diff --git a/lib/Slic3r/GUI/Plater.pm b/lib/Slic3r/GUI/Plater.pm
index 166a21a69..4677f59fd 100644
--- a/lib/Slic3r/GUI/Plater.pm
+++ b/lib/Slic3r/GUI/Plater.pm
@@ -565,8 +565,9 @@ sub rotate {
         $_->set_rotation($new_angle) for @{ $model_object->instances };
         $model_object->update_bounding_box;
         
-        # update print
+        # update print and start background processing
         $self->{print}->add_model_object($model_object, $obj_idx);
+        $self->start_background_process;
         
         $object->transform_thumbnail($self->{model}, $obj_idx);
     }
@@ -600,8 +601,9 @@ sub changescale {
         $_->set_scaling_factor($scale) for @{ $model_object->instances };
         $model_object->update_bounding_box;
         
-        # update print
+        # update print and start background processing
         $self->{print}->add_model_object($model_object, $obj_idx);
+        $self->start_background_process;
         
         $object->transform_thumbnail($self->{model}, $obj_idx);
     }
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index 56664fb54..f2b1332d0 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -219,9 +219,6 @@ sub add_model_object {
     # apply config to print object
     $o->config->apply($self->default_object_config);
     $o->config->apply_dynamic($object_config);
-    
-    $self->invalidate_step(STEP_SKIRT);
-    $self->invalidate_step(STEP_BRIM);
 }
 
 sub reload_object {
@@ -328,12 +325,17 @@ sub extruders {
 sub init_extruders {
     my $self = shift;
     
+    return if $self->step_done(STEP_INIT_EXTRUDERS);
+    $self->set_step_started(STEP_INIT_EXTRUDERS);
+    
     # enforce tall skirt if using ooze_prevention
     # FIXME: this is not idempotent (i.e. switching ooze_prevention off will not revert skirt settings)
     if ($self->config->ooze_prevention && @{$self->extruders} > 1) {
         $self->config->set('skirt_height', -1);
         $self->config->set('skirts', 1) if $self->config->skirts == 0;
     }
+    
+    $self->set_step_done(STEP_INIT_EXTRUDERS);
 }
 
 # this value is not supposed to be compared with $layer->id
@@ -380,131 +382,11 @@ sub _simplify_slices {
 sub process {
     my ($self) = @_;
     
-    my $status_cb = $self->status_cb // sub {};
-    
-    my $print_step = sub {
-        my ($step, $cb) = @_;
-        if (!$self->step_done($step)) {
-            $self->set_step_started($step);
-            $cb->();
-            $self->set_step_done($step);
-        }
-    };
-    my $object_step = sub {
-        my ($step, $cb) = @_;
-        for my $obj_idx (0..($self->object_count - 1)) {
-            my $object = $self->objects->[$obj_idx];
-            if (!$object->step_done($step)) {
-                $object->set_step_started($step);
-                $cb->($obj_idx);
-                $object->set_step_done($step);
-            }
-        }
-    };
-    
-    # STEP_INIT_EXTRUDERS
-    $print_step->(STEP_INIT_EXTRUDERS, sub {
-        $self->init_extruders;
-    });
-    
-    # STEP_SLICE
-    # skein the STL into layers
-    # each layer has surfaces with holes
-    $status_cb->(10, "Processing triangulated mesh");
-    $object_step->(STEP_SLICE, sub {
-        $self->objects->[$_[0]]->slice;
-    });
-    
-    die "No layers were detected. You might want to repair your STL file(s) or check their size and retry.\n"
-        if !grep @{$_->layers}, @{$self->objects};
-    
-    # make perimeters
-    # this will add a set of extrusion loops to each layer
-    # as well as generate infill boundaries
-    $status_cb->(20, "Generating perimeters");
-    $object_step->(STEP_PERIMETERS, sub {
-        $self->objects->[$_[0]]->make_perimeters;
-    });
-    
-    $status_cb->(30, "Preparing infill");
-    $object_step->(STEP_PREPARE_INFILL, sub {
-        my $object = $self->objects->[$_[0]];
-        
-        # this will assign a type (top/bottom/internal) to $layerm->slices
-        # and transform $layerm->fill_surfaces from expolygon 
-        # to typed top/bottom/internal surfaces;
-        $object->detect_surfaces_type;
-    
-        # decide what surfaces are to be filled
-        $_->prepare_fill_surfaces for map @{$_->regions}, @{$object->layers};
-    
-        # this will detect bridges and reverse bridges
-        # and rearrange top/bottom/internal surfaces
-        $object->process_external_surfaces;
-    
-        # detect which fill surfaces are near external layers
-        # they will be split in internal and internal-solid surfaces
-        $object->discover_horizontal_shells;
-        $object->clip_fill_surfaces;
-        
-        # the following step needs to be done before combination because it may need
-        # to remove only half of the combined infill
-        $object->bridge_over_infill;
-    
-        # combine fill surfaces to honor the "infill every N layers" option
-        $object->combine_infill;
-    });
-    
-    # this will generate extrusion paths for each layer
-    $status_cb->(70, "Infilling layers");
-    $object_step->(STEP_INFILL, sub {
-        my $object = $self->objects->[$_[0]];
-        
-        Slic3r::parallelize(
-            threads => $self->config->threads,
-            items => sub {
-                my @items = ();  # [layer_id, region_id]
-                for my $region_id (0 .. ($self->regions_count-1)) {
-                    push @items, map [$_, $region_id], 0..($object->layer_count - 1);
-                }
-                @items;
-            },
-            thread_cb => sub {
-                my $q = shift;
-                while (defined (my $obj_layer = $q->dequeue)) {
-                    my ($i, $region_id) = @$obj_layer;
-                    my $layerm = $object->layers->[$i]->regions->[$region_id];
-                    $layerm->fills->clear;
-                    $layerm->fills->append( $object->fill_maker->make_fill($layerm) );
-                }
-            },
-            collect_cb => sub {},
-            no_threads_cb => sub {
-                foreach my $layerm (map @{$_->regions}, @{$object->layers}) {
-                    $layerm->fills->clear;
-                    $layerm->fills->append($object->fill_maker->make_fill($layerm));
-                }
-            },
-        );
-    
-        ### we could free memory now, but this would make this step not idempotent
-        ### $_->fill_surfaces->clear for map @{$_->regions}, @{$object->layers};
-    });
-    
-    # generate support material
-    $status_cb->(85, "Generating support material") if $self->has_support_material;
-    $object_step->(STEP_SUPPORTMATERIAL, sub {
-        $self->objects->[$_[0]]->generate_support_material;
-    });
-    
-    # make skirt
-    $status_cb->(88, "Generating skirt/brim");
-    $print_step->(STEP_SKIRT, sub {
-        $self->make_skirt;
-    });
-    $print_step->(STEP_BRIM, sub {
-        $self->make_brim;  # must come after make_skirt
-    });
+    $_->make_perimeters for @{$self->objects};
+    $_->infill for @{$self->objects};
+    $_->generate_support_material for @{$self->objects};
+    $self->make_skirt;
+    $self->make_brim;  # must come after make_skirt
     
     # time to make some statistics
     if (0) {
@@ -639,6 +521,15 @@ EOF
 sub make_skirt {
     my $self = shift;
     
+    # prerequisites
+    $_->make_perimeters for @{$self->objects};
+    $_->infill for @{$self->objects};
+    $_->generate_support_material for @{$self->objects};
+    
+    return if $self->step_done(STEP_SKIRT);
+    $self->set_step_started(STEP_SKIRT);
+    $self->status_cb->(88, "Generating skirt/brim");
+    
     # since this method must be idempotent, we clear skirt paths *before*
     # checking whether we need to generate them
     $self->skirt->clear;
@@ -749,11 +640,23 @@ sub make_skirt {
     }
     
     $self->skirt->reverse;
+    
+    $self->set_step_done(STEP_SKIRT);
 }
 
 sub make_brim {
     my $self = shift;
     
+    # prerequisites
+    $_->make_perimeters for @{$self->objects};
+    $_->infill for @{$self->objects};
+    $_->generate_support_material for @{$self->objects};
+    $self->make_skirt;
+    
+    return if $self->step_done(STEP_BRIM);
+    $self->set_step_started(STEP_BRIM);
+    $self->status_cb->(88, "Generating skirt/brim");
+    
     # since this method must be idempotent, we clear brim paths *before*
     # checking whether we need to generate them
     $self->brim->clear;
@@ -818,6 +721,8 @@ sub make_brim {
             height          => $first_layer_height,
         ),
     ), reverse @{union_pt_chained(\@loops)});
+    
+    $self->set_step_done(STEP_BRIM);
 }
 
 sub write_gcode {
diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index f372ecda1..878544706 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -97,7 +97,10 @@ sub bounding_box {
 # this should be idempotent
 sub slice {
     my $self = shift;
-    my %params = @_;
+    
+    return if $self->step_done(STEP_SLICE);
+    $self->set_step_started(STEP_SLICE);
+    $self->print->status_cb->(10, "Processing triangulated mesh");
     
     # init layers
     {
@@ -357,6 +360,11 @@ sub slice {
     if ($self->print->config->resolution) {
         $self->_simplify_slices(scale($self->print->config->resolution));
     }
+    
+    die "No layers were detected. You might want to repair your STL file(s) or check their size and retry.\n"
+        if !@{$self->layers};
+    
+    $self->set_step_done(STEP_SLICE);
 }
 
 sub _slice_region {
@@ -394,6 +402,14 @@ sub _slice_region {
 sub make_perimeters {
     my $self = shift;
     
+    # prerequisites
+    $self->print->init_extruders;
+    $self->slice;
+    
+    return if $self->step_done(STEP_PERIMETERS);
+    $self->set_step_started(STEP_PERIMETERS);
+    $self->print->status_cb->(20, "Generating perimeters");
+    
     # compare each layer to the one below, and mark those slices needing
     # one additional inner perimeter, like the top of domed objects-
     
@@ -473,6 +489,125 @@ sub make_perimeters {
     # we only need the max resolution for perimeters
     ### This makes this method not-idempotent, so we keep it disabled for now.
     ###$self->_simplify_slices(&Slic3r::SCALED_RESOLUTION);
+    
+    $self->set_step_done(STEP_PERIMETERS);
+}
+
+sub prepare_infill {
+    my ($self) = @_;
+    
+    # prerequisites
+    $self->make_perimeters;
+    
+    return if $self->step_done(STEP_PREPARE_INFILL);
+    $self->set_step_started(STEP_PREPARE_INFILL);
+    $self->print->status_cb->(30, "Preparing infill");
+    
+    # this will assign a type (top/bottom/internal) to $layerm->slices
+    # and transform $layerm->fill_surfaces from expolygon 
+    # to typed top/bottom/internal surfaces;
+    $self->detect_surfaces_type;
+
+    # decide what surfaces are to be filled
+    $_->prepare_fill_surfaces for map @{$_->regions}, @{$self->layers};
+
+    # this will detect bridges and reverse bridges
+    # and rearrange top/bottom/internal surfaces
+    $self->process_external_surfaces;
+
+    # detect which fill surfaces are near external layers
+    # they will be split in internal and internal-solid surfaces
+    $self->discover_horizontal_shells;
+    $self->clip_fill_surfaces;
+    
+    # the following step needs to be done before combination because it may need
+    # to remove only half of the combined infill
+    $self->bridge_over_infill;
+
+    # combine fill surfaces to honor the "infill every N layers" option
+    $self->combine_infill;
+    
+    $self->set_step_done(STEP_PREPARE_INFILL);
+}
+
+sub infill {
+    my ($self) = @_;
+    
+    # prerequisites
+    $self->prepare_infill;
+    
+    return if $self->step_done(STEP_INFILL);
+    $self->set_step_started(STEP_INFILL);
+    $self->print->status_cb->(70, "Infilling layers");
+    
+    Slic3r::parallelize(
+        threads => $self->print->config->threads,
+        items => sub {
+            my @items = ();  # [layer_id, region_id]
+            for my $region_id (0 .. ($self->print->regions_count-1)) {
+                push @items, map [$_, $region_id], 0..($self->layer_count - 1);
+            }
+            @items;
+        },
+        thread_cb => sub {
+            my $q = shift;
+            while (defined (my $obj_layer = $q->dequeue)) {
+                my ($i, $region_id) = @$obj_layer;
+                my $layerm = $self->layers->[$i]->regions->[$region_id];
+                $layerm->fills->clear;
+                $layerm->fills->append( $self->fill_maker->make_fill($layerm) );
+            }
+        },
+        collect_cb => sub {},
+        no_threads_cb => sub {
+            foreach my $layerm (map @{$_->regions}, @{$self->layers}) {
+                $layerm->fills->clear;
+                $layerm->fills->append($self->fill_maker->make_fill($layerm));
+            }
+        },
+    );
+
+    ### we could free memory now, but this would make this step not idempotent
+    ### $_->fill_surfaces->clear for map @{$_->regions}, @{$object->layers};
+    
+    $self->set_step_done(STEP_INFILL);
+}
+
+sub generate_support_material {
+    my $self = shift;
+    
+    # prerequisites
+    $self->print->init_extruders;
+    $self->slice;
+    
+    return if $self->step_done(STEP_SUPPORTMATERIAL);
+    $self->set_step_started(STEP_SUPPORTMATERIAL);
+    $self->print->status_cb->(85, "Generating support material");
+    
+    $self->clear_support_layers;
+    
+    return unless ($self->config->support_material || $self->config->raft_layers > 0)
+        && scalar(@{$self->layers}) >= 2;
+    
+    my $first_layer_flow = Slic3r::Flow->new_from_width(
+        width               => ($self->config->first_layer_extrusion_width || $self->config->support_material_extrusion_width),
+        role                => FLOW_ROLE_SUPPORT_MATERIAL,
+        nozzle_diameter     => $self->print->config->nozzle_diameter->[ $self->config->support_material_extruder-1 ]
+                                // $self->print->config->nozzle_diameter->[0],
+        layer_height        => $self->config->get_abs_value('first_layer_height'),
+        bridge_flow_ratio   => 0,
+    );
+    
+    my $s = Slic3r::Print::SupportMaterial->new(
+        print_config        => $self->print->config,
+        object_config       => $self->config,
+        first_layer_flow    => $first_layer_flow,
+        flow                => $self->support_material_flow,
+        interface_flow      => $self->support_material_flow(FLOW_ROLE_SUPPORT_MATERIAL_INTERFACE),
+    );
+    $s->generate($self);
+    
+    $self->set_step_done(STEP_SUPPORTMATERIAL);
 }
 
 sub detect_surfaces_type {
@@ -1005,33 +1140,6 @@ sub combine_infill {
     }
 }
 
-sub generate_support_material {
-    my $self = shift;
-    
-    $self->clear_support_layers;
-    
-    return unless ($self->config->support_material || $self->config->raft_layers > 0)
-        && scalar(@{$self->layers}) >= 2;
-    
-    my $first_layer_flow = Slic3r::Flow->new_from_width(
-        width               => ($self->config->first_layer_extrusion_width || $self->config->support_material_extrusion_width),
-        role                => FLOW_ROLE_SUPPORT_MATERIAL,
-        nozzle_diameter     => $self->print->config->nozzle_diameter->[ $self->config->support_material_extruder-1 ]
-                                // $self->print->config->nozzle_diameter->[0],
-        layer_height        => $self->config->get_abs_value('first_layer_height'),
-        bridge_flow_ratio   => 0,
-    );
-    
-    my $s = Slic3r::Print::SupportMaterial->new(
-        print_config        => $self->print->config,
-        object_config       => $self->config,
-        first_layer_flow    => $first_layer_flow,
-        flow                => $self->support_material_flow,
-        interface_flow      => $self->support_material_flow(FLOW_ROLE_SUPPORT_MATERIAL_INTERFACE),
-    );
-    $s->generate($self);
-}
-
 sub _simplify_slices {
     my ($self, $distance) = @_;
     
diff --git a/xs/src/Print.cpp b/xs/src/Print.cpp
index 90d758b6d..67663ef44 100644
--- a/xs/src/Print.cpp
+++ b/xs/src/Print.cpp
@@ -333,11 +333,15 @@ Print::get_object(size_t idx)
 }
 
 PrintObject*
-Print::add_object(ModelObject *model_object,
-        const BoundingBoxf3 &modobj_bbox)
+Print::add_object(ModelObject *model_object, const BoundingBoxf3 &modobj_bbox)
 {
     PrintObject *object = new PrintObject(this, model_object, modobj_bbox);
     objects.push_back(object);
+    
+    // invalidate steps
+    this->invalidate_step(psSkirt);
+    this->invalidate_step(psBrim);
+    
     return object;
 }
 
@@ -347,6 +351,9 @@ Print::set_new_object(size_t idx, ModelObject *model_object, const BoundingBoxf3
     if (idx >= this->objects.size()) throw "bad idx";
 
     PrintObjectPtrs::iterator old_it = this->objects.begin() + idx;
+    // before deleting object, invalidate all of its steps in order to 
+    // invalidate all of the dependent ones in Print
+    (*old_it)->invalidate_all_steps();
     delete *old_it;
 
     PrintObject *object = new PrintObject(this, model_object, modobj_bbox);