diff --git a/src/libslic3r/Arrange.cpp b/src/libslic3r/Arrange.cpp
index 99645f29d..a70c208bc 100644
--- a/src/libslic3r/Arrange.cpp
+++ b/src/libslic3r/Arrange.cpp
@@ -148,8 +148,9 @@ protected:
     double    m_norm;           // A coefficient to scale distances
     MultiPolygon m_merged_pile; // The already merged pile (vector of items)
     Box          m_pilebb;      // The bounding box of the merged pile.
-    ItemGroup m_remaining; // Remaining items (m_items at the beginning)
-    ItemGroup m_items;     // The items to be packed
+    ItemGroup m_remaining;      // Remaining items
+    ItemGroup m_items;          // allready packed items
+    size_t    m_item_count = 0; // Number of all items to be packed
     
     template<class T> ArithmeticOnly<T, double> norm(T val)
     {
@@ -167,7 +168,6 @@ protected:
         const double bin_area = m_bin_area;
         const SpatIndex& spatindex = m_rtree;
         const SpatIndex& smalls_spatindex = m_smallsrtree;
-        const ItemGroup& remaining = m_remaining;
         
         // We will treat big items (compared to the print bed) differently
         auto isBig = [bin_area](double a) {
@@ -209,8 +209,8 @@ protected:
         } compute_case;
         
         bool bigitems = isBig(item.area()) || spatindex.empty();
-        if(bigitems && !remaining.empty()) compute_case = BIG_ITEM;
-        else if (bigitems && remaining.empty()) compute_case = LAST_BIG_ITEM;
+        if(bigitems && !m_remaining.empty()) compute_case = BIG_ITEM;
+        else if (bigitems && m_remaining.empty()) compute_case = LAST_BIG_ITEM;
         else compute_case = SMALL_ITEM;
         
         switch (compute_case) {
@@ -235,7 +235,7 @@ protected:
             // The smalles distance from the arranged pile center:
             double dist = norm(*(std::min_element(dists.begin(), dists.end())));
             double bindist = norm(pl::distance(ibb.center(), bincenter));
-            dist = 0.8 * dist + 0.2*bindist;
+            dist = 0.8 * dist + 0.2 * bindist;
 
             // Prepare a variable for the alignment score.
             // This will indicate: how well is the candidate item
@@ -267,29 +267,24 @@ protected:
                     if(ascore < alignment_score) alignment_score = ascore;
                 }
             }
-
+            
             density = std::sqrt(norm(fullbb.width()) * norm(fullbb.height()));
-
+            double R = double(m_remaining.size()) / m_item_count;
+            
             // The final mix of the score is the balance between the
             // distance from the full pile center, the pack density and
             // the alignment with the neighbors
             if (result.empty())
-                score = 0.5 * dist + 0.5 * density;
+                score = 0.50 * dist + 0.50 * density;
             else
-                score = 0.40 * dist + 0.40 * density + 0.2 * alignment_score;
+                score = R * 0.60 * dist +
+                        (1.0 - R) * 0.20 * density +
+                        0.20 * alignment_score;
             
             break;
         }
         case LAST_BIG_ITEM: {
-            auto mp = m_merged_pile;
-            mp.emplace_back(item.transformedShape());
-            auto chull = sl::convexHull(mp);
-    
-            placers::EdgeCache<clppr::Polygon> ec(chull);
-            
-            double circ  = norm(ec.circumference());
-            double bcirc = 2.0 * norm(fullbb.width() + fullbb.height());
-            score = 0.5 * circ + 0.5 * bcirc;
+            score = norm(pl::distance(ibb.center(), m_pilebb.center()));
             break;
         }
         case SMALL_ITEM: {
@@ -355,9 +350,11 @@ public:
         m_pck.configure(m_pconf);
     }
     
-    template<class...Args> inline void operator()(Args&&...args) {
-        m_rtree.clear(); /*m_preload_idx.clear();*/
-        m_pck.execute(std::forward<Args>(args)...);
+    template<class It> inline void operator()(It from, It to) {
+        m_rtree.clear();
+        m_item_count += size_t(to - from);
+        m_pck.execute(from, to);
+        m_item_count = 0;
     }
     
     inline void preload(std::vector<Item>& fixeditems) {
@@ -376,6 +373,7 @@ public:
         }
 
         m_pck.configure(m_pconf);
+        m_item_count += fixeditems.size();
     }
 };
 
diff --git a/src/libslic3r/Print.cpp b/src/libslic3r/Print.cpp
index c5b619891..71529cff1 100644
--- a/src/libslic3r/Print.cpp
+++ b/src/libslic3r/Print.cpp
@@ -262,8 +262,14 @@ std::vector<unsigned int> Print::object_extruders() const
 {
     std::vector<unsigned int> extruders;
     extruders.reserve(m_regions.size() * 3);
-    for (const PrintRegion *region : m_regions)
-        region->collect_object_printing_extruders(extruders);
+    std::vector<unsigned char> region_used(m_regions.size(), false);
+    for (const PrintObject *object : m_objects)
+		for (const std::vector<std::pair<t_layer_height_range, int>> &volumes_per_region : object->region_volumes)
+        	if (! volumes_per_region.empty())
+        		region_used[&volumes_per_region - &object->region_volumes.front()] = true;
+    for (size_t idx_region = 0; idx_region < m_regions.size(); ++ idx_region)
+    	if (region_used[idx_region])
+        	m_regions[idx_region]->collect_object_printing_extruders(extruders);
     sort_remove_duplicates(extruders);
     return extruders;
 }
@@ -273,17 +279,24 @@ std::vector<unsigned int> Print::support_material_extruders() const
 {
     std::vector<unsigned int> extruders;
     bool support_uses_current_extruder = false;
+    auto num_extruders = (unsigned int)m_config.nozzle_diameter.size();
 
     for (PrintObject *object : m_objects) {
         if (object->has_support_material()) {
+        	assert(object->config().support_material_extruder >= 0);
             if (object->config().support_material_extruder == 0)
                 support_uses_current_extruder = true;
-            else
-                extruders.push_back(object->config().support_material_extruder - 1);
+            else {
+            	unsigned int i = (unsigned int)object->config().support_material_extruder - 1;
+                extruders.emplace_back((i >= num_extruders) ? 0 : i);
+            }
+        	assert(object->config().support_material_interface_extruder >= 0);
             if (object->config().support_material_interface_extruder == 0)
                 support_uses_current_extruder = true;
-            else
-                extruders.push_back(object->config().support_material_interface_extruder - 1);
+            else {
+            	unsigned int i = (unsigned int)object->config().support_material_interface_extruder - 1;
+                extruders.emplace_back((i >= num_extruders) ? 0 : i);
+            }
         }
     }
 
@@ -577,6 +590,8 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
 
     // Apply variables to placeholder parser. The placeholder parser is used by G-code export,
     // which should be stopped if print_diff is not empty.
+    size_t num_extruders = m_config.nozzle_diameter.size();
+    bool   num_extruders_changed = false;
     if (! full_config_diff.empty() || ! placeholder_parser_overrides.empty()) {
         update_apply_status(this->invalidate_step(psGCodeExport));
 		m_placeholder_parser.apply_config(std::move(placeholder_parser_overrides));
@@ -592,6 +607,10 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
 	    // Handle changes to regions config defaults
 	    m_default_region_config.apply_only(new_full_config, region_diff, true);
         m_full_print_config = std::move(new_full_config);
+        if (num_extruders != m_config.nozzle_diameter.size()) {
+        	num_extruders = m_config.nozzle_diameter.size();
+        	num_extruders_changed = true;
+        }
     }
     
     class LayerRanges
@@ -768,7 +787,6 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
         print_object_status.emplace(PrintObjectStatus(print_object));
 
     // 3) Synchronize ModelObjects & PrintObjects.
-    size_t num_extruders = m_config.nozzle_diameter.size();
     for (size_t idx_model_object = 0; idx_model_object < model.objects.size(); ++ idx_model_object) {
         ModelObject &model_object = *m_model.objects[idx_model_object];
         auto it_status = model_object_status.find(ModelObjectStatus(model_object.id()));
@@ -815,7 +833,7 @@ Print::ApplyStatus Print::apply(const Model &model, DynamicPrintConfig new_full_
             bool object_config_changed = model_object.config != model_object_new.config;
 			if (object_config_changed)
 				static_cast<DynamicPrintConfig&>(model_object.config) = static_cast<const DynamicPrintConfig&>(model_object_new.config);
-            if (! object_diff.empty() || object_config_changed) {
+            if (! object_diff.empty() || object_config_changed || num_extruders_changed) {
                 PrintObjectConfig new_config = PrintObject::object_config_from_model_object(m_default_object_config, model_object, num_extruders);
                 auto range = print_object_status.equal_range(PrintObjectStatus(model_object.id()));
                 for (auto it = range.first; it != range.second; ++ it) {
@@ -1144,7 +1162,12 @@ std::string Print::validate() const
         // #4043
         if (total_copies_count > 1 && ! m_config.complete_objects.value)
             return L("The Spiral Vase option can only be used when printing a single object.");
-        if (m_regions.size() > 1)
+        assert(m_objects.size() == 1);
+        size_t num_regions = 0;
+        for (const std::vector<std::pair<t_layer_height_range, int>> &volumes_per_region : m_objects.front()->region_volumes)
+        	if (! volumes_per_region.empty())
+        		++ num_regions;
+        if (num_regions > 1)
             return L("The Spiral Vase option can only be used when printing single material objects.");
     }
 
diff --git a/src/libslic3r/PrintRegion.cpp b/src/libslic3r/PrintRegion.cpp
index 73b40487b..fc2bdfa7d 100644
--- a/src/libslic3r/PrintRegion.cpp
+++ b/src/libslic3r/PrintRegion.cpp
@@ -46,7 +46,7 @@ Flow PrintRegion::flow(FlowRole role, double layer_height, bool bridge, bool fir
     }
     double nozzle_diameter = m_print->config().nozzle_diameter.get_at(extruder-1);
     
-    return Flow::new_from_config_width(role, config_width, nozzle_diameter, layer_height, bridge ? (float)m_config.bridge_flow_ratio : 0.0);
+    return Flow::new_from_config_width(role, config_width, (float)nozzle_diameter, (float)layer_height, bridge ? (float)m_config.bridge_flow_ratio : 0.0f);
 }
 
 coordf_t PrintRegion::nozzle_dmr_avg(const PrintConfig &print_config) const
@@ -64,16 +64,27 @@ coordf_t PrintRegion::bridging_height_avg(const PrintConfig &print_config) const
 void PrintRegion::collect_object_printing_extruders(const PrintConfig &print_config, const PrintRegionConfig &region_config, std::vector<unsigned int> &object_extruders)
 {
     // These checks reflect the same logic used in the GUI for enabling/disabling extruder selection fields.
+    auto num_extruders = (int)print_config.nozzle_diameter.size();
+    auto emplace_extruder = [num_extruders, &object_extruders](int extruder_id) {
+    	int i = std::max(0, extruder_id - 1);
+        object_extruders.emplace_back((i >= num_extruders) ? 0 : i);
+    };
     if (region_config.perimeters.value > 0 || print_config.brim_width.value > 0)
-        object_extruders.emplace_back(region_config.perimeter_extruder - 1);
+    	emplace_extruder(region_config.perimeter_extruder);
     if (region_config.fill_density.value > 0)
-        object_extruders.emplace_back(region_config.infill_extruder - 1);
+    	emplace_extruder(region_config.infill_extruder);
     if (region_config.top_solid_layers.value > 0 || region_config.bottom_solid_layers.value > 0)
-        object_extruders.emplace_back(region_config.solid_infill_extruder - 1);
+    	emplace_extruder(region_config.solid_infill_extruder);
 }
 
 void PrintRegion::collect_object_printing_extruders(std::vector<unsigned int> &object_extruders) const
 {
+    auto num_extruders = (int)print()->config().nozzle_diameter.size();
+    // PrintRegion, if used by some PrintObject, shall have all the extruders set to an existing printer extruder.
+    // If not, then there must be something wrong with the Print::apply() function.
+    assert(this->config().perimeter_extruder    <= num_extruders);
+    assert(this->config().infill_extruder       <= num_extruders);
+    assert(this->config().solid_infill_extruder <= num_extruders);
     collect_object_printing_extruders(print()->config(), this->config(), object_extruders);
 }