More fixes.

lib/Slic3r/Geometry.pm

@@ -596,7 +596,6 @@ sub polygon_remove_parallel_continuous_edges {
 
 sub polyline_remove_acute_vertices {
     my ($points, $isPolygon) = @_;
-    
     for (my $i = $isPolygon ? -1 : 1; $i < $#$points; $i++) {
         my $angle = angle3points($points->[$i], $points->[$i-1], $points->[$i+1]);
         if ($angle < 0.01 || $angle >= 2*PI - 0.01) {

lib/Slic3r/Geometry/Clipper.pm

@@ -0,0 +1,32 @@
+package Slic3r::Geometry::Clipper;
+use strict;
+use warnings;
+
+require Exporter;
+our @ISA = qw(Exporter);
+our @EXPORT_OK = qw(diff_ex diff union_ex);
+
+use Math::Clipper ':all';
+our $clipper = Math::Clipper->new;
+
+sub diff_ex {
+    my ($subject, $clip) = @_;
+    
+    $clipper->clear;
+    $clipper->add_subject_polygons($subject);
+    $clipper->add_clip_polygons($clip);
+    return $clipper->ex_execute(CT_DIFFERENCE, PFT_NONZERO, PFT_NONZERO);
+}
+
+sub diff {
+    return [ map { $_->{outer}, $_->{holes} } diff_ex(@_) ];
+}
+
+sub union_ex {
+    my ($polygons) = @_;
+    $clipper->clear;
+    $clipper->add_subject_polygons($polygons);
+    return $clipper->ex_execute(CT_UNION, PFT_NONZERO, PFT_NONZERO);
+}
+
+1;

lib/Slic3r/Polyline/Closed.pm

@@ -35,4 +35,9 @@ sub is_printable {
     return @$offsets ? 1 : 0;
 }
 
+sub is_valid {
+    my $self = shift;
+    return @{$self->points} >= 3;
+}
+
 1;

lib/Slic3r/Print.pm

@@ -134,8 +134,13 @@ sub detect_surfaces_type {
         if ($lower_layer) {
             @bottom = $surface_difference->($layer->surfaces, $lower_layer->surfaces, 'bottom');
             
+            $_->contour->merge_continuous_lines for @bottom;
+            
+            # merge_continuous_lines could return polylines with less than 3 points (thus invalid)
+            # actually, this shouldn't happen so it deserves further investigation
+            @bottom = grep $_->contour->is_valid, @bottom;
+            
             for (@bottom) {
-                $_->contour->merge_continuous_lines;
                 $_->contour->remove_acute_vertices;
         
                 # okay, this is an Ugly Hack(tm) to avoid floating point math problems