<size_t>& r) 
      {
        for (size_t objectID=r.begin(); objectID<r.end(); objectID++)
        {
          /* ignore if no triangle mesh or not enabled */
          TriangleMesh* mesh = scene->getTriangleMeshSafe(objectID);
          if (mesh == nullptr || !mesh->isEnabled() || mesh->numTimeSteps != 1) 
            continue;
        
          BVH4*    object  = objects [objectID]; assert(object);
          Builder* builder = builders[objectID]; assert(builder);
          
          /* build object if it got modified */
#if !PROFILE 
          if (mesh->isModified()) 
#endif
            builder->build(0,0);
          
          /* create build primitive */
          if (!object->bounds.empty())
            refs[nextRef++] = BVH4BuilderTwoLevel::BuildRef(object->bounds,object->root);
        }
      });
      
      /* fast path for single geometry scenes */
      if (nextRef == 1) { 
        bvh->set(refs[0].node,refs[0].bounds(),numPrimitives);
        return;
      }

      /* open all large nodes */
      refs.resize(nextRef);
      open_sequential(numPrimitives); 
      
      /* fast path for small geometries */
      if (refs.size() == 1) { 
        bvh->set(refs[0].node,refs[0].bounds(),numPrimitives);
        return;
      }

      /* compute PrimRefs */
      prims.resize(refs.size());
      const PrimInfo pinfo = parallel_reduce(size_t(0), refs.size(), size_t(1024), PrimInfo(empty), [&] (const range<size_t>& r) -> PrimInfo
      {
        PrimInfo pinfo(empty);
        for (size_t i=r.begin(); i<r.end(); i++) {
          pinfo.add(refs[i].bounds());
          prims[i] = PrimRef(refs[i].bounds(),(size_t)refs[i].node);
        }
        return pinfo;
      }, [] (const PrimInfo& a, const PrimInfo& b) { return PrimInfo::merge(a,b); });

      /* skip if all objects where empty */
      if (pinfo.size() == 0)
        bvh->set(BVH4::emptyNode,empty,0);

      /* otherwise build toplevel hierarchy */
      else
      {
        BVH4::NodeRef root;
        BVHBuilderBinnedSAH::build<BVH4::NodeRef>
          (root,
           [&] { return bvh->alloc.threadLocal2(); },
           [&] (const isa::BVHBuilderBinnedSAH::BuildRecord& current, BVHBuilderBinnedSAH::BuildRecord* children, const size_t N, FastAllocator::ThreadLocal2* alloc) -> int
           {
             BVH4::Node* node = (BVH4::Node*) alloc->alloc0.malloc(sizeof(BVH4::Node)); node->clear();
             for (size_t i=0; i<N; i++) {
               node->set(i,children[i].pinfo.geomBounds);
               children[i].parent = (size_t*)&node->child(i);
             }
             *current.parent = bvh->encodeNode(node);
             return 0;
           },
           [&] (const BVHBuilderBinnedSAH::BuildRecord& current, FastAllocator::ThreadLocal2* alloc) -> int
           {
             assert(current.prims.size() == 1);
             *current.parent = (BVH4::NodeRef) prims[current.prims.begin()].ID();
             return 1;
           },
           [&] (size_t dn) { bvh->scene->progressMonitor(0); },
           prims.data(),pinfo,BVH4::N,BVH4::maxBuildDepthLeaf,4,1,1,1.0f,1.0f);
        
        bvh->set(root,pinfo.geomBounds,numPrimitives);
      }

#if PROFILE
      }); 
#endif

      bvh->alloc.cleanup();
      bvh->postBuild(t0);
    }