diff --git a/src/libslic3r/Model.cpp b/src/libslic3r/Model.cpp
index 1cd20c6fa6..2ff6a34435 100644
--- a/src/libslic3r/Model.cpp
+++ b/src/libslic3r/Model.cpp
@@ -242,6 +242,14 @@ BoundingBoxf3 Model::bounding_box() const
     return bb;
 }
 
+unsigned int Model::update_print_volume_state(const BoundingBoxf3 &print_volume) 
+{
+    unsigned int num_printable = 0;
+    for (ModelObject *model_object : this->objects)
+        num_printable += model_object->check_instances_print_volume_state(print_volume);
+    return num_printable;
+}
+
 void Model::center_instances_around_point(const Vec2d &point)
 {
     BoundingBoxf3 bb;
@@ -875,26 +883,32 @@ void ModelObject::repair()
         v->mesh.repair();
 }
 
-// Called by Print::validate() from the UI thread.
-void ModelObject::check_instances_print_volume_state(const BoundingBoxf3& print_volume)
+unsigned int ModelObject::check_instances_print_volume_state(const BoundingBoxf3& print_volume)
 {
-    for (const ModelVolume* vol : this->volumes)
-    {
-        if (vol->is_model_part())
-        {
-            for (ModelInstance* inst : this->instances)
-            {
-                BoundingBoxf3 bb = vol->get_convex_hull().transformed_bounding_box(inst->world_matrix());
-
+    unsigned int num_printable = 0;
+    enum {
+        INSIDE  = 1,
+        OUTSIDE = 2
+    };
+    for (ModelInstance *model_instance : this->instances) {
+        unsigned int inside_outside = 0;
+        for (const ModelVolume *vol : this->volumes)
+            if (vol->is_model_part()) {
+                BoundingBoxf3 bb = vol->get_convex_hull().transformed_bounding_box(model_instance->world_matrix());
                 if (print_volume.contains(bb))
-                    inst->print_volume_state = ModelInstance::PVS_Inside;
+                    inside_outside |= INSIDE;
                 else if (print_volume.intersects(bb))
-                    inst->print_volume_state = ModelInstance::PVS_Partly_Outside;
+                    inside_outside |= INSIDE | OUTSIDE;
                 else
-                    inst->print_volume_state = ModelInstance::PVS_Fully_Outside;
+                    inside_outside |= OUTSIDE;
             }
-        }
+        model_instance->print_volume_state = 
+            (inside_outside == (INSIDE | OUTSIDE)) ? ModelInstance::PVS_Partly_Outside :
+            (inside_outside == INSIDE) ? ModelInstance::PVS_Inside : ModelInstance::PVS_Fully_Outside;
+        if (inside_outside == INSIDE)
+            ++ num_printable;
     }
+    return num_printable;
 }
 
 void ModelObject::print_info() const
diff --git a/src/libslic3r/Model.hpp b/src/libslic3r/Model.hpp
index 81f101636d..9635c2fd2f 100644
--- a/src/libslic3r/Model.hpp
+++ b/src/libslic3r/Model.hpp
@@ -156,7 +156,7 @@ public:
     void repair();
 
     // Called by Print::validate() from the UI thread.
-    void check_instances_print_volume_state(const BoundingBoxf3& print_volume);
+    unsigned int check_instances_print_volume_state(const BoundingBoxf3& print_volume);
 
     // Print object statistics to console.
     void print_info() const;
@@ -411,6 +411,9 @@ public:
     bool add_default_instances();
     // Returns approximate axis aligned bounding box of this model
     BoundingBoxf3 bounding_box() const;
+    // Set the print_volume_state of PrintObject::instances, 
+    // return total number of printable objects.
+    unsigned int update_print_volume_state(const BoundingBoxf3 &print_volume);
     void center_instances_around_point(const Vec2d &point);
     void translate(coordf_t x, coordf_t y, coordf_t z) { for (ModelObject *o : this->objects) o->translate(x, y, z); }
     TriangleMesh mesh() const;
diff --git a/src/libslic3r/Print.cpp b/src/libslic3r/Print.cpp
index cfbbda44b4..d4fed0d5f9 100644
--- a/src/libslic3r/Print.cpp
+++ b/src/libslic3r/Print.cpp
@@ -852,7 +852,11 @@ bool Print::apply(const Model &model, const DynamicPrintConfig &config_in)
         bool modifiers_differ           = model_volume_list_changed(model_object, model_object_new, ModelVolume::PARAMETER_MODIFIER);
         bool support_blockers_differ    = model_volume_list_changed(model_object, model_object_new, ModelVolume::SUPPORT_BLOCKER);
         bool support_enforcers_differ   = model_volume_list_changed(model_object, model_object_new, ModelVolume::SUPPORT_ENFORCER);
-        if (model_parts_differ || modifiers_differ) {
+        if (model_parts_differ || modifiers_differ || 
+            model_object.origin_translation         != model_object_new.origin_translation   ||
+            model_object.layer_height_ranges        != model_object_new.layer_height_ranges  || 
+            model_object.layer_height_profile       != model_object_new.layer_height_profile ||
+            model_object.layer_height_profile_valid != model_object_new.layer_height_profile_valid) {
             // The very first step (the slicing step) is invalidated. One may freely remove all associated PrintObjects.
             auto range = print_object_status.equal_range(PrintObjectStatus(model_object.id()));
             for (auto it = range.first; it != range.second; ++ it) {
@@ -889,6 +893,9 @@ bool Print::apply(const Model &model, const DynamicPrintConfig &config_in)
                     it->print_object->config_apply_only(new_config, diff, true);
                 }
             }
+            model_object.name       = model_object_new.name;
+            model_object.input_file = model_object_new.input_file;
+            model_object.instances  = model_object_new.instances;
         }
     }
 
@@ -936,11 +943,10 @@ bool Print::apply(const Model &model, const DynamicPrintConfig &config_in)
                     print_objects_new.emplace_back(print_object);
                     print_object_status.emplace(PrintObjectStatus(print_object, PrintObjectStatus::New));
                 } else if ((*it_old)->print_object->copies() != new_instances.copies) {
-                    // The PrintObject already exists and the copies differ. The only step currently sensitive to the order is the G-code generator.
-                    // Stop it.
-                    this->invalidate_step(psGCodeExport);
+                    // The PrintObject already exists and the copies differ.
                     (*it_old)->print_object->set_copies(new_instances.copies);
-                }
+					print_objects_new.emplace_back((*it_old)->print_object);
+				}
             }
         }
         if (m_objects != print_objects_new) {
@@ -958,7 +964,7 @@ bool Print::apply(const Model &model, const DynamicPrintConfig &config_in)
         int idx_region = 0;
         for (const auto &volumes : print_object->region_volumes) {
             if (! volumes.empty())
-                ++ m_regions[idx_region];
+				++ m_regions[idx_region]->m_refcnt;
             ++ idx_region;
         }
     }
@@ -1110,23 +1116,7 @@ bool Print::has_skirt() const
 
 std::string Print::validate() const
 {
-    BoundingBox bed_box_2D = get_extents(Polygon::new_scale(m_config.bed_shape.values));
-	BoundingBoxf3 print_volume(unscale(bed_box_2D.min(0), bed_box_2D.min(1), 0.0), unscale(bed_box_2D.max(0), bed_box_2D.max(1), scale_(m_config.max_print_height)));
-    // Allow the objects to protrude below the print bed, only the part of the object above the print bed will be sliced.
-    print_volume.min(2) = -1e10;
-	unsigned int printable_count = 0;
-	{
-		// Lock due to the po->reload_model_instances()
-		tbb::mutex::scoped_lock lock(m_mutex);
-		for (PrintObject *po : m_objects) {
-			po->model_object()->check_instances_print_volume_state(print_volume);
-			po->reload_model_instances();
-			if (po->is_printable())
-				++ printable_count;
-		}
-	}
-
-    if (printable_count == 0)
+    if (m_objects.empty())
         return L("All objects are outside of the print volume.");
 
     if (m_config.complete_objects) {
diff --git a/src/libslic3r/Print.hpp b/src/libslic3r/Print.hpp
index f4fd2235be..e765b8801e 100644
--- a/src/libslic3r/Print.hpp
+++ b/src/libslic3r/Print.hpp
@@ -96,6 +96,7 @@ public:
 #endif
             mtx.unlock();
             cancel();
+			m_state[step] = INVALID;
             mtx.lock();
         }
         return invalidated;
diff --git a/src/libslic3r/PrintObject.cpp b/src/libslic3r/PrintObject.cpp
index c62dd000ca..38b2e678b9 100644
--- a/src/libslic3r/PrintObject.cpp
+++ b/src/libslic3r/PrintObject.cpp
@@ -87,23 +87,25 @@ bool PrintObject::delete_last_copy()
 
 bool PrintObject::set_copies(const Points &points)
 {
-    bool copies_num_changed = m_copies.size() != points.size();
-    
-    // order copies with a nearest neighbor search and translate them by _copies_shift
-    m_copies.clear();
-    m_copies.reserve(points.size());
-    
-    // order copies with a nearest-neighbor search
-    std::vector<Points::size_type> ordered_copies;
-    Slic3r::Geometry::chained_path(points, ordered_copies);
-    
-    for (size_t point_idx : ordered_copies)
-        m_copies.push_back(points[point_idx] + m_copies_shift);
-    
-    bool invalidated = m_print->invalidate_step(psSkirt);
-    invalidated |= m_print->invalidate_step(psBrim);
-    if (copies_num_changed)
-        invalidated |= m_print->invalidate_step(psWipeTower);
+    // Order copies with a nearest-neighbor search.
+    std::vector<Point> copies;
+    {
+        std::vector<Points::size_type> ordered_copies;
+        Slic3r::Geometry::chained_path(points, ordered_copies);
+        copies.reserve(ordered_copies.size());
+        for (size_t point_idx : ordered_copies)
+            copies.emplace_back(points[point_idx] + m_copies_shift);
+    }
+    // Invalidate and set copies.
+    bool invalidated = false;
+    if (copies != m_copies) {
+        invalidated = m_print->invalidate_step(psSkirt);
+        invalidated |= m_print->invalidate_step(psBrim);
+        if (copies.size() != m_copies.size())
+            invalidated |= m_print->invalidate_step(psWipeTower);
+        invalidated |= m_print->invalidate_step(psGCodeExport);
+        m_copies = copies;
+    }
     return invalidated;
 }
 
diff --git a/src/libslic3r/TriangleMesh.cpp b/src/libslic3r/TriangleMesh.cpp
index 665351c393..c8a6e2130b 100644
--- a/src/libslic3r/TriangleMesh.cpp
+++ b/src/libslic3r/TriangleMesh.cpp
@@ -858,12 +858,12 @@ void TriangleMeshSlicer::_slice_do(size_t facet_idx, std::vector<IntersectionLin
     // find layer extents
     std::vector<float>::const_iterator min_layer, max_layer;
     min_layer = std::lower_bound(z.begin(), z.end(), min_z); // first layer whose slice_z is >= min_z
-    max_layer = std::upper_bound(z.begin() + (min_layer - z.begin()), z.end(), max_z) - 1; // last layer whose slice_z is <= max_z
+    max_layer = std::upper_bound(z.begin() + (min_layer - z.begin()), z.end(), max_z); // first layer, whose slice_z is > max_z
     #ifdef SLIC3R_TRIANGLEMESH_DEBUG
-    printf("layers: min = %d, max = %d\n", (int)(min_layer - z.begin()), (int)(max_layer - z.begin()));
+    printf("layers: min = %d, max = %d\n", (int)(min_layer - z.begin()), (int)(max_layer - z.begin()) - 1);
     #endif /* SLIC3R_TRIANGLEMESH_DEBUG */
     
-    for (std::vector<float>::const_iterator it = min_layer; it != max_layer + 1; ++it) {
+    for (std::vector<float>::const_iterator it = min_layer; it != max_layer; ++it) {
         std::vector<float>::size_type layer_idx = it - z.begin();
         IntersectionLine il;
         if (this->slice_facet(*it / SCALING_FACTOR, facet, facet_idx, min_z, max_z, &il) == TriangleMeshSlicer::Slicing) {
diff --git a/src/slic3r/GUI/BackgroundSlicingProcess.cpp b/src/slic3r/GUI/BackgroundSlicingProcess.cpp
index 7c4a4c0399..71250598b7 100644
--- a/src/slic3r/GUI/BackgroundSlicingProcess.cpp
+++ b/src/slic3r/GUI/BackgroundSlicingProcess.cpp
@@ -57,12 +57,20 @@ void BackgroundSlicingProcess::thread_proc()
 		    if (! m_print->canceled()) {
                 wxQueueEvent(GUI::wxGetApp().mainframe->m_plater, new wxCommandEvent(m_event_sliced_id));
 			    m_print->export_gcode(m_temp_output_path, m_gcode_preview_data);
-			    if (! m_print->canceled() && ! m_output_path.empty()) {
-			    	if (copy_file(m_temp_output_path, m_output_path) != 0)
-		    			throw std::runtime_error("Copying of the temporary G-code to the output G-code failed");
-		    		m_print->set_status(95, "Running post-processing scripts");
-		    		run_post_process_scripts(m_output_path, m_print->config());
-		    	}
+			    if (! m_print->canceled() && ! this->is_step_done(bspsGCodeFinalize)) {
+			    	this->set_step_started(bspsGCodeFinalize);
+			    	if (! m_export_path.empty()) {
+			    		//FIXME localize the messages
+				    	if (copy_file(m_temp_output_path, m_export_path) != 0)
+			    			throw std::runtime_error("Copying of the temporary G-code to the output G-code failed");
+			    		m_print->set_status(95, "Running post-processing scripts");
+			    		run_post_process_scripts(m_export_path, m_print->config());
+			    		m_print->set_status(100, "G-code file exported to " + m_export_path);
+			    	} else {
+			    		m_print->set_status(100, "Slicing complete");
+			    	}
+					this->set_step_done(bspsGCodeFinalize);
+			    }
 		    }
 		} catch (CanceledException &ex) {
 			// Canceled, this is all right.
@@ -133,7 +141,7 @@ bool BackgroundSlicingProcess::stop()
 {
 	std::unique_lock<std::mutex> lck(m_mutex);
 	if (m_state == STATE_INITIAL) {
-		this->m_output_path.clear();
+//		this->m_export_path.clear();
 		return false;
 	}
 //	assert(this->running());
@@ -147,7 +155,7 @@ bool BackgroundSlicingProcess::stop()
 		// In the "Finished" or "Canceled" state. Reset the state to "Idle".
 		m_state = STATE_IDLE;
 	}
-	this->m_output_path.clear();
+//	this->m_export_path.clear();
 	return true;
 }
 
@@ -169,4 +177,53 @@ bool BackgroundSlicingProcess::apply(const Model &model, const DynamicPrintConfi
 	return invalidated;
 }
 
+// Set the output path of the G-code.
+void BackgroundSlicingProcess::schedule_export(const std::string &path)
+{ 
+	assert(m_export_path.empty());
+	if (! m_export_path.empty())
+		return;
+
+	// Guard against entering the export step before changing the export path.
+	tbb::mutex::scoped_lock lock(m_step_state_mutex);
+	this->invalidate_step(bspsGCodeFinalize);
+	m_export_path = path;
+}
+
+void BackgroundSlicingProcess::reset_export()
+{
+	assert(! this->running());
+	if (! this->running()) {
+		m_export_path.clear();
+		// invalidate_step expects the mutex to be locked.
+		tbb::mutex::scoped_lock lock(m_step_state_mutex);
+		this->invalidate_step(bspsGCodeFinalize);
+	}
+}
+
+void BackgroundSlicingProcess::set_step_started(BackgroundSlicingProcessStep step)
+{ 
+	m_step_state.set_started(step, m_step_state_mutex);
+	if (m_print->canceled())
+		throw CanceledException();
+}
+
+void BackgroundSlicingProcess::set_step_done(BackgroundSlicingProcessStep step)
+{ 
+	m_step_state.set_done(step, m_step_state_mutex);
+	if (m_print->canceled())
+		throw CanceledException();
+}
+
+bool BackgroundSlicingProcess::invalidate_step(BackgroundSlicingProcessStep step)
+{
+    bool invalidated = m_step_state.invalidate(step, m_step_state_mutex, [this](){ this->stop(); });
+    return invalidated;
+}
+
+bool BackgroundSlicingProcess::invalidate_all_steps()
+{ 
+	return m_step_state.invalidate_all(m_step_state_mutex, [this](){ this->stop(); });
+}
+
 }; // namespace Slic3r
diff --git a/src/slic3r/GUI/BackgroundSlicingProcess.hpp b/src/slic3r/GUI/BackgroundSlicingProcess.hpp
index 758acd682a..4265d1227f 100644
--- a/src/slic3r/GUI/BackgroundSlicingProcess.hpp
+++ b/src/slic3r/GUI/BackgroundSlicingProcess.hpp
@@ -6,6 +6,8 @@
 #include <mutex>
 #include <thread>
 
+#include "Print.hpp"
+
 namespace Slic3r {
 
 class DynamicPrintConfig;
@@ -13,6 +15,11 @@ class GCodePreviewData;
 class Model;
 class Print;
 
+// Print step IDs for keeping track of the print state.
+enum BackgroundSlicingProcessStep {
+    bspsGCodeFinalize, bspsCount,
+};
+
 // Support for the GUI background processing (Slicing and G-code generation).
 // As of now this class is not declared in Slic3r::GUI due to the Perl bindings limits.
 class BackgroundSlicingProcess
@@ -32,8 +39,6 @@ public:
 	// The wxCommandEvent is sent to the UI thread asynchronously without waiting for the event to be processed.
 	void set_finished_event(int event_id) { m_event_finished_id = event_id; }
 
-	// Set the output path of the G-code.
-	void set_output_path(const std::string &path) { m_output_path = path; }
 	// Start the background processing. Returns false if the background processing was already running.
 	bool start();
 	// Cancel the background processing. Returns false if the background processing was not running.
@@ -46,6 +51,13 @@ public:
 	// Apply config over the print. Returns false, if the new config values caused any of the already
 	// processed steps to be invalidated, therefore the task will need to be restarted.
 	bool apply(const Model &model, const DynamicPrintConfig &config);
+	// Set the export path of the G-code.
+	// Once the path is set, the G-code 
+	void schedule_export(const std::string &path);
+	// Clear m_export_path.
+	void reset_export();
+	// Once the G-code export is scheduled, the apply() methods will do nothing.
+	bool is_export_scheduled() const { return ! m_export_path.empty(); }
 
 	enum State {
 		// m_thread  is not running yet, or it did not reach the STATE_IDLE yet (it does not wait on the condition yet).
@@ -74,8 +86,11 @@ private:
 	Print 					   *m_print 			 = nullptr;
 	// Data structure, to which the G-code export writes its annotations.
 	GCodePreviewData 		   *m_gcode_preview_data = nullptr;
+	// Temporary G-code, there is one defined for the BackgroundSlicingProcess, differentiated from the other processes by a process ID.
 	std::string 				m_temp_output_path;
-	std::string 				m_output_path;
+	// Output path provided by the user. The output path may be set even if the slicing is running,
+	// but once set, it cannot be re-set.
+	std::string 				m_export_path;
 	// Thread, on which the background processing is executed. The thread will always be present
 	// and ready to execute the slicing process.
 	std::thread		 			m_thread;
@@ -84,6 +99,14 @@ private:
 	std::condition_variable		m_condition;
 	State 						m_state = STATE_INITIAL;
 
+    PrintState<BackgroundSlicingProcessStep, bspsCount>   	m_step_state;
+    mutable tbb::mutex                      				m_step_state_mutex;
+	void                set_step_started(BackgroundSlicingProcessStep step);
+	void                set_step_done(BackgroundSlicingProcessStep step);
+    bool 				is_step_done(BackgroundSlicingProcessStep step) const { return m_step_state.is_done(step); }
+	bool                invalidate_step(BackgroundSlicingProcessStep step);
+    bool                invalidate_all_steps();
+
 	// wxWidgets command ID to be sent to the platter to inform that the slicing is finished, and the G-code export will continue.
 	int 						m_event_sliced_id 	 = 0;
 	// wxWidgets command ID to be sent to the platter to inform that the task finished.
diff --git a/src/slic3r/GUI/GLCanvas3D.cpp b/src/slic3r/GUI/GLCanvas3D.cpp
index 5db94b95f6..9928e43343 100644
--- a/src/slic3r/GUI/GLCanvas3D.cpp
+++ b/src/slic3r/GUI/GLCanvas3D.cpp
@@ -5145,7 +5145,7 @@ void GLCanvas3D::_render_layer_editing_overlay() const
 #else
     int object_idx = int(volume->select_group_id / 1000000);
 #endif // ENABLE_EXTENDED_SELECTION
-    if ((int)m_print->objects().size() < object_idx)
+    if ((int)m_print->objects().size() <= object_idx)
         return;
 
     const PrintObject* print_object = m_print->get_object(object_idx);
diff --git a/src/slic3r/GUI/GUI.cpp b/src/slic3r/GUI/GUI.cpp
index 0e23585f7b..9e18cac020 100644
--- a/src/slic3r/GUI/GUI.cpp
+++ b/src/slic3r/GUI/GUI.cpp
@@ -261,18 +261,6 @@ void warning_catcher(wxWindow* parent, const wxString& message){
 	msg.ShowModal();
 }
 
-// Assign a Lambda to the print object to emit a wxWidgets Command with the provided ID
-// to deliver a progress status message.
-void set_print_callback_event(Print *print, int id)
-{
-	print->set_status_callback([id](int percent, const std::string &message){
-		wxCommandEvent event(id);
-		event.SetInt(percent);
-		event.SetString(message);
-        wxQueueEvent(wxGetApp().mainframe, event.Clone());
-	});
-}
-
 void create_combochecklist(wxComboCtrl* comboCtrl, std::string text, std::string items, bool initial_value)
 {
     if (comboCtrl == nullptr)
diff --git a/src/slic3r/GUI/GUI.hpp b/src/slic3r/GUI/GUI.hpp
index 3203fd8b25..cbc20c3c1a 100644
--- a/src/slic3r/GUI/GUI.hpp
+++ b/src/slic3r/GUI/GUI.hpp
@@ -80,10 +80,6 @@ void show_error_id(int id, const std::string& message);   // For Perl
 void show_info(wxWindow* parent, const wxString& message, const wxString& title);
 void warning_catcher(wxWindow* parent, const wxString& message);
 
-// Assign a Lambda to the print object to emit a wxWidgets Command with the provided ID
-// to deliver a progress status message.
-void set_print_callback_event(Print *print, int id);
-
 // Creates a wxCheckListBoxComboPopup inside the given wxComboCtrl, filled with the given text and items.
 // Items are all initialized to the given value.
 // Items must be separated by '|', for example "Item1|Item2|Item3", and so on.
diff --git a/src/slic3r/GUI/Plater.cpp b/src/slic3r/GUI/Plater.cpp
index e5668732ac..fa8368b4dc 100644
--- a/src/slic3r/GUI/Plater.cpp
+++ b/src/slic3r/GUI/Plater.cpp
@@ -63,10 +63,10 @@ namespace Slic3r {
 namespace GUI {
 
 
+wxDEFINE_EVENT(EVT_PROGRESS_BAR,      wxCommandEvent);
 wxDEFINE_EVENT(EVT_SLICING_COMPLETED, wxCommandEvent);
 wxDEFINE_EVENT(EVT_PROCESS_COMPLETED, wxCommandEvent);
 
-
 // Sidebar widgets
 
 // struct InfoBox : public wxStaticBox
@@ -741,9 +741,6 @@ struct Plater::priv
     std::vector<PlaterObject> objects;
 #endif // !ENABLE_EXTENDED_SELECTION
 
-    fs::path export_gcode_output_file;
-    fs::path send_gcode_file;
-
     // GUI elements
     wxNotebook *notebook;
     Sidebar *sidebar;
@@ -809,7 +806,7 @@ struct Plater::priv
 
     void on_notebook_changed(wxBookCtrlEvent&);
     void on_select_preset(wxCommandEvent&);
-    void on_progress_event();
+    void on_progress_event(wxCommandEvent&);
     void on_update_print_preview(wxCommandEvent&);
     void on_process_completed(wxCommandEvent&);
     void on_layer_editing_toggled(bool enable);
@@ -880,6 +877,14 @@ Plater::priv::priv(Plater *q, MainFrame *main_frame) :
     background_process.set_gcode_preview_data(&gcode_preview_data);
     background_process.set_sliced_event(EVT_SLICING_COMPLETED);
     background_process.set_finished_event(EVT_PROCESS_COMPLETED);
+    // Register progress callback from the Print class to the Platter.
+    print.set_status_callback([this](int percent, const std::string &message){
+        wxCommandEvent event(EVT_PROGRESS_BAR);
+        event.SetInt(percent);
+        event.SetString(message);
+        wxQueueEvent(this->q, event.Clone());
+    });
+    this->q->Bind(EVT_PROGRESS_BAR, &priv::on_progress_event, this);
 
     _3DScene::add_canvas(canvas3D);
     _3DScene::allow_multisample(canvas3D, GLCanvas3DManager::can_multisample());
@@ -902,7 +907,8 @@ Plater::priv::priv(Plater *q, MainFrame *main_frame) :
     _3DScene::enable_shader(canvas3D, true);
     _3DScene::enable_force_zoom_to_bed(canvas3D, true);
 
-    background_process_timer.Bind(wxEVT_TIMER, [this](wxTimerEvent &evt){ this->async_apply_config(); }, 0);
+    this->background_process_timer.SetOwner(this->q, 0);
+    this->q->Bind(wxEVT_TIMER, [this](wxTimerEvent &evt){ this->async_apply_config(); });
 
     auto *bed_shape = config->opt<ConfigOptionPoints>("bed_shape");
     _3DScene::set_bed_shape(canvas3D, bed_shape->values);
@@ -938,7 +944,7 @@ Plater::priv::priv(Plater *q, MainFrame *main_frame) :
     canvas3D->Bind(EVT_GLCANVAS_ARRANGE, [this](SimpleEvent&) { arrange(); });
 #if !ENABLE_EXTENDED_SELECTION
     canvas3D->Bind(EVT_GLCANVAS_ROTATE_OBJECT, [this](Event<int> &evt) { /*TODO: call rotate */ });
-    canvas3D->Bind(EVT_GLCANVAS_SCALE_UNIFORMLY, [this](SimpleEvent&) { scale(); });
+    canvas3D->Bind(EVT_GLCANVAS_SCALE_UNIFORMLY, [this](SimpleEvent&) { this->scale(); });
 #endif // !ENABLE_EXTENDED_SELECTION
     canvas3D->Bind(EVT_GLCANVAS_INCREASE_INSTANCES, [q](Event<int> &evt) { evt.data == 1 ? q->increase_instances() : q->decrease_instances(); });
     canvas3D->Bind(EVT_GLCANVAS_INSTANCE_MOVED, [this](SimpleEvent&) { update(); });
@@ -998,7 +1004,7 @@ void Plater::priv::update(bool force_autocenter)
     }
 
     // stop_background_process();   // TODO
-    print.reload_model_instances();
+//    print.reload_model_instances();
 
 #if !ENABLE_EXTENDED_SELECTION
     const auto selections = collect_selections();
@@ -1008,7 +1014,7 @@ void Plater::priv::update(bool force_autocenter)
     preview->reset_gcode_preview_data();
     preview->reload_print();
 
-    schedule_background_process();
+    this->schedule_background_process();
 }
 
 void Plater::priv::select_view(const std::string& direction)
@@ -1034,7 +1040,7 @@ void Plater::priv::update_ui_from_settings()
     // }
 }
 
-ProgressStatusBar*  Plater::priv::statusbar()
+ProgressStatusBar* Plater::priv::statusbar()
 {
     return main_frame->m_statusbar;
 }
@@ -1257,7 +1263,7 @@ std::unique_ptr<CheckboxFileDialog> Plater::priv::get_export_file(GUI::FileType
         case FT_GCODE:
             wildcard = file_wildcards[file_type];
         break;
-
+// FT_GCODE
         default:
             wildcard = file_wildcards[FT_MODEL];
         break;
@@ -1290,7 +1296,6 @@ std::unique_ptr<CheckboxFileDialog> Plater::priv::get_export_file(GUI::FileType
 
     fs::path path(dlg->GetPath());
     wxGetApp().app_config->update_last_output_dir(path.parent_path().string());
-    export_gcode_output_file = path;
 
     return dlg;
 }
@@ -1446,7 +1451,7 @@ void Plater::priv::object_list_changed()
     _3DScene::enable_toolbar_item(canvas3D, "arrange", have_objects);
 #endif // ENABLE_EXTENDED_SELECTION
 
-    const bool export_in_progress = !(export_gcode_output_file.empty() && send_gcode_file.empty());
+    const bool export_in_progress = this->background_process.is_export_scheduled(); // || ! send_gcode_file.empty());
     // XXX: is this right?
     const bool model_fits = _3DScene::check_volumes_outside_state(canvas3D, config) == ModelInstance::PVS_Inside;
 
@@ -1478,7 +1483,7 @@ void Plater::priv::remove(size_t obj_idx)
     objects.erase(objects.begin() + obj_idx);
 #endif // !ENABLE_EXTENDED_SELECTION
     model.delete_object(obj_idx);
-    print.delete_object(obj_idx);
+//    print.delete_object(obj_idx);
     // Delete object from Sidebar list
     sidebar->obj_list()->delete_object_from_list(obj_idx);
 
@@ -1504,7 +1509,7 @@ void Plater::priv::reset()
     objects.clear();
 #endif // !ENABLE_EXTENDED_SELECTION
     model.clear_objects();
-    print.clear_objects();
+//    print.clear_objects();
 
     // Delete all objects from list on c++ side
     sidebar->obj_list()->delete_all_objects_from_list();
@@ -1609,7 +1614,6 @@ void Plater::priv::split_object()
     if (new_objects.size() == 1)
     {
         Slic3r::GUI::warning_catcher(q, _(L("The selected object couldn't be split because it contains only one part.")));
-//        $self->schedule_background_process;
     }
     else
     {
@@ -1635,9 +1639,16 @@ void Plater::priv::schedule_background_process()
 
 void Plater::priv::async_apply_config()
 {
+    DynamicPrintConfig config = wxGetApp().preset_bundle->full_config();
+    BoundingBox     bed_box_2D = get_extents(Polygon::new_scale(config.opt<ConfigOptionPoints>("bed_shape")->values));
+    BoundingBoxf3   print_volume(unscale(bed_box_2D.min(0), bed_box_2D.min(1), 0.0), unscale(bed_box_2D.max(0), bed_box_2D.max(1), scale_(config.opt_float("max_print_height"))));
+    // Allow the objects to protrude below the print bed, only the part of the object above the print bed will be sliced.
+    print_volume.min(2) = -1e10;
+    this->q->model().update_print_volume_state(print_volume);
+
     // Apply new config to the possibly running background task.
     bool was_running = this->background_process.running();
-    bool invalidated = this->background_process.apply(this->q->model(), wxGetApp().preset_bundle->full_config());
+    bool invalidated = this->background_process.apply(this->q->model(), std::move(config));
     // Just redraw the 3D canvas without reloading the scene to consume the update of the layer height profile.
     if (Slic3r::_3DScene::is_layers_editing_enabled(this->canvas3D))
         this->canvas3D->Refresh();
@@ -1669,19 +1680,19 @@ void Plater::priv::async_apply_config()
 
 void Plater::priv::start_background_process()
 {
+	if (this->background_process.running())
+		return;
     // return if ! @{$self->{objects}} || $self->{background_slicing_process}->running;
-    // Don't start process thread if config is not valid.
-    std::string err = wxGetApp().preset_bundle->full_config().validate();
-    if (err.empty())
-        err = this->q->print().validate();
+    // Don't start process thread if Print is not valid.
+    std::string err = this->q->print().validate();
     if (! err.empty()) {
-        // $self->statusbar->SetStatusText(err);
-        return;
-    }   
-    // Copy the names of active presets into the placeholder parser.
-    wxGetApp().preset_bundle->export_selections(this->q->print().placeholder_parser());
-    // Start the background process.
-    this->background_process.start();
+        this->statusbar()->set_status_text(err);
+	} else {
+		// Copy the names of active presets into the placeholder parser.
+		wxGetApp().preset_bundle->export_selections(this->q->print().placeholder_parser());
+		// Start the background process.
+		this->background_process.start();
+	}
 }
 
 void Plater::priv::stop_background_process()
@@ -1795,9 +1806,10 @@ void Plater::priv::on_select_preset(wxCommandEvent &evt)
     wxGetApp().plater()->on_config_change(wxGetApp().preset_bundle->full_config());
 }
 
-void Plater::priv::on_progress_event()
+void Plater::priv::on_progress_event(wxCommandEvent &evt)
 {
-    // TODO
+    this->statusbar()->set_progress(evt.GetInt());
+    this->statusbar()->set_status_text(evt.GetString() + wxString::FromUTF8("…"));
 }
 
 void Plater::priv::on_update_print_preview(wxCommandEvent &)
@@ -1813,62 +1825,25 @@ void Plater::priv::on_update_print_preview(wxCommandEvent &)
 #endif // !ENABLE_EXTENDED_SELECTION
 }
 
-void Plater::priv::on_process_completed(wxCommandEvent &)
+void Plater::priv::on_process_completed(wxCommandEvent &evt)
 {
     // Stop the background task, wait until the thread goes into the "Idle" state.
     // At this point of time the thread should be either finished or canceled,
     // so the following call just confirms, that the produced data were consumed.
     this->background_process.stop();
-    //$self->statusbar->ResetCancelCallback();
-    //$self->statusbar->StopBusy;
-    //$self->statusbar->SetStatusText("");
-    
-/*
-    my $message;
-    my $send_gcode = 0;
-    my $do_print = 0;
-#    print "Process completed, message: ", $message, "\n";
-    if (defined($result)) {
-        $message = L("Export failed");
-    } else {
-        # G-code file exported successfully.
-        if ($self->{print_file}) {
-            $message = L("File added to print queue");
-            $do_print = 1;
-        } elsif ($self->{send_gcode_file}) {
-            $message = L("Sending G-code file to the Printer Host ...");
-            $send_gcode = 1;
-        } elsif (defined $self->{export_gcode_output_file}) {
-            $message = L("G-code file exported to ") . $self->{export_gcode_output_file};
-        } else {
-            $message = L("Slicing complete");
-        }
+    this->statusbar()->reset_cancel_callback();
+    this->statusbar()->stop_busy();
+  
+    bool success = evt.GetInt();
+    // Reset the "export G-code path" name, so that the automatic background processing will be enabled again.
+    this->background_process.reset_export();
+    if (! success) {
+        wxString message = evt.GetString();
+        if (message.IsEmpty())
+            message = _(L("Export failed"));
+        this->statusbar()->set_status_text(message);
     }
-    $self->{export_gcode_output_file} = undef;
-    wxTheApp->notify($message);
-    
-    $self->do_print if $do_print;
 
-    # Send $self->{send_gcode_file} to OctoPrint.
-    if ($send_gcode) {
-        my $host = Slic3r::PrintHost::get_print_host($self->{config});
-        if ($host->send_gcode($self->{send_gcode_file})) {
-            $message = L("Upload to host finished.");
-        } else {
-            $message = "";
-        }
-    }
-*/
-
-    // As of now, the BackgroundProcessing thread posts status bar update messages to a queue on the MainFrame.pm,
-    // but the "Processing finished" message is posted to this window.
-    // Delay the following status bar update, so it will be called later than what is received by MainFrame.pm.
-    //wxTheApp->CallAfter(sub {
-//        $self->statusbar->SetStatusText($message);
-//    });
-
-    //$self->{print_file} = undef;
-    //$self->{send_gcode_file} = undef;
     //$self->print_info_box_show(1);
 
     // this updates buttons status
@@ -2209,9 +2184,9 @@ void Plater::increase_instances(size_t num)
         Vec3d offset_vec = model_instance->get_offset() + Vec3d(offset, offset, 0.0);
         model_object->add_instance(offset_vec, model_instance->get_scaling_factor(), model_instance->get_rotation());
 #if ENABLE_EXTENDED_SELECTION
-        p->print.get_object(obj_idx)->add_copy(Slic3r::to_2d(offset_vec));
+//        p->print.get_object(obj_idx)->add_copy(Slic3r::to_2d(offset_vec));
 #else
-        p->print.get_object(*obj_idx)->add_copy(Slic3r::to_2d(offset_vec));
+//        p->print.get_object(*obj_idx)->add_copy(Slic3r::to_2d(offset_vec));
 #endif // ENABLE_EXTENDED_SELECTION
     }
 
@@ -2254,9 +2229,9 @@ void Plater::decrease_instances(size_t num)
         for (size_t i = 0; i < num; i++) {
             model_object->delete_last_instance();
 #if ENABLE_EXTENDED_SELECTION
-            p->print.get_object(obj_idx)->delete_last_copy();
+//            p->print.get_object(obj_idx)->delete_last_copy();
 #else
-            p->print.get_object(*obj_idx)->delete_last_copy();
+//            p->print.get_object(*obj_idx)->delete_last_copy();
 #endif // ENABLE_EXTENDED_SELECTION
         }
 #if ENABLE_EXTENDED_SELECTION
@@ -2281,8 +2256,7 @@ void Plater::decrease_instances(size_t num)
 #endif // ENABLE_EXTENDED_SELECTION
 
     p->selection_changed();
-
-    // $self->schedule_background_process;
+    this->p->schedule_background_process();
 }
 
 void Plater::set_number_of_copies(size_t num)
@@ -2307,38 +2281,35 @@ void Plater::set_number_of_copies(size_t num)
         decrease_instances(-diff);
 }
 
-fs::path Plater::export_gcode(const fs::path &output_path)
+void Plater::export_gcode(fs::path output_path)
 {
 #if ENABLE_EXTENDED_SELECTION
-    if (p->model.objects.empty()) { return ""; }
+    if (p->model.objects.empty())
+        return;
 #else
-    if (p->objects.empty()) { return ""; }
+    if (p->objects.empty())
+        return;
 #endif // ENABLE_EXTENDED_SELECTION
 
-    if (! p->export_gcode_output_file.empty()) {
+    if (this->p->background_process.is_export_scheduled()) {
         GUI::show_error(this, _(L("Another export job is currently running.")));
-        return "";
+        return;
     }
 
     std::string err = wxGetApp().preset_bundle->full_config().validate();
-    if (err.empty()) {
+    if (err.empty())
         err = p->print.validate();
-    }
     if (! err.empty()) {
         // The config is not valid
         GUI::show_error(this, _(err));
-        return fs::path();
+        return;
     }
 
     // Copy the names of active presets into the placeholder parser.
     wxGetApp().preset_bundle->export_selections(p->print.placeholder_parser());
 
     // select output file
-    if (! output_path.empty()) {
-        p->export_gcode_output_file = fs::path(p->print.output_filepath(output_path.string()));
-        // FIXME: ^ errors to handle?
-    } else {
-
+    if (output_path.empty()) {
         // XXX: take output path from CLI opts? Ancient Slic3r versions used to do that...
 
         // If possible, remove accents from accented latin characters.
@@ -2355,13 +2326,17 @@ fs::path Plater::export_gcode(const fs::path &output_path)
             wxFD_SAVE | wxFD_OVERWRITE_PROMPT
         );
 
-        if (dlg.ShowModal() != wxID_OK) { return ""; }
-        fs::path path(dlg.GetPath());
-        wxGetApp().app_config->update_last_output_dir(path.parent_path().string());
-        p->export_gcode_output_file = path;
+        if (dlg.ShowModal() == wxID_OK) {
+            fs::path path(dlg.GetPath());
+            wxGetApp().app_config->update_last_output_dir(path.parent_path().string());
+            output_path = path;
+        }
     }
 
-    return p->export_gcode_output_file;
+    if (! output_path.empty()) {
+        this->p->background_process.schedule_export(p->print.output_filepath(output_path.string()));
+        this->p->background_process.start();
+    }
 }
 
 void Plater::export_stl()
@@ -2452,7 +2427,7 @@ void Plater::reslice()
 
 void Plater::send_gcode()
 {
-    p->send_gcode_file = export_gcode();
+//    p->send_gcode_file = export_gcode();
 }
 
 void Plater::on_extruders_change(int num_extruders)
diff --git a/src/slic3r/GUI/Plater.hpp b/src/slic3r/GUI/Plater.hpp
index 544d4672ad..ab912f5e3a 100644
--- a/src/slic3r/GUI/Plater.hpp
+++ b/src/slic3r/GUI/Plater.hpp
@@ -118,7 +118,7 @@ public:
     void set_number_of_copies(size_t num);
 
     // Note: empty path means "use the default"
-    boost::filesystem::path export_gcode(const boost::filesystem::path &output_path = boost::filesystem::path());
+    void export_gcode(boost::filesystem::path output_path = boost::filesystem::path());
     void export_stl();
     void export_amf();
     void export_3mf();
diff --git a/src/slic3r/GUI/Preset.cpp b/src/slic3r/GUI/Preset.cpp
index 1a6063dc73..07410e325f 100644
--- a/src/slic3r/GUI/Preset.cpp
+++ b/src/slic3r/GUI/Preset.cpp
@@ -884,7 +884,8 @@ bool PresetCollection::update_dirty_ui(wxBitmapComboBox *ui)
         std::string   old_label    = ui->GetString(ui_id).utf8_str().data();
         std::string   preset_name  = Preset::remove_suffix_modified(old_label);
         const Preset *preset       = this->find_preset(preset_name, false);
-        assert(preset != nullptr);
+//		The old_label could be the "----- system presets ------" or the "------- user presets --------" separator.
+//      assert(preset != nullptr);
 		if (preset != nullptr) {
 			std::string new_label = preset->is_dirty ? preset->name + g_suffix_modified : preset->name;
 			if (old_label != new_label)
diff --git a/src/slic3r/GUI/ProgressStatusBar.cpp b/src/slic3r/GUI/ProgressStatusBar.cpp
index 45c3aaa76b..44a7b06c51 100644
--- a/src/slic3r/GUI/ProgressStatusBar.cpp
+++ b/src/slic3r/GUI/ProgressStatusBar.cpp
@@ -53,8 +53,8 @@ ProgressStatusBar::ProgressStatusBar(wxWindow *parent, int id):
     });
 
     m_cancelbutton->Bind(wxEVT_BUTTON, [this](const wxCommandEvent&) {
-        if(m_cancel_cb) m_cancel_cb();
-        m_perl_cancel_callback.call();
+        if (m_cancel_cb) 
+            m_cancel_cb();
         m_cancelbutton->Hide();
     });
 }
@@ -136,7 +136,17 @@ void ProgressStatusBar::embed(wxFrame *frame)
 
 void ProgressStatusBar::set_status_text(const wxString& txt)
 {
-    self->SetStatusText(wxString::FromUTF8(txt.c_str()));
+	self->SetStatusText(txt);
+}
+
+void ProgressStatusBar::set_status_text(const std::string& txt)
+{ 
+    this->set_status_text(txt.c_str());
+}
+
+void ProgressStatusBar::set_status_text(const char *txt)
+{ 
+    this->set_status_text(wxString::FromUTF8(txt));
 }
 
 void ProgressStatusBar::show_cancel_button()
diff --git a/src/slic3r/GUI/ProgressStatusBar.hpp b/src/slic3r/GUI/ProgressStatusBar.hpp
index 57ec9ae2bb..f33a70ed30 100644
--- a/src/slic3r/GUI/ProgressStatusBar.hpp
+++ b/src/slic3r/GUI/ProgressStatusBar.hpp
@@ -4,8 +4,6 @@
 #include <memory>
 #include <functional>
 
-#include "callback.hpp"
-
 class wxTimer;
 class wxGauge;
 class wxButton;
@@ -22,7 +20,8 @@ namespace Slic3r {
  * of the Slicer main window. It consists of a message area to the left and a
  * progress indication area to the right with an optional cancel button.
  */
-class ProgressStatusBar {
+class ProgressStatusBar 
+{
     wxStatusBar *self;      // we cheat! It should be the base class but: perl!
     wxTimer *m_timer;
     wxGauge *m_prog;
@@ -35,25 +34,26 @@ public:
     ProgressStatusBar(wxWindow *parent = nullptr, int id = -1);
     ~ProgressStatusBar();
 
-    int  get_progress() const;
-    void set_progress(int);
-    int  get_range() const;
-    void set_range(int = 100);
-    void show_progress(bool);
-    void start_busy(int = 100);
-    void stop_busy();
+    int         get_progress() const;
+    void        set_progress(int);
+    int         get_range() const;
+    void        set_range(int = 100);
+    void        show_progress(bool);
+    void        start_busy(int = 100);
+    void        stop_busy();
     inline bool is_busy() const { return m_busy; }
-    void set_cancel_callback(CancelFn = CancelFn());
-    inline void remove_cancel_callback() { set_cancel_callback(); }
-    void run(int rate);
-    void embed(wxFrame *frame = nullptr);
-    void set_status_text(const wxString& txt);
+    void        set_cancel_callback(CancelFn = CancelFn());
+    inline void reset_cancel_callback() { set_cancel_callback(); }
+    void        run(int rate);
+    void        embed(wxFrame *frame = nullptr);
+    void        set_status_text(const wxString& txt);
+    void        set_status_text(const std::string& txt);
+    void        set_status_text(const char *txt);
 
     // Temporary methods to satisfy Perl side
-    void show_cancel_button();
-    void hide_cancel_button();
+    void        show_cancel_button();
+    void        hide_cancel_button();
 
-    PerlCallback m_perl_cancel_callback;
 private:
     bool m_busy = false;
     CancelFn m_cancel_cb;
diff --git a/xs/xsp/Print.xsp b/xs/xsp/Print.xsp
index 0424d8b7b2..39708ca7a9 100644
--- a/xs/xsp/Print.xsp
+++ b/xs/xsp/Print.xsp
@@ -185,22 +185,6 @@ _constant()
             }
         %};
 
-    void export_gcode_with_preview_data(char *path_template, GCodePreviewData *preview_data) %code%{
-            try {
-                THIS->export_gcode(path_template, preview_data);
-            } catch (std::exception& e) {
-                croak(e.what());
-            }
-        %};
-
-    void export_gcode(char *path_template) %code%{
-            try {
-                THIS->export_gcode(path_template, nullptr);
-            } catch (std::exception& e) {
-                croak(e.what());
-            }
-        %};
-
     void export_png(char *path) %code%{
             try {
                 THIS->export_png(path);