diff --git a/lib/Slic3r.pm b/lib/Slic3r.pm
index b345f0d7d..17acd0e48 100644
--- a/lib/Slic3r.pm
+++ b/lib/Slic3r.pm
@@ -143,6 +143,7 @@ sub thread_cleanup {
     *Slic3r::Polygon::DESTROY               = sub {};
     *Slic3r::Polyline::DESTROY              = sub {};
     *Slic3r::Polyline::Collection::DESTROY  = sub {};
+    *Slic3r::Print::State::DESTROY          = sub {};
     *Slic3r::Surface::DESTROY               = sub {};
     *Slic3r::Surface::Collection::DESTROY   = sub {};
     *Slic3r::TriangleMesh::DESTROY          = sub {};
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index b68758587..5c30f84a4 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -9,6 +9,7 @@ use Slic3r::Geometry qw(X Y Z X1 Y1 X2 Y2 MIN MAX PI scale unscale move_points c
     convex_hull);
 use Slic3r::Geometry::Clipper qw(diff_ex union_ex union_pt intersection_ex intersection offset
     offset2 union union_pt_chained JT_ROUND JT_SQUARE);
+use Slic3r::Print::State ':steps';
 
 has 'config'                 => (is => 'rw', default => sub { Slic3r::Config->new_from_defaults }, trigger => \&init_config);
 has 'extra_variables'        => (is => 'rw', default => sub {{}});
@@ -19,6 +20,7 @@ has 'regions'                => (is => 'rw', default => sub {[]});
 has 'support_material_flow'  => (is => 'rw');
 has 'first_layer_support_material_flow' => (is => 'rw');
 has 'has_support_material'   => (is => 'lazy');
+has '_state'                 => (is => 'ro', default => sub { Slic3r::Print::State->new });
 
 # ordered collection of extrusion paths to build skirt loops
 has 'skirt' => (is => 'rw', default => sub { Slic3r::ExtrusionPath::Collection->new });
@@ -135,19 +137,29 @@ sub add_model_object {
             @{$self->extra_variables}{qw(input_filename input_filename_base)} = parse_filename($input_file);
         }
     }
-    # TODO: invalidate skirt and brim
+    
+    $self->_state->invalidate(STEP_SKIRT);
+    $self->_state->invalidate(STEP_BRIM);
 }
 
 sub delete_object {
     my ($self, $obj_idx) = @_;
+    
     splice @{$self->objects}, $obj_idx, 1;
-    # TODO: invalidate skirt and brim
+    # TODO: purge unused regions
+    
+    $self->_state->invalidate(STEP_SKIRT);
+    $self->_state->invalidate(STEP_BRIM);
 }
 
 sub delete_all_objects {
     my ($self) = @_;
+    
     @{$self->objects} = ();
-    # TODO: invalidate skirt and brim
+    @{$self->regions} = ();
+    
+    $self->_state->invalidate(STEP_SKIRT);
+    $self->_state->invalidate(STEP_BRIM);
 }
 
 sub validate {
@@ -324,108 +336,132 @@ sub _simplify_slices {
 sub process {
     my ($self) = @_;
     
-    $self->init_extruders;
     my $status_cb = $self->status_cb // sub {};
     
+    my $print_step = sub {
+        my ($step, $cb) = @_;
+        if (!$self->_state->done($step)) {
+            $self->_state->set_started($step);
+            $cb->();
+            ### Re-enable this for step-based slicing:
+            ### $self->_state->set_done($step);
+        }
+    };
+    my $object_step = sub {
+        my ($step, $cb) = @_;
+        for my $obj_idx (0..$#{$self->objects}) {
+            my $object = $self->objects->[$obj_idx];
+            if (!$object->_state->done($step)) {
+                $object->_state->set_started($step);
+                $cb->($obj_idx);
+                ### Re-enable this for step-based slicing:
+                ### $object->_state->set_done($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");
-    $_->slice for @{$self->objects};
+    $object_step->(STEP_SLICE, sub {
+        $self->objects->[$_[0]]->slice;
+    });
     
-    # remove empty layers and abort if there are no more
-    # as some algorithms assume all objects have at least one layer
-    # note: this will change object indexes
-    @{$self->objects} = grep @{$_->layers}, @{$self->objects};
     die "No layers were detected. You might want to repair your STL file(s) or check their size and retry.\n"
-        if !@{$self->objects};
-    
-    if ($Slic3r::Config->resolution) {
-        $status_cb->(15, "Simplifying input");
-        $self->_simplify_slices(scale $Slic3r::Config->resolution);
-    }
+        if !grep @{$_->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");
-    $_->make_perimeters for @{$self->objects};
+    $object_step->(STEP_PERIMETERS, sub {
+        $self->objects->[$_[0]]->make_perimeters;
+    });
     
-    # simplify slices (both layer and region slices),
-    # we only need the max resolution for perimeters
-    $self->_simplify_slices(&Slic3r::SCALED_RESOLUTION);
+    $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;
     
-    # this will assign a type (top/bottom/internal) to $layerm->slices
-    # and transform $layerm->fill_surfaces from expolygon 
-    # to typed top/bottom/internal surfaces;
-    $status_cb->(30, "Detecting solid surfaces");
-    $_->detect_surfaces_type for @{$self->objects};
+        # decide what surfaces are to be filled
+        $_->prepare_fill_surfaces for map @{$_->regions}, @{$object->layers};
     
-    # decide what surfaces are to be filled
-    $status_cb->(35, "Preparing infill surfaces");
-    $_->prepare_fill_surfaces for map @{$_->regions}, map @{$_->layers}, @{$self->objects};
+        # this will detect bridges and reverse bridges
+        # and rearrange top/bottom/internal surfaces
+        $object->process_external_surfaces;
     
-    # this will detect bridges and reverse bridges
-    # and rearrange top/bottom/internal surfaces
-    $status_cb->(45, "Detect bridges");
-    $_->process_external_surfaces for @{$self->objects};
+        # 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;
     
-    # detect which fill surfaces are near external layers
-    # they will be split in internal and internal-solid surfaces
-    $status_cb->(60, "Generating horizontal shells");
-    $_->discover_horizontal_shells for @{$self->objects};
-    $_->clip_fill_surfaces for @{$self->objects};
-    # the following step needs to be done before combination because it may need
-    # to remove only half of the combined infill
-    $_->bridge_over_infill for @{$self->objects};
-    
-    # combine fill surfaces to honor the "infill every N layers" option
-    $status_cb->(70, "Combining infill");
-    $_->combine_infill for @{$self->objects};
+        # combine fill surfaces to honor the "infill every N layers" option
+        $object->combine_infill;
+    });
     
     # this will generate extrusion paths for each layer
-    $status_cb->(80, "Infilling layers");
-    {
+    $status_cb->(70, "Infilling layers");
+    $object_step->(STEP_INFILL, sub {
+        my $object = $self->objects->[$_[0]];
+        
         Slic3r::parallelize(
             items => sub {
-                my @items = ();  # [obj_idx, layer_id]
-                for my $obj_idx (0 .. $#{$self->objects}) {
-                    for my $region_id (0 .. ($self->regions_count-1)) {
-                        push @items, map [$obj_idx, $_, $region_id], 0..($self->objects->[$obj_idx]->layer_count-1);
-                    }
+                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 = $q->dequeue)) {
-                    my ($obj_idx, $layer_id, $region_id) = @$obj_layer;
-                    my $object = $self->objects->[$obj_idx];
+                    my ($layer_id, $region_id) = @$obj_layer;
                     my $layerm = $object->layers->[$layer_id]->regions->[$region_id];
                     $layerm->fills->append( $object->fill_maker->make_fill($layerm) );
                 }
             },
             collect_cb => sub {},
             no_threads_cb => sub {
-                foreach my $layerm (map @{$_->regions}, map @{$_->layers}, @{$self->objects}) {
-                    $layerm->fills->append($layerm->layer->object->fill_maker->make_fill($layerm));
+                foreach my $layerm (map @{$_->regions}, @{$object->layers}) {
+                    $layerm->fills->append($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
-    if ($self->has_support_material) {
-        $status_cb->(85, "Generating support material");
-        $_->generate_support_material for @{$self->objects};
-    }
-    
-    # free memory (note that support material needs fill_surfaces)
-    $_->fill_surfaces->clear for map @{$_->regions}, map @{$_->layers}, @{$self->objects};
+    $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");
-    $self->make_skirt;
-    $self->make_brim;  # must come after make_skirt
+    $print_step->(STEP_SKIRT, sub {
+        $self->make_skirt;
+    });
+    
+    $status_cb->(88, "Generating skirt");
+    $print_step->(STEP_BRIM, sub {
+        $self->make_brim;  # must come after make_skirt
+    });
     
     # time to make some statistics
     if (0) {
@@ -945,4 +981,25 @@ sub apply_extra_variables {
     $self->extra_variables->{$_} = $extra->{$_} for keys %$extra;
 }
 
+sub invalidate_step {
+    my ($self, $step, $obj_idx) = @_;
+    
+    # invalidate $step in the correct state object
+    if ($Slic3r::Print::State::print_step->{$step}) {
+        $self->_state->invalidate($step);
+    } else {
+        # object step
+        if (defined $obj_idx) {
+            $self->objects->[$obj_idx]->_state->invalidate($step);
+        } else {
+            $_->_state->invalidate($step) for @{$self->objects};
+        }
+    }
+    
+    # recursively invalidate steps depending on $step
+    $self->invalidate_step($_)
+        for grep { grep { $_ == $step } @{$Slic3r::Print::State::prereqs{$_}} }
+            keys %Slic3r::Print::State::prereqs;
+}
+
 1;
diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index 8fb6cc2e9..30b577fb8 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -5,6 +5,7 @@ use List::Util qw(min max sum first);
 use Slic3r::Geometry qw(X Y Z PI scale unscale deg2rad rad2deg scaled_epsilon chained_path);
 use Slic3r::Geometry::Clipper qw(diff diff_ex intersection intersection_ex union union_ex 
     offset offset_ex offset2 offset2_ex CLIPPER_OFFSET_SCALE JT_MITER);
+use Slic3r::Print::State ':steps';
 use Slic3r::Surface ':types';
 
 has 'print'             => (is => 'ro', weak_ref => 1, required => 1);
@@ -21,6 +22,7 @@ has '_shifted_copies'   => (is => 'rw');  # Slic3r::Point objects in scaled G-co
 has 'layers'            => (is => 'rw', default => sub { [] });
 has 'support_layers'    => (is => 'rw', default => sub { [] });
 has 'fill_maker'        => (is => 'lazy');
+has '_state'            => (is => 'ro', default => sub { Slic3r::Print::State->new });
 
 sub BUILD {
     my $self = shift;
@@ -70,6 +72,9 @@ sub _trigger_copies {
             $c;
         } @{$self->copies}[@{chained_path($self->copies)}]
     ]);
+    
+    $self->print->_state->invalidate(STEP_SKIRT);
+    $self->print->_state->invalidate(STEP_BRIM);
 }
 
 # in unscaled coordinates
@@ -273,6 +278,11 @@ sub slice {
             $self->layers->[$i]->id($i);
         }
     }
+    
+    # simplify slices if required
+    if ($self->config->resolution) {
+        $self->_simplify_slices(scale($self->config->resolution));
+    }
 }
 
 sub make_perimeters {
@@ -348,6 +358,11 @@ sub make_perimeters {
             $_->make_perimeters for @{$self->layers};
         },
     );
+    
+    # simplify slices (both layer and region slices),
+    # 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);
 }
 
 sub detect_surfaces_type {
@@ -850,4 +865,13 @@ sub generate_support_material {
         ->generate($self);
 }
 
+sub _simplify_slices {
+    my ($self, $distance) = @_;
+    
+    foreach my $layer (@{$self->layers}) {
+        $layer->slices->simplify($distance);
+        $_->slices->simplify($distance) for @{$layer->regions};
+    }
+}
+
 1;
diff --git a/lib/Slic3r/Print/State.pm b/lib/Slic3r/Print/State.pm
new file mode 100644
index 000000000..bd76e3805
--- /dev/null
+++ b/lib/Slic3r/Print/State.pm
@@ -0,0 +1,28 @@
+package Slic3r::Print::State;
+use strict;
+use warnings;
+
+require Exporter;
+our @ISA = qw(Exporter);
+our @EXPORT_OK   = qw(STEP_INIT_EXTRUDERS STEP_SLICE STEP_PERIMETERS STEP_PREPARE_INFILL 
+                    STEP_INFILL STEP_SUPPORTMATERIAL STEP_SKIRT STEP_BRIM);
+our %EXPORT_TAGS = (steps => \@EXPORT_OK);
+
+our %print_steps = map { $_ => 1 } (
+    STEP_INIT_EXTRUDERS,
+    STEP_SKIRT,
+    STEP_BRIM,
+);
+
+our %prereqs = (
+    STEP_INIT_EXTRUDERS     => [],
+    STEP_SLICE              => [],
+    STEP_PERIMETERS         => [STEP_SLICE, STEP_INIT_EXTRUDERS],
+    STEP_PREPARE_INFILL     => [STEP_PERIMETERS],
+    STEP_INFILL             => [STEP_INFILL],
+    STEP_SUPPORTMATERIAL    => [STEP_SLICE, STEP_INIT_EXTRUDERS],
+    STEP_SKIRT              => [STEP_PERIMETERS, STEP_INFILL],
+    STEP_BRIM               => [STEP_PERIMETERS, STEP_INFILL, STEP_SKIRT],
+);
+
+1;
diff --git a/xs/MANIFEST b/xs/MANIFEST
index 5256bba42..903b39deb 100644
--- a/xs/MANIFEST
+++ b/xs/MANIFEST
@@ -35,6 +35,8 @@ src/Polyline.cpp
 src/Polyline.hpp
 src/PolylineCollection.cpp
 src/PolylineCollection.hpp
+src/PrintState.cpp
+src/PrintState.hpp
 src/ppport.h
 src/Surface.cpp
 src/Surface.hpp
@@ -72,6 +74,7 @@ xsp/Point.xsp
 xsp/Polygon.xsp
 xsp/Polyline.xsp
 xsp/PolylineCollection.xsp
+xsp/PrintState.xsp
 xsp/Surface.xsp
 xsp/SurfaceCollection.xsp
 xsp/TriangleMesh.xsp
diff --git a/xs/src/Print.cpp b/xs/src/Print.cpp
new file mode 100644
index 000000000..f5dd9fe7d
--- /dev/null
+++ b/xs/src/Print.cpp
@@ -0,0 +1,36 @@
+#include "Print.hpp"
+
+namespace Slic3r {
+
+bool
+PrintState::started(PrintStep step) const
+{
+    return this->_started.find(step) != this->_started.end();
+}
+
+bool
+PrintState::done(PrintStep step) const
+{
+    return this->_done.find(step) != this->_done.end();
+}
+
+void
+PrintState::set_started(PrintStep step)
+{
+    this->_started.insert(step);
+}
+
+void
+PrintState::set_done(PrintStep step)
+{
+    this->_done.insert(step);
+}
+
+void
+PrintState::invalidate(PrintStep step)
+{
+    this->_started.erase(step);
+    this->_done.erase(step);
+}
+
+}
diff --git a/xs/src/Print.hpp b/xs/src/Print.hpp
new file mode 100644
index 000000000..a79d544df
--- /dev/null
+++ b/xs/src/Print.hpp
@@ -0,0 +1,29 @@
+#ifndef slic3r_Print_hpp_
+#define slic3r_Print_hpp_
+
+#include <set>
+
+namespace Slic3r {
+
+enum PrintStep {
+    psInitExtruders, psSlice, psPerimeters, prPrepareInfill,
+    psInfill, psSupportMaterial, psSkirt, psBrim,
+};
+
+class PrintState
+{
+    private:
+    std::set<PrintStep> _started;
+    std::set<PrintStep> _done;
+    
+    public:
+    bool started(PrintStep step) const;
+    bool done(PrintStep step) const;
+    void set_started(PrintStep step);
+    void set_done(PrintStep step);
+    void invalidate(PrintStep step);
+};
+
+}
+
+#endif
diff --git a/xs/xsp/Print.xsp b/xs/xsp/Print.xsp
new file mode 100644
index 000000000..0eef527ea
--- /dev/null
+++ b/xs/xsp/Print.xsp
@@ -0,0 +1,41 @@
+%module{Slic3r::XS};
+
+%{
+#include <myinit.h>
+#include "Print.hpp"
+%}
+
+%name{Slic3r::Print::State} class PrintState {
+    PrintState();
+    ~PrintState();
+    bool started(PrintStep step) const;
+    bool done(PrintStep step) const;
+    void set_started(PrintStep step);
+    void set_done(PrintStep step);
+    void invalidate(PrintStep step);
+%{
+
+%}
+};
+
+%package{Slic3r::Print::State};
+%{
+
+IV
+_constant()
+  ALIAS:
+    STEP_INIT_EXTRUDERS     = psInitExtruders
+    STEP_SLICE              = psSlice
+    STEP_PERIMETERS         = psPerimeters
+    STEP_PREPARE_INFILL     = prPrepareInfill
+    STEP_INFILL             = psInfill
+    STEP_SUPPORTMATERIAL    = psSupportMaterial
+    STEP_SKIRT              = psSkirt
+    STEP_BRIM               = psBrim
+  PROTOTYPE:
+  CODE:
+    RETVAL = ix;
+  OUTPUT: RETVAL
+
+%}
+
diff --git a/xs/xsp/my.map b/xs/xsp/my.map
index dcd0293f8..415b9a58c 100644
--- a/xs/xsp/my.map
+++ b/xs/xsp/my.map
@@ -12,10 +12,12 @@ ExPolygonCollection*    O_OBJECT
 ExtrusionEntityCollection*    O_OBJECT
 ExtrusionPath*  O_OBJECT
 ExtrusionLoop*  O_OBJECT
+PrintState*  O_OBJECT
 Surface*        O_OBJECT
 SurfaceCollection*      O_OBJECT
 
 ExtrusionRole     T_UV
+PrintStep     T_UV
 SurfaceType     T_UV
 ClipperLib::JoinType		T_UV
 ClipperLib::PolyFillType		T_UV
diff --git a/xs/xsp/typemap.xspt b/xs/xsp/typemap.xspt
index 559dba843..6457717e3 100644
--- a/xs/xsp/typemap.xspt
+++ b/xs/xsp/typemap.xspt
@@ -18,6 +18,7 @@
 %typemap{Lines};
 %typemap{Polygons};
 %typemap{Polylines};
+%typemap{PrintState};
 %typemap{ExPolygons};
 %typemap{Surfaces};
 %typemap{Polygons*};
@@ -35,3 +36,9 @@
     $CVar = (ExtrusionRole)SvUV($PerlVar);
   %};
 };
+%typemap{PrintStep}{parsed}{
+  %cpp_type{PrintStep};
+  %precall_code{%
+    $CVar = (PrintStep)SvUV($PerlVar);
+  %};
+};