Merge new-support2

13 files changed, 559 insertions, 290 deletions

diff --git a/README.markdown b/README.markdown
index 3d1ef2848..46fbd0e63 100644
--- a/README.markdown
+++ b/README.markdown
@@ -322,6 +322,8 @@ The author of the Silk icon set is Mark James.
         --infill-extruder   Extruder to use for infill (1+, default: 1)
         --support-material-extruder
                             Extruder to use for support material (1+, default: 1)
+        --support-material-interface-extruder
+                            Extruder to use for support material interface (1+, default: 1)
 
 If you want to change a preset file, just do
 
diff --git a/lib/Slic3r/Config.pm b/lib/Slic3r/Config.pm
index 03b568339..b45ef47ee 100644
--- a/lib/Slic3r/Config.pm
+++ b/lib/Slic3r/Config.pm
@@ -204,11 +204,18 @@ our $Options = {
     },
     'support_material_extruder' => {
         label   => 'Support material extruder',
-        tooltip => 'The extruder to use when printing support material. This affects brim too.',
+        tooltip => 'The extruder to use when printing support material. This affects brim and raft too.',
         cli     => 'support-material-extruder=i',
         type    => 'i',
         default => 1,
     },
+    'support_material_interface_extruder' => {
+        label   => 'Support material interface extruder',
+        tooltip => 'The extruder to use when printing support material interface. This affects raft too.',
+        cli     => 'support-material-interface-extruder=i',
+        type    => 'i',
+        default => 1,
+    },
     
     # filament options
     'first_layer_bed_temperature' => {
@@ -652,7 +659,7 @@ our $Options = {
         type    => 'select',
         values  => [qw(rectilinear rectilinear-grid honeycomb)],
         labels  => ['rectilinear', 'rectilinear grid', 'honeycomb'],
-        default => 'rectilinear',
+        default => 'honeycomb',
     },
     'support_material_spacing' => {
         label   => 'Pattern spacing',
@@ -676,7 +683,7 @@ our $Options = {
         sidetext => 'layers',
         cli     => 'support-material-interface-layers=i',
         type    => 'i',
-        default => 0,
+        default => 3,
     },
     'support_material_interface_spacing' => {
         label   => 'Interface pattern spacing',
diff --git a/lib/Slic3r/GCode/CoolingBuffer.pm b/lib/Slic3r/GCode/CoolingBuffer.pm
index a4f44fe26..45283bef6 100644
--- a/lib/Slic3r/GCode/CoolingBuffer.pm
+++ b/lib/Slic3r/GCode/CoolingBuffer.pm
@@ -18,8 +18,6 @@ sub append {
     my $self = shift;
     my ($gcode, $obj_id, $layer_id, $print_z) = @_;
     
-    # TODO: differentiate $obj_id between normal layers and support layers
-    
     my $return = "";
     if (exists $self->last_z->{$obj_id} && $self->last_z->{$obj_id} != $print_z) {
         $return = $self->flush;
diff --git a/lib/Slic3r/GCode/Layer.pm b/lib/Slic3r/GCode/Layer.pm
index af296edf0..307ab0e32 100644
--- a/lib/Slic3r/GCode/Layer.pm
+++ b/lib/Slic3r/GCode/Layer.pm
@@ -97,17 +97,15 @@ sub process_layer {
         
         # extrude support material before other things because it might use a lower Z
         # and also because we avoid travelling on other things when printing it
-        if ($self->print->has_support_material) {
-            $gcode .= $self->gcodegen->move_z($layer->support_material_contact_z)
-                if ($layer->support_contact_fills && @{ $layer->support_contact_fills->paths });
-            $gcode .= $self->gcodegen->set_extruder($self->extruders->[$Slic3r::Config->support_material_extruder-1]);
-            if ($layer->support_contact_fills) {
-                $gcode .= $self->gcodegen->extrude_path($_, 'support material contact area') 
-                    for $layer->support_contact_fills->chained_path($self->gcodegen->last_pos); 
-            }
-            
+        if ($self->print->has_support_material && $layer->isa('Slic3r::Layer::Support')) {
             $gcode .= $self->gcodegen->move_z($layer->print_z);
+            if ($layer->support_interface_fills) {
+                $gcode .= $self->gcodegen->set_extruder($self->extruders->[$Slic3r::Config->support_material_interface_extruder-1]);
+                $gcode .= $self->gcodegen->extrude_path($_, 'support material interface') 
+                    for $layer->support_interface_fills->chained_path($self->gcodegen->last_pos); 
+            }
             if ($layer->support_fills) {
+                $gcode .= $self->gcodegen->set_extruder($self->extruders->[$Slic3r::Config->support_material_extruder-1]);
                 $gcode .= $self->gcodegen->extrude_path($_, 'support material') 
                     for $layer->support_fills->chained_path($self->gcodegen->last_pos);
             }
@@ -125,7 +123,7 @@ sub process_layer {
         }
         
         foreach my $region_id (@region_ids) {
-            my $layerm = $layer->regions->[$region_id];
+            my $layerm = $layer->regions->[$region_id] or next;
             my $region = $self->print->regions->[$region_id];
             
             my @islands = ();
diff --git a/lib/Slic3r/GUI/Tab.pm b/lib/Slic3r/GUI/Tab.pm
index 27ff3c56e..70fd3962a 100644
--- a/lib/Slic3r/GUI/Tab.pm
+++ b/lib/Slic3r/GUI/Tab.pm
@@ -526,7 +526,7 @@ sub build {
     $self->add_options_page('Multiple Extruders', 'funnel.png', optgroups => [
         {
             title => 'Extruders',
-            options => [qw(perimeter_extruder infill_extruder support_material_extruder)],
+            options => [qw(perimeter_extruder infill_extruder support_material_extruder support_material_interface_extruder)],
         },
     ]);
     
diff --git a/lib/Slic3r/Geometry/Clipper.pm b/lib/Slic3r/Geometry/Clipper.pm
index 35d13c874..fc8d35b8a 100644
--- a/lib/Slic3r/Geometry/Clipper.pm
+++ b/lib/Slic3r/Geometry/Clipper.pm
@@ -7,7 +7,7 @@ our @ISA = qw(Exporter);
 our @EXPORT_OK = qw(safety_offset safety_offset_ex offset offset_ex collapse_ex
     diff_ex diff union_ex intersection_ex xor_ex PFT_EVENODD JT_MITER JT_ROUND
     JT_SQUARE is_counter_clockwise union_pt offset2 offset2_ex traverse_pt
-    intersection);
+    intersection union);
 
 use Math::Clipper 1.22 qw(:cliptypes :polyfilltypes :jointypes is_counter_clockwise area);
 use Slic3r::Geometry qw(scale);
@@ -88,6 +88,17 @@ sub diff {
     ];
 }
 
+sub union {
+    my ($polygons, $jointype, $safety_offset) = @_;
+    $jointype = PFT_NONZERO unless defined $jointype;
+    $clipper->clear;
+    $clipper->add_subject_polygons($safety_offset ? safety_offset($polygons) : $polygons);
+    return [
+        map Slic3r::Polygon->new(@$_),
+            @{ $clipper->execute(CT_UNION, $jointype, $jointype) },
+    ];
+}
+
 sub union_ex {
     my ($polygons, $jointype, $safety_offset) = @_;
     $jointype = PFT_NONZERO unless defined $jointype;
diff --git a/lib/Slic3r/Layer.pm b/lib/Slic3r/Layer.pm
index ece506253..d1bc2818b 100644
--- a/lib/Slic3r/Layer.pm
+++ b/lib/Slic3r/Layer.pm
@@ -18,42 +18,11 @@ has 'height'            => (is => 'ro', required => 1); # layer height in unscal
 # also known as 'islands' (all regions and surface types are merged here)
 has 'slices'            => (is => 'rw');
 
-# ordered collection of extrusion paths to fill surfaces for support material
-has 'support_islands'           => (is => 'rw');
-has 'support_fills'             => (is => 'rw');
-has 'support_contact_fills'     => (is => 'rw');
-
 sub _trigger_id {
     my $self = shift;
     $_->_trigger_layer for @{$self->regions || []};
 }
 
-# layer height of contact paths in unscaled coordinates
-sub support_material_contact_height {
-    my $self = shift;
-    
-    return $self->height if $self->id == 0;
-    
-    # TODO: check what upper region applies instead of considering the first one
-    my $upper_layer = $self->object->layers->[ $self->id + 1 ] // $self;
-    my $h = ($self->height + $upper_layer->height) - $upper_layer->regions->[0]->extruders->{infill}->bridge_flow->width;
-    
-    # If layer height is less than half the bridge width then we'll get a negative height for contact area.
-    # The optimal solution would be to skip some layers during support material generation, but for now
-    # we'll apply a (dirty) workaround that should still work.
-    if ($h <= 0) {
-        $h = $self->height;
-    }
-    
-    return $h;
-}
-
-# Z used for printing support material contact in scaled coordinates
-sub support_material_contact_z {
-    my $self = shift;
-    return ($self->print_z - ($self->height - $self->support_material_contact_height)) / &Slic3r::SCALING_FACTOR;
-}
-
 sub upper_layer_slices {
     my $self = shift;
     
@@ -94,4 +63,13 @@ sub support_islands_enclose_line {
     return (first { $_->encloses_line($line) } @{$self->support_islands}) ? 1 : 0;
 }
 
+package Slic3r::Layer::Support;
+use Moo;
+extends 'Slic3r::Layer';
+
+# ordered collection of extrusion paths to fill surfaces for support material
+has 'support_islands'           => (is => 'rw');
+has 'support_fills'             => (is => 'rw');
+has 'support_interface_fills'   => (is => 'rw');
+
 1;
diff --git a/lib/Slic3r/Polyline.pm b/lib/Slic3r/Polyline.pm
index fd4bd571f..0196cf677 100644
--- a/lib/Slic3r/Polyline.pm
+++ b/lib/Slic3r/Polyline.pm
@@ -211,6 +211,38 @@ sub clip_start {
     return (ref $self)->new($points);
 }
 
+# this method returns a collection of points picked on the polygon contour
+# so that they are evenly spaced according to the input distance
+# (find a better name!)
+sub regular_points {
+    my $self = shift;
+    my ($distance) = @_;
+    
+    my @points = ($self->[0]);
+    my $len = 0;
+    
+    for (my $i = 1; $i <= $#$self; $i++) {
+        my $point = $self->[$i];
+        my $segment_length = $point->distance_to($self->[$i-1]);
+        $len += $segment_length;
+        next if $len < $distance;
+        
+        if ($len == $distance) {
+            push @points, $point;
+            $len = 0;
+            next;
+        }
+        
+        my $take = $segment_length - ($len - $distance);  # how much we take of this segment
+        my $new_point = Slic3r::Geometry::point_along_segment($self->[$i-1], $point, $take);
+        push @points, Slic3r::Point->new($new_point);
+        $i--;
+        $len = -$take;
+    }
+    
+    return @points;
+}
+
 package Slic3r::Polyline::Collection;
 use Moo;
 
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index 49b25ccdd..d12a6e87f 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -3,7 +3,7 @@ use Moo;
 
 use File::Basename qw(basename fileparse);
 use File::Spec;
-use List::Util qw(max first);
+use List::Util qw(min max first);
 use Math::ConvexHull::MonotoneChain qw(convex_hull);
 use Slic3r::ExtrusionPath ':roles';
 use Slic3r::Geometry qw(X Y Z X1 Y1 X2 Y2 MIN MAX PI scale unscale move_points
@@ -263,6 +263,7 @@ sub init_extruders {
     }
     
     # calculate support material flow
+    # Note: we should calculate a different flow for support material interface
     if ($self->has_support_material) {
         my $extruder = $self->extruders->[$self->config->support_material_extruder-1];
         $self->support_material_flow($extruder->make_flow(
@@ -581,15 +582,18 @@ sub make_skirt {
     # collect points from all layers contained in skirt height
     my @points = ();
     foreach my $obj_idx (0 .. $#{$self->objects}) {
-        my $skirt_height = $Slic3r::Config->skirt_height;
-        $skirt_height = $self->objects->[$obj_idx]->layer_count if $skirt_height > $self->objects->[$obj_idx]->layer_count;
-        my @layers = map $self->objects->[$obj_idx]->layers->[$_], 0..($skirt_height-1);
+        my $object = $self->objects->[$obj_idx];
+        my @layers = map $object->layers->[$_], 0..min($Slic3r::Config->skirt_height-1, $#{$object->layers});
         my @layer_points = (
             (map @$_, map @$_, map @{$_->slices}, @layers),
             (map @$_, map @{$_->thin_walls}, map @{$_->regions}, @layers),
-            (map @{$_->unpack->polyline}, map @{$_->support_fills->paths}, grep $_->support_fills, @layers),
         );
-        push @points, map move_points($_, @layer_points), @{$self->objects->[$obj_idx]->copies};
+        if (@{ $object->support_layers }) {
+            my @support_layers = map $object->support_layers->[$_], 0..min($Slic3r::Config->skirt_height-1, $#{$object->support_layers});
+            push @layer_points,
+                (map @{$_->unpack->polyline}, map @{$_->support_fills->paths}, grep $_->support_fills, @support_layers);
+        }
+        push @points, map move_points($_, @layer_points), @{$object->copies};
     }
     return if @points < 3;  # at least three points required for a convex hull
     
@@ -644,13 +648,19 @@ sub make_brim {
     my $grow_distance = $flow->scaled_width / 2;
     my @islands = (); # array of polygons
     foreach my $obj_idx (0 .. $#{$self->objects}) {
-        my $layer0 = $self->objects->[$obj_idx]->layers->[0];
+        my $object = $self->objects->[$obj_idx];
+        my $layer0 = $object->layers->[0];
         my @object_islands = (
             (map $_->contour, @{$layer0->slices}),
             (map { $_->isa('Slic3r::Polygon') ? $_ : $_->grow($grow_distance) } map @{$_->thin_walls}, @{$layer0->regions}),
-            (map $_->unpack->polyline->grow($grow_distance), map @{$_->support_fills->paths}, grep $_->support_fills, $layer0),
         );
-        foreach my $copy (@{$self->objects->[$obj_idx]->copies}) {
+        if (@{ $object->support_layers }) {
+            my $support_layer0 = $object->support_layers->[0];
+            push @object_islands,
+                (map $_->unpack->polyline->grow($grow_distance), @{$support_layer0->support_fills->paths})
+                if $support_layer0->support_fills;
+        }
+        foreach my $copy (@{$object->copies}) {
             push @islands, map $_->clone->translate(@$copy), @object_islands;
         }
     }
@@ -812,7 +822,9 @@ sub write_gcode {
                     gcodegen    => $gcodegen,
                 );
                 
-                for my $layer (@{$self->objects->[$obj_idx]->layers}) {
+                my $object = $self->objects->[$obj_idx];
+                my @layers = sort { $a->print_z <=> $b->print_z } @{$object->layers}, @{$object->support_layers};
+                for my $layer (@layers) {
                     # if we are printing the bottom layer of an object, and we have already finished
                     # another one, set first layer temperatures. this happens before the Z move
                     # is triggered, so machine has more time to reach such temperatures
@@ -837,11 +849,13 @@ sub write_gcode {
         my @obj_idx = chained_path([ map $_->copies->[0], @{$self->objects} ]);
         
         # sort layers by Z
-        my %layers = ();  # print_z => [ layer, layer, layer ]  by obj_idx
+        my %layers = ();  # print_z => [ [layers], [layers], [layers] ]  by obj_idx
         foreach my $obj_idx (0 .. $#{$self->objects}) {
-            foreach my $layer (@{$self->objects->[$obj_idx]->layers}) {
+            my $object = $self->objects->[$obj_idx];
+            foreach my $layer (@{$object->layers}, @{$object->support_layers}) {
                 $layers{ $layer->print_z } ||= [];
-                $layers{ $layer->print_z }[$obj_idx] = $layer;  # turn this into [$layer] when merging support layers
+                $layers{ $layer->print_z }[$obj_idx] ||= [];
+                push @{$layers{ $layer->print_z }[$obj_idx]}, $layer;
             }
         }
         
@@ -851,13 +865,14 @@ sub write_gcode {
         );
         foreach my $print_z (sort { $a <=> $b } keys %layers) {
             foreach my $obj_idx (@obj_idx) {
-                next unless my $layer = $layers{$print_z}[$obj_idx];
-                print $fh $buffer->append(
-                    $layer_gcode->process_layer($layer, $layer->object->copies),
-                    $layer->object."",
-                    $layer->id,
-                    $layer->print_z,
-                );
+                foreach my $layer (@{ $layers{$print_z}[$obj_idx] // [] }) {
+                    print $fh $buffer->append(
+                        $layer_gcode->process_layer($layer, $layer->object->copies),
+                        $layer->object . ref($layer),  # differentiate $obj_id between normal layers and support layers
+                        $layer->id,
+                        $layer->print_z,
+                    );
+                }
             }
         }
         print $fh $buffer->flush;
diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index 8bd80d151..d71ff140d 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -1,11 +1,11 @@
 package Slic3r::Print::Object;
 use Moo;
 
-use List::Util qw(min sum first);
+use List::Util qw(min max sum first);
 use Slic3r::ExtrusionPath ':roles';
 use Slic3r::Geometry qw(Z PI scale unscale deg2rad rad2deg scaled_epsilon chained_path_points);
-use Slic3r::Geometry::Clipper qw(diff_ex intersection_ex union_ex offset collapse_ex
-    offset2 diff intersection);
+use Slic3r::Geometry::Clipper qw(diff diff_ex intersection intersection_ex union union_ex 
+    offset offset_ex offset2);
 use Slic3r::Surface ':types';
 
 has 'print'             => (is => 'ro', weak_ref => 1, required => 1);
@@ -14,6 +14,7 @@ has 'meshes'            => (is => 'rw', default => sub { [] });  # by region_id
 has 'size'              => (is => 'rw', required => 1); # XYZ in scaled coordinates
 has 'copies'            => (is => 'rw', trigger => 1);  # in scaled coordinates
 has 'layers'            => (is => 'rw', default => sub { [] });
+has 'support_layers'    => (is => 'rw', default => sub { [] });
 has 'layer_height_ranges' => (is => 'rw', default => sub { [] }); # [ z_min, z_max, layer_height ]
 has 'fill_maker'        => (is => 'lazy');
 has '_slice_z_table'    => (is => 'lazy');
@@ -808,264 +809,446 @@ sub generate_support_material {
     my $self = shift;
     return if $self->layer_count < 2;
     
+    my $flow = $self->print->support_material_flow;
+    
+    # how much we extend support around the actual contact area
+    #my $margin      = $flow->scaled_width / 2;
+    my $margin      = scale 3;
+    
+    # increment used to reach $margin in steps to avoid trespassing thin objects
+    my $margin_step = $margin/3;
+    
+    # if user specified a custom angle threshold, convert it to radians
     my $threshold_rad;
     if ($Slic3r::Config->support_material_threshold) {
         $threshold_rad = deg2rad($Slic3r::Config->support_material_threshold + 1);  # +1 makes the threshold inclusive
         Slic3r::debugf "Threshold angle = %d°\n", rad2deg($threshold_rad);
     }
-    my $flow                    = $self->print->support_material_flow;
-    my $distance_from_object    = 1.5 * $flow->scaled_width;
-    my $pattern_spacing = ($Slic3r::Config->support_material_spacing > $flow->spacing)
-        ? $Slic3r::Config->support_material_spacing
-        : $flow->spacing;
     
-    # determine support regions in each layer (for upper layers)
-    Slic3r::debugf "Detecting regions\n";
-    my %layers = ();            # this represents the areas of each layer having to support upper layers (excluding interfaces)
-    my %layers_interfaces = (); # this represents the areas of each layer to be filled with interface pattern, excluding the contact areas which are stored separately
-    my %layers_contact_areas = (); # this represents the areas of each layer having an overhang in the immediately upper layer
+    # shape of contact area
+    my $contact_loops = 1;
+    my $circle_distance = 3 * $flow->scaled_width;
+    my $circle;
     {
-        my @current_support_regions = ();   # expolygons we've started to support (i.e. below the empty interface layers)
-        my @upper_layers_overhangs = (map [], 1..$Slic3r::Config->support_material_interface_layers);
-        for my $i (reverse 0 .. $#{$self->layers}) {
-            next unless $Slic3r::Config->support_material
-                || ($i <= $Slic3r::Config->raft_layers)  # <= because we need to start from the first non-raft layer
-                || ($i <= $Slic3r::Config->support_material_enforce_layers + $Slic3r::Config->raft_layers);
-            
-            my $layer = $self->layers->[$i];
-            my $lower_layer = $i > 0 ? $self->layers->[$i-1] : undef;
+        # TODO: make sure teeth between circles are compatible with support material flow
+        my $r = 1.5 * $flow->scaled_width;
+        $circle = Slic3r::Polygon->new(map [ $r * cos $_, $r * sin $_ ], (5*PI/3, 4*PI/3, PI, 2*PI/3, PI/3, 0));
+    }
+    
+    # determine contact areas
+    my %contact  = ();  # contact_z => [ polygons ]
+    my %overhang = ();  # contact_z => [ expolygons ] - this stores the actual overhang supported by each contact layer
+    for my $layer_id (1 .. $#{$self->layers}) {
+        my $layer = $self->layers->[$layer_id];
+        my $lower_layer = $self->layers->[$layer_id-1];
+        
+        # detect overhangs and contact areas needed to support them
+        my (@overhang, @contact) = ();
+        foreach my $layerm (@{$layer->regions}) {
+            my $fw = $layerm->perimeter_flow->scaled_width;
+            my $diff;
             
-            my @current_layer_offsetted_slices = map $_->offset_ex($distance_from_object), @{$layer->slices};
+            # If a threshold angle was specified, use a different logic for detecting overhangs.
+            if (defined $threshold_rad || $layer_id <= $Slic3r::Config->support_material_enforce_layers) {
+                my $d = defined $threshold_rad
+                    ? scale $lower_layer->height * ((cos $threshold_rad) / (sin $threshold_rad))
+                    : 0;
+                
+                $diff = diff(
+                    [ offset([ map $_->p, @{$layerm->slices} ], -$d) ],
+                    [ map @$_, @{$lower_layer->slices} ],
+                );
+                
+                # only enforce spacing from the object ($fw/2) if the threshold angle
+                # is not too high: in that case, $d will be very small (as we need to catch
+                # very short overhangs), and such contact area would be eaten by the
+                # enforced spacing, resulting in high threshold angles to be almost ignored
+                $diff = diff(
+                    [ offset($diff, $d - $fw/2) ],
+                    [ map @$_, @{$lower_layer->slices} ],
+                ) if $d > $fw/2;
+            } else {
+                $diff = diff(
+                    [ offset([ map $_->p, @{$layerm->slices} ], -$fw/2) ],
+                    [ map @$_, @{$lower_layer->slices} ],
+                );
+                # $diff now contains the ring or stripe comprised between the boundary of 
+                # lower slices and the centerline of the last perimeter in this overhanging layer.
+                # Void $diff means that there's no upper perimeter whose centerline is
+                # outside the lower slice boundary, thus no overhang
+            }
             
-            # $upper_layers_overhangs[-1] contains the overhangs of the upper layer, regardless of any interface layers
-            # $upper_layers_overhangs[0] contains the overhangs of the first upper layer above the interface layers
+            next if !@$diff;
+            push @overhang, @{union_ex($diff)};  # NOTE: this is not the full overhang as it misses the outermost half of the perimeter width!
             
-            # we only consider the overhangs of the upper layer to define contact areas of the current one
-            $layers_contact_areas{$i} = diff_ex(
-                [ map @$_, @{ $upper_layers_overhangs[-1] || [] } ],
-                [ map @$_, @current_layer_offsetted_slices ],
-            );
-            $layers_contact_areas{$i} = [
-                @{collapse_ex([ map @$_, @{$layers_contact_areas{$i}} ], $flow->scaled_width)},
-            ];
+            # Let's define the required contact area by using a max gap of half the upper 
+            # extrusion width and extending the area according to the configured margin.
+            # We increment the area in steps because we don't want our support to overflow
+            # on the other side of the object (if it's very thin).
+            {
+                my @slices_margin = offset([ map @$_, @{$lower_layer->slices} ], $fw/2);
+                for ($fw/2, map {$margin_step} 1..($margin / $margin_step)) {
+                    $diff = diff(
+                        [ offset($diff, $_) ],
+                        \@slices_margin,
+                    );
+                }
+            }
+            push @contact, @$diff;
+        }
+        next if !@contact;
+        
+        # now apply the contact areas to the layer were they need to be made
+        {
+            # get the average nozzle diameter used on this layer
+            my @nozzle_diameters = map $_->nozzle_diameter,
+                map { $_->perimeter_flow, $_->solid_infill_flow }
+                @{$layer->regions};
+            my $nozzle_diameter = sum(@nozzle_diameters)/@nozzle_diameters;
             
-            # to define interface regions of this layer we consider the overhangs of all the upper layers
-            # minus the first one
-            $layers_interfaces{$i} = diff_ex(
-                [ map @$_, map @$_, @upper_layers_overhangs[0 .. $#upper_layers_overhangs-1] ],
+            my $contact_z = $layer->print_z - $nozzle_diameter * 1.5;
+            ###$contact_z = $layer->print_z - $layer->height;
+            $contact{$contact_z}  = [ @contact ];
+            $overhang{$contact_z} = [ @overhang ];
+        }
+    }
+    my @contact_z = sort keys %contact;
+    
+    # find object top surfaces
+    # we'll use them to clip our support and detect where does it stick
+    my %top = ();  # print_z => [ expolygons ]
+    {
+        my $projection = [];
+        foreach my $layer (reverse @{$self->layers}) {
+            if (my @top = grep $_->surface_type == S_TYPE_TOP, map @{$_->slices}, @{$layer->regions}) {
+                # compute projection of the contact areas above this top layer
+                # first add all the 'new' contact areas to the current projection
+                # ('new' means all the areas that are lower than the last top layer
+                # we considered)
+                my $min_top = min(keys %top) // max(keys %contact);
+                push @$projection, map @{$contact{$_}}, grep { $_ > $layer->print_z && $_ < $min_top } keys %contact;
+                
+                # now find whether any projection falls onto this top surface
+                my $touching = intersection($projection, [ map $_->p, @top ]);
+                if (@$touching) {
+                    $top{ $layer->print_z } = $touching;
+                }
+                
+                # remove the areas that touched from the projection that will continue on 
+                # next, lower, top surfaces
+                $projection = diff($projection, $touching);
+            }
+        }
+    }
+    my @top_z = sort keys %top;
+    
+    # we now know the upper and lower boundaries for our support material object
+    # (@contact_z and @top_z), so we can generate intermediate layers
+    my @support_layers = _compute_support_layers(\@contact_z, \@top_z, $Slic3r::Config, $flow);
+    
+    # if we wanted to apply some special logic to the first support layers lying on
+    # object's top surfaces this is the place to detect them
+    
+    # Let's now determine shells (interface layers) and normal support below them.
+    # Let's now fill each support layer by generating shells (interface layers) and
+    # clipping support area to the actual object boundaries.
+    my %interface = ();  # layer_id => [ polygons ]
+    my %support   = ();  # layer_id => [ polygons ]
+    my $interface_layers = $Slic3r::Config->support_material_interface_layers;
+    for my $layer_id (0 .. $#support_layers) {
+        my $z = $support_layers[$layer_id];
+        my $this = $contact{$z} // next;
+        # count contact layer as interface layer
+        for (my $i = $layer_id; $i >= 0 && $i > $layer_id-$interface_layers; $i--) {
+            $z = $support_layers[$i];
+            # Compute interface area on this layer as diff of upper contact area
+            # (or upper interface area) and layer slices.
+            # This diff is responsible of the contact between support material and
+            # the top surfaces of the object. We should probably offset the top 
+            # surfaces before performing the diff, but this needs investigation.
+            $this = $interface{$i} = diff(
                 [
-                    (map @$_, @current_layer_offsetted_slices),
-                    (map @$_, @{ $layers_contact_areas{$i} }),
+                    @$this,
+                    @{ $interface{$i} || [] },
+                ],
+                [
+                    @{ $top{$z} || [] },
                 ],
             );
-            $layers_interfaces{$i} = [
-                @{collapse_ex([ map @$_, @{$layers_interfaces{$i}} ], $flow->scaled_width)},
-            ];
-            
-            # generate support material in current layer (for upper layers)
-            @current_support_regions = @{diff_ex(
+        }
+        
+        # determine what layers does our support belong to
+        for (my $i = $layer_id-$interface_layers; $i >= 0; $i--) {
+            $z = $support_layers[$i];
+            # Compute support area on this layer as diff of upper support area
+            # and layer slices.
+            $this = $support{$i} = diff(
                 [
-                    (map @$_, @current_support_regions),
-                    (map @$_, @{ $upper_layers_overhangs[-1] || [] }),   # only considering -1 instead of the whole array contents is just an optimization
+                    @$this,
+                    @{ $support{$i} || [] },
                 ],
-                [ map @$_, @{$layer->slices} ],
-            )};
-            shift @upper_layers_overhangs;
-            
-            $layers{$i} = diff_ex(
-                [ map @$_, @current_support_regions ],
                 [
-                    (map @$_, @current_layer_offsetted_slices),
-                    (map @$_, @{ $layers_interfaces{$i} }),
+                    @{ $top{$z} || [] },
+                    @{ $interface{$i} || [] },
                 ],
             );
-            $layers{$i} = [
-                @{collapse_ex([ map @$_, @{$layers{$i}} ], $flow->scaled_width)},
-            ];
+        }
+    }
+    
+    push @{$self->support_layers}, map Slic3r::Layer::Support->new(
+        object  => $self,
+        id      => $_,
+        height  => ($_ == 0) ? $support_layers[$_] : ($support_layers[$_] - $support_layers[$_-1]),
+        print_z => $support_layers[$_],
+        slice_z => -1,
+        slices  => [],
+    ), 0 .. $#support_layers;
+
+    Slic3r::debugf "Generating patterns\n";
+    
+    # prepare fillers
+    my $pattern = $Slic3r::Config->support_material_pattern;
+    my @angles = ($Slic3r::Config->support_material_angle);
+    if ($pattern eq 'rectilinear-grid') {
+        $pattern = 'rectilinear';
+        push @angles, $angles[0] + 90;
+    }
+    
+    my %fillers = (
+        interface   => $self->fill_maker->filler('rectilinear'),
+        support     => $self->fill_maker->filler($pattern),
+    );
+    
+    my $interface_angle = $Slic3r::Config->support_material_angle + 90;
+    my $interface_spacing = $Slic3r::Config->support_material_interface_spacing + $flow->spacing;
+    my $interface_density = $interface_spacing == 0 ? 1 : $flow->spacing / $interface_spacing;
+    my $support_spacing = $Slic3r::Config->support_material_spacing + $flow->spacing;
+    my $support_density = $support_spacing == 0 ? 1 : $flow->spacing / $support_spacing;
+    
+    my $process_layer = sub {
+        my ($layer_id) = @_;
+        my $result = { contact => [], interface => [], support => [] };
+        
+        $contact{$layer_id}     ||= [];
+        $interface{$layer_id}   ||= [];
+        $support{$layer_id}     ||= [];
+        
+        # contact
+        if ((my $contact = $contact{$support_layers[$layer_id]}) && $contact_loops > 0) {
+            my $overhang = $overhang{$support_layers[$layer_id]};
+            $contact = [ grep $_->is_counter_clockwise, @$contact ];
             
-            # get layer overhangs and put them into queue for adding support inside lower layers;
-            # we need an angle threshold for this
-            my @overhangs = ();
-            if ($lower_layer) {
-                # consider all overhangs regardless of their angle if we're told to enforce support on this layer
-                my $distance = $i <= ($Slic3r::Config->support_material_enforce_layers + $Slic3r::Config->raft_layers)
-                    ? 0
-                    : $Slic3r::Config->support_material_threshold
-                        ? scale $lower_layer->height * ((cos $threshold_rad) / (sin $threshold_rad))
-                        : $self->layers->[1]->regions->[0]->overhang_width;
+            # generate the outermost loop
+            my @loops0;
+            {
+                # find centerline of the external loop of the contours
+                my @external_loops = offset($contact, -$flow->scaled_width/2);
                 
-                @overhangs = map $_->offset_ex(+$distance), @{diff_ex(
-                    [ map @$_, @{$layer->slices} ],
-                    [ map @$_, @{$lower_layer->slices} ],
-                    1,
+                # apply a pattern to the loop
+                my @positions = map Slic3r::Polygon->new(@$_)->split_at_first_point->regular_points($circle_distance), @external_loops;
+                @loops0 = @{diff(
+                    [ @external_loops ],
+                    [ map $circle->clone->translate(@$_), @positions ],
                 )};
             }
-            push @upper_layers_overhangs, [@overhangs];
             
-            if ($Slic3r::debug) {
-                printf "Layer %d (z = %.2f) has %d generic support areas, %d normal interface areas, %d contact areas\n",
-                    $i, $layer->print_z, scalar(@{$layers{$i}}), scalar(@{$layers_interfaces{$i}}), scalar(@{$layers_contact_areas{$i}});
+            # make more loops
+            my @loops = @loops0;
+            for my $i (2..$contact_loops) {
+                my $d = ($i-1) * $flow->scaled_spacing;
+                push @loops, offset2(\@loops0, -$d -0.5*$flow->scaled_spacing, +0.5*$flow->scaled_spacing);
             }
+            
+            # clip such loops to the side oriented towards the object
+            @loops = map Slic3r::Polyline->new(@$_),
+                @{ Boost::Geometry::Utils::multi_polygon_multi_linestring_intersection(
+                    [ offset_ex([ map @$_, @$overhang ], +scale 3) ],
+                    [ map Slic3r::Polygon->new(@$_)->split_at_first_point, @loops ],
+                ) };
+            
+            # subtract loops from the contact area to detect the remaining part
+            $interface{$layer_id} = intersection(
+                $interface{$layer_id},
+                [ offset2(\@loops0, -($contact_loops) * $flow->scaled_spacing, +0.5*$flow->scaled_spacing) ],
+            );
+            
+            # transform loops into ExtrusionPath objects
+            @loops = map Slic3r::ExtrusionPath->pack(
+                polyline        => $_,
+                role            => EXTR_ROLE_SUPPORTMATERIAL,
+                flow_spacing    => $flow->spacing,
+            ), @loops;
+            
+            $result->{contact} = [ @loops ];
         }
-    }
-    return if !map @$_, values %layers;
-    
-    # generate paths for the pattern that we're going to use
-    Slic3r::debugf "Generating patterns\n";
-    my $support_patterns = [];
-    my $support_interface_patterns = [];
-    {
-        # 0.5 ensures the paths don't get clipped externally when applying them to layers
-        my @areas = map $_->offset_ex(- 0.5 * $flow->scaled_width),
-            @{union_ex([ map $_->contour, map @$_, values %layers, values %layers_interfaces, values %layers_contact_areas ])};
         
-        my $pattern = $Slic3r::Config->support_material_pattern;
-        my @angles = ($Slic3r::Config->support_material_angle);
-        if ($pattern eq 'rectilinear-grid') {
-            $pattern = 'rectilinear';
-            push @angles, $angles[0] + 90;
+        # interface
+        if (@{$interface{$layer_id}}) {
+            $fillers{interface}->angle($interface_angle);
+            
+            # steal some space from support
+            $interface{$layer_id} = intersection(
+                [ offset($interface{$layer_id}, scale 3) ],
+                [ @{$interface{$layer_id}}, @{$support{$layer_id}} ],
+            );
+            $support{$layer_id} = diff(
+                $support{$layer_id},
+                $interface{$layer_id},
+            );
+            
+            my @paths = ();
+            foreach my $expolygon (offset_ex($interface{$layer_id}, -$flow->scaled_width/2)) {
+                my @p = $fillers{interface}->fill_surface(
+                    Slic3r::Surface->new(expolygon => $expolygon),
+                    density         => $interface_density,
+                    flow_spacing    => $flow->spacing,
+                    complete        => 1,
+                );
+                my $params = shift @p;
+                
+                push @paths, map Slic3r::ExtrusionPath->pack(
+                    polyline        => Slic3r::Polyline->new(@$_),
+                    role            => EXTR_ROLE_SUPPORTMATERIAL,
+                    height          => undef,
+                    flow_spacing    => $params->{flow_spacing},
+                ), @p;
+            }            
+            $result->{interface} = [ @paths ];
         }
         
-        my $filler = $self->fill_maker->filler($pattern);
-        my $make_pattern = sub {
-            my ($expolygon, $density) = @_;
+        # support or flange
+        if (@{$support{$layer_id}}) {
+            my $filler = $fillers{support};
+            $filler->angle($angles[ ($layer_id) % @angles ]);
+            my $density         = $support_density;
+            my $flow_spacing    = $flow->spacing;
             
-            my @paths = $filler->fill_surface(
-                Slic3r::Surface->new(expolygon => $expolygon),
-                density         => $density,
-                flow_spacing    => $flow->spacing,
-            );
-            my $params = shift @paths;
+            # TODO: use offset2_ex()
+            my $to_infill = [ offset_ex(union($support{$layer_id}), -$flow->scaled_width/2) ];
+            my @paths = ();
             
-            return map Slic3r::ExtrusionPath->new(
-                polyline        => Slic3r::Polyline->new(@$_),
-                role            => EXTR_ROLE_SUPPORTMATERIAL,
-                height          => undef,
-                flow_spacing    => $params->{flow_spacing},
-            ), @paths;
-        };
-        foreach my $angle (@angles) {
-            $filler->angle($angle);
-            {
-                my $density = $flow->spacing / $pattern_spacing;
-                push @$support_patterns, [ map $make_pattern->($_, $density), @areas ];
+            # base flange
+            if ($layer_id == 0) {
+                $filler = $fillers{interface};
+                $filler->angle($Slic3r::Config->support_material_angle + 90);
+                $density        = 0.5;
+                $flow_spacing   = $self->print->first_layer_support_material_flow->spacing;
+            } else {
+                # draw a perimeter all around support infill
+                # TODO: use brim ordering algorithm
+                push @paths, map Slic3r::ExtrusionPath->pack(
+                    polyline        => $_->split_at_first_point,
+                    role            => EXTR_ROLE_SUPPORTMATERIAL,
+                    height          => undef,
+                    flow_spacing    => $flow->spacing,
+                ), map @$_, @$to_infill;
+                
+                # TODO: use offset2_ex()
+                $to_infill = [ offset_ex([ map @$_, @$to_infill ], -$flow->scaled_spacing) ];
             }
             
-            if ($Slic3r::Config->support_material_interface_layers > 0) {
-                # if pattern is not cross-hatched, rotate the interface pattern by 90° degrees
-                $filler->angle($angle + 90) if @angles == 1;
+            foreach my $expolygon (@$to_infill) {
+                my @p = $filler->fill_surface(
+                    Slic3r::Surface->new(expolygon => $expolygon),
+                    density         => $density,
+                    flow_spacing    => $flow_spacing,
+                    complete        => 1,
+                );
+                my $params = shift @p;
                 
-                my $spacing = $Slic3r::Config->support_material_interface_spacing;
-                my $density = $spacing == 0 ? 1 : $flow->spacing / $spacing;
-                push @$support_interface_patterns, [ map $make_pattern->($_, $density), @areas ];
+                push @paths, map Slic3r::ExtrusionPath->pack(
+                    polyline        => Slic3r::Polyline->new(@$_),
+                    role            => EXTR_ROLE_SUPPORTMATERIAL,
+                    height          => undef,
+                    flow_spacing    => $params->{flow_spacing},
+                ), @p;
             }
+            
+            $result->{support} = [ @paths ];
         }
+        
+        # islands
+        $result->{islands} = union_ex([
+            @{$interface{$layer_id} || []},
+            @{$support{$layer_id}   || []},
+        ]);
+        
+        return $result;
+    };
+    
+    my $apply = sub {
+        my ($layer_id, $result) = @_;
+        my $layer = $self->support_layers->[$layer_id];
+        
+        my $interface_collection = Slic3r::ExtrusionPath::Collection->new(paths => [ @{$result->{contact}}, @{$result->{interface}} ]);
+        $layer->support_interface_fills($interface_collection) if @{$interface_collection->paths} > 0;
+        
+        my $support_collection = Slic3r::ExtrusionPath::Collection->new(paths => $result->{support});
+        $layer->support_fills($support_collection) if @{$support_collection->paths} > 0;
+        
+        $layer->support_islands($result->{islands});
+    };
+    Slic3r::parallelize(
+        items => [ 0 .. $#{$self->support_layers} ],
+        thread_cb => sub {
+            my $q = shift;
+            $Slic3r::Geometry::Clipper::clipper = Math::Clipper->new;
+            my $result = {};
+            while (defined (my $layer_id = $q->dequeue)) {
+                $result->{$layer_id} = $process_layer->($layer_id);
+            }
+            return $result;
+        },
+        collect_cb => sub {
+            my $result = shift;
+            $apply->($_, $result->{$_}) for keys %$result;
+        },
+        no_threads_cb => sub {
+            $apply->($_, $process_layer->($_)) for 0 .. $#{$self->support_layers};
+        },
+    );
+}
+
+sub _compute_support_layers {
+    my ($contact_z, $top_z, $config, $flow) = @_;
     
-        if (0) {
-            require "Slic3r/SVG.pm";
-            Slic3r::SVG::output("support_$_.svg",
-                polylines        => [ map $_->polyline, map @$_, $support_patterns->[$_] ],
-                red_polylines    => [ map $_->polyline, map @$_, $support_interface_patterns->[$_] ],
-                polygons         => [ map @$_, @areas ],
-            ) for 0 .. $#$support_patterns;
+    # quick table to check whether a given Z is a top surface
+    my %top = map { $_ => 1 } @$top_z;
+    
+    # determine layer height for any non-contact layer
+    # we use max() to prevent many ultra-thin layers to be inserted in case
+    # layer_height > nozzle_diameter * 0.75
+    my $support_material_height = max($config->layer_height, $flow->nozzle_diameter * 0.75);
+    
+    my @support_layers = sort { $a <=> $b } @$contact_z, @$top_z,
+        (map { $_ + $flow->nozzle_diameter } @$top_z);
+    
+    # enforce first layer height
+    my $first_layer_height = $config->get_value('first_layer_height');
+    shift @support_layers while @support_layers && $support_layers[0] < $first_layer_height;
+    unshift @support_layers, $first_layer_height;
+    
+    for (my $i = $#support_layers; $i >= 0; $i--) {
+        my $target_height = $support_material_height;
+        if ($i > 0 && $top{ $support_layers[$i-1] }) {
+            $target_height = $flow->nozzle_diameter;
+        }
+        
+        # enforce first layer height
+        if (($i == 0 && $support_layers[$i] > $target_height + $first_layer_height)
+            || ($support_layers[$i] - $support_layers[$i-1] > $target_height + Slic3r::Geometry::epsilon)) {
+            splice @support_layers, $i, 0, ($support_layers[$i] - $target_height);
+            $i++;
         }
     }
     
-    # apply the pattern to layers
-    Slic3r::debugf "Applying patterns\n";
+    # remove duplicates
     {
-        my $clip_pattern = sub {
-            my ($layer_id, $expolygons, $height, $is_interface) = @_;
-            my @paths = ();
-            foreach my $expolygon (@$expolygons) {
-                push @paths,
-                    map $_->pack,
-                    map {
-                        $_->height($height);
-                        
-                        # useless line because this coderef isn't called for layer 0 anymore;
-                        # let's keep it here just in case we want to make the base flange optional
-                        # in the future
-                        $_->flow_spacing($self->print->first_layer_support_material_flow->spacing)
-                            if $layer_id == 0;
-                        
-                        $_;
-                    }
-                    map $_->clip_with_expolygon($expolygon),
-                    ###map $_->clip_with_polygon($expolygon->bounding_box->polygon),  # currently disabled as a workaround for Boost failing at being idempotent
-                    ($is_interface && @$support_interface_patterns)
-                        ? @{$support_interface_patterns->[ $layer_id % @$support_interface_patterns ]}
-                        : @{$support_patterns->[ $layer_id % @$support_patterns ]};
-            };
-            return @paths;
-        };
-        my %layer_paths             = ();
-        my %layer_contact_paths     = ();
-        my %layer_islands           = ();
-        my $process_layer = sub {
-            my ($layer_id) = @_;
-            my $layer = $self->layers->[$layer_id];
-            
-            my ($paths, $contact_paths) = ([], []);
-            my $islands = union_ex([ map @$_, map @$_, $layers{$layer_id}, $layers_contact_areas{$layer_id} ]);
-            
-            # make a solid base on bottom layer
-            if ($layer_id == 0) {
-                my $filler = $self->fill_maker->filler('rectilinear');
-                $filler->angle($Slic3r::Config->support_material_angle + 90);
-                foreach my $expolygon (@$islands) {
-                    my @paths = $filler->fill_surface(
-                        Slic3r::Surface->new(expolygon => $expolygon),
-                        density         => 0.5,
-                        flow_spacing    => $self->print->first_layer_support_material_flow->spacing,
-                    );
-                    my $params = shift @paths;
-                    
-                    push @$paths, map Slic3r::ExtrusionPath->new(
-                        polyline        => Slic3r::Polyline->new(@$_),
-                        role            => EXTR_ROLE_SUPPORTMATERIAL,
-                        height          => undef,
-                        flow_spacing    => $params->{flow_spacing},
-                    ), @paths;
-                }
-            } else {
-                $paths           = [
-                    $clip_pattern->($layer_id, $layers{$layer_id}, $layer->height),
-                    $clip_pattern->($layer_id, $layers_interfaces{$layer_id}, $layer->height, 1),
-                ];
-                $contact_paths   = [ $clip_pattern->($layer_id, $layers_contact_areas{$layer_id}, $layer->support_material_contact_height, 1) ];
-            }
-            return ($paths, $contact_paths, $islands);
-        };
-        Slic3r::parallelize(
-            items => [ keys %layers ],
-            thread_cb => sub {
-                my $q = shift;
-                $Slic3r::Geometry::Clipper::clipper = Math::Clipper->new;
-                my $result = {};
-                while (defined (my $layer_id = $q->dequeue)) {
-                    $result->{$layer_id} = [ $process_layer->($layer_id) ];
-                }
-                return $result;
-            },
-            collect_cb => sub {
-                my $result = shift;
-                ($layer_paths{$_}, $layer_contact_paths{$_}, $layer_islands{$_}) = @{$result->{$_}} for keys %$result;
-            },
-            no_threads_cb => sub {
-                ($layer_paths{$_}, $layer_contact_paths{$_}, $layer_islands{$_}) = $process_layer->($_) for keys %layers;
-            },
-        );
-        
-        foreach my $layer_id (keys %layer_paths) {
-            my $layer = $self->layers->[$layer_id];
-            $layer->support_islands($layer_islands{$layer_id});
-            $layer->support_fills(Slic3r::ExtrusionPath::Collection->new);
-            $layer->support_contact_fills(Slic3r::ExtrusionPath::Collection->new);
-            push @{$layer->support_fills->paths}, @{$layer_paths{$layer_id}};
-            push @{$layer->support_contact_fills->paths}, @{$layer_contact_paths{$layer_id}};
-        }
+        my %sl = map { $_ => 1 } @support_layers;
+        @support_layers = sort { $a <=> $b } keys %sl;
     }
+    
+    return @support_layers;
 }
 
 1;
diff --git a/lib/Slic3r/Test/SectionCut.pm b/lib/Slic3r/Test/SectionCut.pm
index 97d76af8b..0ee1c2b0f 100644
--- a/lib/Slic3r/Test/SectionCut.pm
+++ b/lib/Slic3r/Test/SectionCut.pm
@@ -58,7 +58,7 @@ sub export_svg {
     );
     
     $group->(
-        filter => sub { $_[0]->support_fills, $_[0]->support_contact_fills },
+        filter => sub { $_[0]->isa('Slic3r::Layer::Support') ? ($_[0]->support_fills, $_[0]->support_interface_fills) : () },
         style  => {
             'stroke-width'  => 1,
             'stroke'        => '#444444',
@@ -80,7 +80,7 @@ sub _plot {
     
     foreach my $object (@{$self->print->objects}) {
         foreach my $copy (@{$object->copies}) {
-            foreach my $layer (@{$object->layers}) {
+            foreach my $layer (@{$object->layers}, @{$object->support_layers}) {
                 # get all ExtrusionPath objects
                 my @paths = 
                     map { $_->polyline->translate(@$copy); $_ }
diff --git a/slic3r.pl b/slic3r.pl
index 4a654b31e..52becfdd1 100755
--- a/slic3r.pl
+++ b/slic3r.pl
@@ -407,6 +407,8 @@ $j
     --infill-extruder   Extruder to use for infill (1+, default: 1)
     --support-material-extruder
                         Extruder to use for support material (1+, default: 1)
+    --support-material-interface-extruder
+                        Extruder to use for support material interface (1+, default: 1)
     
 EOF
     exit ($exit_code || 0);
diff --git a/t/support.t b/t/support.t
index bee1987f1..9ed16395a 100644
--- a/t/support.t
+++ b/t/support.t
@@ -1,4 +1,4 @@
-use Test::More tests => 1;
+use Test::More tests => 13;
 use strict;
 use warnings;
 
@@ -7,15 +7,58 @@ BEGIN {
     use lib "$FindBin::Bin/../lib";
 }
 
+use List::Util qw(first);
 use Slic3r;
+use Slic3r::Geometry qw(epsilon);
 use Slic3r::Test;
 
 {
     my $config = Slic3r::Config->new_from_defaults;
+    my @contact_z = my @top_z = ();
+    
+    my $test = sub {
+        my $flow = Slic3r::Flow->new(nozzle_diameter => $config->nozzle_diameter->[0], layer_height => $config->layer_height);
+        my @support_layers = Slic3r::Print::Object::_compute_support_layers(\@contact_z, \@top_z, $config, $flow);
+        
+        is $support_layers[0], $config->first_layer_height,
+            'first layer height is honored';
+        is scalar(grep { $support_layers[$_]-$support_layers[$_-1] <= 0 } 1..$#support_layers), 0,
+            'no null or negative support layers';
+        is scalar(grep { $support_layers[$_]-$support_layers[$_-1] > $flow->nozzle_diameter + epsilon } 1..$#support_layers), 0,
+            'no layers thicker than nozzle diameter';
+        
+        my $wrong_top_spacing = 0;
+        foreach my $top_z (@top_z) {
+            # find layer index of this top surface
+            my $layer_id = first { abs($support_layers[$_] - $top_z) < epsilon } 0..$#support_layers;
+            
+            # check that first support layer above this top surface is spaced with nozzle diameter
+            $wrong_top_spacing = 1
+                if ($support_layers[$layer_id+1] - $support_layers[$layer_id]) != $flow->nozzle_diameter;
+        }
+        ok !$wrong_top_spacing, 'layers above top surfaces are spaced correctly';
+    };
+    
+    $config->set('layer_height', 0.2);
+    $config->set('first_layer_height', 0.3);
+    @contact_z = (1.9);
+    @top_z = (1.1);
+    $test->();
+    
+    $config->set('first_layer_height', 0.4);
+    $test->();
+    
+    $config->set('layer_height', $config->nozzle_diameter->[0]);
+    $test->();
+}
+
+{
+    my $config = Slic3r::Config->new_from_defaults;
     $config->set('raft_layers', 3);
     $config->set('brim_width',  6);
     $config->set('skirts', 0);
     $config->set('support_material_extruder', 2);
+    $config->set('support_material_interface_extruder', 2);
     $config->set('layer_height', 0.4);
     $config->set('first_layer_height', '100%');
     my $print = Slic3r::Test::init_print('20mm_cube', config => $config);