diff --git a/lib/Slic3r/GCode.pm b/lib/Slic3r/GCode.pm
index 38ce4ccb8..934388147 100644
--- a/lib/Slic3r/GCode.pm
+++ b/lib/Slic3r/GCode.pm
@@ -158,24 +158,40 @@ sub extrude_loop {
     if ($self->config->spiral_vase) {
         $loop->split_at($last_pos);
     } elsif ($self->config->seam_position eq 'nearest' || $self->config->seam_position eq 'aligned') {
+        # simplify polygon in order to skip false positives in concave/convex detection
         my $polygon = $loop->polygon;
-        my @candidates = @{$polygon->concave_points(PI*4/3)};
-        @candidates = @{$polygon->convex_points(PI*2/3)} if !@candidates;
-        @candidates = @{$polygon} if !@candidates;
+        my @simplified = @{$polygon->simplify(scale $self->extruder->nozzle_diameter/2)};
         
-        my @non_overhang = grep !$loop->has_overhang_point($_), @candidates;
-        @candidates = @non_overhang if @non_overhang;
+        # concave vertices have priority
+        my @candidates = map @{$_->concave_points(PI*4/3)}, @simplified;
         
-        if ($self->config->seam_position eq 'nearest') {
-            $loop->split_at_vertex($last_pos->nearest_point(\@candidates));
-        } elsif ($self->config->seam_position eq 'aligned') {
-            my $obj_ptr = $self->layer->object->ptr;
-            if (defined $self->layer && defined $self->_seam_position->{$obj_ptr}) {
+        # if no concave points were found, look for convex vertices
+        @candidates = map @{$_->convex_points(PI*2/3)}, @simplified if !@candidates;
+        
+        # retrieve the last start position for this object
+        my $obj_ptr;
+        if (defined $self->layer) {
+            $obj_ptr = $self->layer->object->ptr;
+            if (defined $self->_seam_position->{$self->layer->object}) {
                 $last_pos = $self->_seam_position->{$obj_ptr};
             }
-            my $point = $self->_seam_position->{$obj_ptr} = $last_pos->nearest_point(\@candidates);
-            $loop->split_at_vertex($point);
         }
+        
+        my $point;
+        if ($self->config->seam_position eq 'nearest') {
+            @candidates = @$polygon if !@candidates;
+            $point = $last_pos->nearest_point(\@candidates);
+            $loop->split_at_vertex($point);
+        } elsif (@candidates) {
+            my @non_overhang = grep !$loop->has_overhang_point($_), @candidates;
+            @candidates = @non_overhang if @non_overhang;
+            $point = $last_pos->nearest_point(\@candidates);
+            $loop->split_at_vertex($point);
+        } else {
+            $point = $last_pos->projection_onto_polygon($polygon);
+            $loop->split_at($point);
+        }
+        $self->_seam_position->{$obj_ptr} = $point;
     } elsif ($self->config->seam_position eq 'random') {
         if ($loop->role == EXTRL_ROLE_CONTOUR_INTERNAL_PERIMETER) {
             my $polygon = $loop->polygon;
diff --git a/lib/Slic3r/Polygon.pm b/lib/Slic3r/Polygon.pm
index 01d72dd47..d3d8e6a8d 100644
--- a/lib/Slic3r/Polygon.pm
+++ b/lib/Slic3r/Polygon.pm
@@ -7,7 +7,7 @@ use parent 'Slic3r::Polyline';
 
 use Slic3r::Geometry qw(
     polygon_segment_having_point
-    PI X1 X2 Y1 Y2 epsilon);
+    PI X1 X2 Y1 Y2 epsilon scaled_epsilon);
 use Slic3r::Geometry::Clipper qw(intersection_pl);
 
 sub wkt {
@@ -54,18 +54,18 @@ sub concave_points {
     # but angle3points measures CCW angle, so we calculate the complementary angle
     my $ccw_angle = 2*PI-$angle;
     
+    my @concave = ();
     my @points = @$self;
     my @points_pp = @{$self->pp};
-    
-    my @concave = ();
+
     for my $i (-1 .. ($#points-1)) {
-        next if $points[$i-1]->coincides_with_epsilon($points[$i]) || $points[$i+1]->coincides_with_epsilon($points[$i]);
         # angle is measured in ccw orientation
         my $vertex_angle = Slic3r::Geometry::angle3points(@points_pp[$i, $i-1, $i+1]);
         if ($vertex_angle <= $ccw_angle) {
             push @concave, $points[$i];
         }
     }
+    
     return [@concave];
 }
 
@@ -78,12 +78,11 @@ sub convex_points {
     # but angle3points measures CCW angle, so we calculate the complementary angle
     my $ccw_angle = 2*PI-$angle;
     
+    my @convex = ();
     my @points = @$self;
     my @points_pp = @{$self->pp};
     
-    my @convex = ();
     for my $i (-1 .. ($#points-1)) {
-        next if $points[$i-1]->coincides_with_epsilon($points[$i]) || $points[$i+1]->coincides_with_epsilon($points[$i]);
         # angle is measured in ccw orientation
         my $vertex_angle = Slic3r::Geometry::angle3points(@points_pp[$i, $i-1, $i+1]);
         if ($vertex_angle >= $ccw_angle) {