diff --git a/xs/src/libslic3r/SVG.cpp b/xs/src/libslic3r/SVG.cpp
index 328d5421a..f196d4713 100644
--- a/xs/src/libslic3r/SVG.cpp
+++ b/xs/src/libslic3r/SVG.cpp
@@ -32,9 +32,17 @@ SVG::draw(const Line &line, std::string stroke)
 }
 
 void
-SVG::AddLine(const IntersectionLine &line)
+SVG::draw(const Lines &lines, std::string stroke)
 {
-    this->draw(Line(line.a, line.b));
+    for (Lines::const_iterator it = lines.begin(); it != lines.end(); ++it)
+        this->draw(*it, stroke);
+}
+
+void
+SVG::draw(const IntersectionLines &lines, std::string stroke)
+{
+    for (IntersectionLines::const_iterator it = lines.begin(); it != lines.end(); ++it)
+        this->draw((Line)*it, stroke);
 }
 
 void
@@ -50,11 +58,25 @@ SVG::draw(const ExPolygon &expolygon, std::string fill)
     this->path(d, true);
 }
 
+void
+SVG::draw(const ExPolygons &expolygons, std::string fill)
+{
+    for (ExPolygons::const_iterator it = expolygons.begin(); it != expolygons.end(); ++it)
+        this->draw(*it, fill);
+}
+
 void
 SVG::draw(const Polygon &polygon, std::string fill)
 {
     this->fill = fill;
-    this->path(this->get_path_d(polygon, true), true);
+    this->path(this->get_path_d(polygon, true), !fill.empty());
+}
+
+void
+SVG::draw(const Polygons &polygons, std::string fill)
+{
+    for (Polygons::const_iterator it = polygons.begin(); it != polygons.end(); ++it)
+        this->draw(*it, fill);
 }
 
 void
@@ -64,6 +86,13 @@ SVG::draw(const Polyline &polyline, std::string stroke)
     this->path(this->get_path_d(polyline, false), false);
 }
 
+void
+SVG::draw(const Polylines &polylines, std::string stroke)
+{
+    for (Polylines::const_iterator it = polylines.begin(); it != polylines.end(); ++it)
+        this->draw(*it, fill);
+}
+
 void
 SVG::draw(const Point &point, std::string fill, unsigned int radius)
 {
@@ -75,6 +104,13 @@ SVG::draw(const Point &point, std::string fill, unsigned int radius)
     fprintf(this->f, "%s\n", svg.str().c_str());
 }
 
+void
+SVG::draw(const Points &points, std::string fill, unsigned int radius)
+{
+    for (Points::const_iterator it = points.begin(); it != points.end(); ++it)
+        this->draw(*it, fill, radius);
+}
+
 void
 SVG::path(const std::string &d, bool fill)
 {
diff --git a/xs/src/libslic3r/SVG.hpp b/xs/src/libslic3r/SVG.hpp
index bb0d5adcf..cbc546402 100644
--- a/xs/src/libslic3r/SVG.hpp
+++ b/xs/src/libslic3r/SVG.hpp
@@ -15,12 +15,17 @@ class SVG
     std::string fill, stroke;
     
     SVG(const char* filename);
-    void AddLine(const IntersectionLine &line);
     void draw(const Line &line, std::string stroke = "black");
+    void draw(const Lines &lines, std::string stroke = "black");
+    void draw(const IntersectionLines &lines, std::string stroke = "black");
     void draw(const ExPolygon &expolygon, std::string fill = "grey");
+    void draw(const ExPolygons &expolygons, std::string fill = "grey");
     void draw(const Polygon &polygon, std::string fill = "grey");
+    void draw(const Polygons &polygons, std::string fill = "grey");
     void draw(const Polyline &polyline, std::string stroke = "black");
+    void draw(const Polylines &polylines, std::string stroke = "black");
     void draw(const Point &point, std::string fill = "black", unsigned int radius = 3);
+    void draw(const Points &points, std::string fill = "black", unsigned int radius = 3);
     void Close();
     
     private:
diff --git a/xs/src/libslic3r/TriangleMesh.cpp b/xs/src/libslic3r/TriangleMesh.cpp
index 89116f17b..b25df7e0e 100644
--- a/xs/src/libslic3r/TriangleMesh.cpp
+++ b/xs/src/libslic3r/TriangleMesh.cpp
@@ -181,6 +181,7 @@ TriangleMesh::WriteOBJFile(char* output_file) {
 void TriangleMesh::scale(float factor)
 {
     stl_scale(&(this->stl), factor);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::scale(const Pointf3 &versor)
@@ -190,41 +191,49 @@ void TriangleMesh::scale(const Pointf3 &versor)
     fversor[1] = versor.y;
     fversor[2] = versor.z;
     stl_scale_versor(&this->stl, fversor);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::translate(float x, float y, float z)
 {
     stl_translate_relative(&(this->stl), x, y, z);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::rotate_x(float angle)
 {
     stl_rotate_x(&(this->stl), angle);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::rotate_y(float angle)
 {
     stl_rotate_y(&(this->stl), angle);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::rotate_z(float angle)
 {
     stl_rotate_z(&(this->stl), angle);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::flip_x()
 {
     stl_mirror_yz(&this->stl);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::flip_y()
 {
     stl_mirror_xz(&this->stl);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::flip_z()
 {
     stl_mirror_xy(&this->stl);
+    stl_invalidate_shared_vertices(&this->stl);
 }
 
 void TriangleMesh::align_to_origin()
@@ -507,7 +516,7 @@ TriangleMeshSlicer::slice(const std::vector<float> &z, std::vector<ExPolygons>*
     for (std::vector<Polygons>::const_iterator loops = layers_p.begin(); loops != layers_p.end(); ++loops) {
         #ifdef SLIC3R_DEBUG
         size_t layer_id = loops - layers_p.begin();
-        printf("Layer %zu (slice_z = %.2f): ", layer_id, z[layer_id]);
+        printf("Layer %zu (slice_z = %.2f):\n", layer_id, z[layer_id]);
         #endif
         
         this->make_expolygons(*loops, &(*layers)[ loops - layers_p.begin() ]);
@@ -542,15 +551,19 @@ TriangleMeshSlicer::slice_facet(float slice_z, const stl_facet &facet, const int
         if (a->z == b->z && a->z == slice_z) {
             // edge is horizontal and belongs to the current layer
             
-            /* We assume that this method is never being called for horizontal
-               facets, so no other edge is going to be on this layer. */
-            stl_vertex* v0 = &this->v_scaled_shared[ this->mesh->stl.v_indices[facet_idx].vertex[0] ];
-            stl_vertex* v1 = &this->v_scaled_shared[ this->mesh->stl.v_indices[facet_idx].vertex[1] ];
-            stl_vertex* v2 = &this->v_scaled_shared[ this->mesh->stl.v_indices[facet_idx].vertex[2] ];
+            stl_vertex &v0 = this->v_scaled_shared[ this->mesh->stl.v_indices[facet_idx].vertex[0] ];
+            stl_vertex &v1 = this->v_scaled_shared[ this->mesh->stl.v_indices[facet_idx].vertex[1] ];
+            stl_vertex &v2 = this->v_scaled_shared[ this->mesh->stl.v_indices[facet_idx].vertex[2] ];
             IntersectionLine line;
             if (min_z == max_z) {
                 line.edge_type = feHorizontal;
-            } else if (v0->z < slice_z || v1->z < slice_z || v2->z < slice_z) {
+                if (this->mesh->stl.facet_start[facet_idx].normal.z < 0) {
+                    /*  if normal points downwards this is a bottom horizontal facet so we reverse
+                        its point order */
+                    std::swap(a, b);
+                    std::swap(a_id, b_id);
+                }
+            } else if (v0.z < slice_z || v1.z < slice_z || v2.z < slice_z) {
                 line.edge_type = feTop;
                 std::swap(a, b);
                 std::swap(a_id, b_id);
@@ -624,12 +637,9 @@ TriangleMeshSlicer::slice_facet(float slice_z, const stl_facet &facet, const int
 void
 TriangleMeshSlicer::make_loops(std::vector<IntersectionLine> &lines, Polygons* loops)
 {
-        
     /*
     SVG svg("lines.svg");
-    for (IntersectionLines::iterator line = lines.begin(); line != lines.end(); ++line) {
-            svg.AddLine(*line);
-    }
+    svg.draw(lines);
     svg.Close();
     */
     
@@ -837,9 +847,9 @@ TriangleMeshSlicer::make_expolygons(const Polygons &loops, ExPolygons* slices)
            of the loops, since the Orientation() function provided by Clipper
            would do the same, thus repeating the calculation */
         Polygons::const_iterator loop = loops.begin() + *loop_idx;
-        if (area[*loop_idx] >= 0) {
+        if (area[*loop_idx] > +EPSILON) {
             p_slices.push_back(*loop);
-        } else {
+        } else if (area[*loop_idx] < -EPSILON) {
             diff(p_slices, *loop, &p_slices);
         }
     }
diff --git a/xs/t/01_trianglemesh.t b/xs/t/01_trianglemesh.t
index b2f1d2849..cc140aa5a 100644
--- a/xs/t/01_trianglemesh.t
+++ b/xs/t/01_trianglemesh.t
@@ -4,7 +4,7 @@ use strict;
 use warnings;
 
 use Slic3r::XS;
-use Test::More tests => 46;
+use Test::More tests => 49;
 
 is Slic3r::TriangleMesh::hello_world(), 'Hello world!',
     'hello world';
@@ -79,7 +79,7 @@ my $cube = {
     my $m = Slic3r::TriangleMesh->new;
     $m->ReadFromPerl($cube->{vertices}, $cube->{facets});
     $m->repair;
-    my @z = (2,4,8,6,8,10,12,14,16,18,20);
+    my @z = (0,2,4,8,6,8,10,12,14,16,18,20);
     my $result = $m->slice(\@z);
     my $SCALING_FACTOR = 0.000001;
     for my $i (0..$#z) {
@@ -95,8 +95,19 @@ my $cube = {
         [ [0,1,2],[2,1,3],[1,0,4],[5,1,4],[0,2,4],[4,2,6],[7,6,8],[4,6,7],[9,4,7],[7,8,10],[2,3,6],[11,3,12],[7,12,9],[13,12,7],[6,3,11],[11,12,13],[3,1,5],[12,3,5],[5,4,9],[12,5,9],[13,7,10],[14,13,10],[8,15,10],[10,15,14],[6,11,8],[8,11,15],[15,11,13],[14,15,13] ],
     );
     $m->repair;
-    my $slices = $m->slice([ 5, 10 ]);
-    is $slices->[0][0]->area, $slices->[1][0]->area, 'slicing a tangent plane includes its area';
+    {
+        # at Z = 10 we have a top horizontal surface
+        my $slices = $m->slice([ 5, 10 ]);
+        is $slices->[0][0]->area, $slices->[1][0]->area, 'slicing a top tangent plane includes its area';
+    }
+    $m->flip_z;
+    {
+        # this second test also checks that performing a second slice on a mesh after
+        # a transformation works properly (shared_vertices is correctly invalidated);
+        # at Z = -10 we have a bottom horizontal surface
+        my $slices = $m->slice([ -5, -10 ]);
+        is $slices->[0][0]->area, $slices->[1][0]->area, 'slicing a bottom tangent plane includes its area';
+    }
 }
 
 {