diff --git a/main/Scene-load.cc b/main/Scene-load.cc
index a8d077a..29714aa 100644
--- a/main/Scene-load.cc
+++ b/main/Scene-load.cc
@@ -642,15 +642,19 @@ refptr<Polygon> Scene::processNGon(refptr<Node> node)
     refptr<Polygon> p = new Polygon();
     double radius = node->getChildren()[0]->getNumber();
     int n = node->getInteger();
+    int step = n < 0 ? -1 : 1;
+    n = abs(n);
     if (n < 3)
         n = 3;
+    int pos = 0;
     double astep = 2.0 * M_PI / n;
     for (int i = 0; i < n; i++)
     {
         p->push_back(new Vector(
-                    radius * cos(i * astep),
-                    radius * sin(i * astep),
+                    radius * cos(pos * astep),
+                    radius * sin(pos * astep),
                     0.0));
+        pos += step;
     }
     return p;
 }
diff --git a/shapes/Extrude.cc b/shapes/Extrude.cc
index e249f73..9d5eadc 100644
--- a/shapes/Extrude.cc
+++ b/shapes/Extrude.cc
@@ -4,6 +4,7 @@
 #include <iostream>
 
 #include "Extrude.h"
+#include "util/Polygon.h"
 #include "util/Solver.h"
 
 using namespace std;
@@ -19,6 +20,63 @@ Shape::IntersectionList Extrude::intersect(refptr<Shape> _this, const Ray & ray)
     Ray ray_inv = m_inverse.transform_ray(ray);
     IntersectionList res;
 
+    int n_polygons = m_polygons.size(), n_offsets = m_offsets.size();
+    for (int p = 0; p < n_polygons; p++)
+    {
+        refptr<Polygon> polygon = m_polygons[p];
+        double distance = 0.0;
+        Vector scale(1.0, 1.0, 1.0);
+        Vector shift(0.0, 0.0, 0.0);
+        for (int o = 0; o < n_offsets; o++)
+        {
+            Offset & offset = m_offsets[o];
+            for (int pt = 0, n_pts = polygon->size(); pt < n_pts; pt++)
+            {
+                Vector p1 = *(*polygon)[pt] * scale + shift;
+                Vector p2 = *(*polygon)[(pt+1) % n_pts] * scale + shift;
+                Vector p3 = *(*polygon)[(pt+1) % n_pts]
+                    * offset.scale + offset.shift;
+                Vector p4 = *(*polygon)[pt] * offset.scale + offset.shift;
+                p3[2] += offset.distance;
+                p4[2] += offset.distance;
+                Vector e1 = p2 - p1;
+                Vector e2 = p3 - p2;
+                Vector normal = (e1 * e2).normalize();
+                double a = normal[0], b = normal[1], c = normal[2];
+                double d = -a * p1[0] - b * p1[1] - c * p1[2];
+                if (ray_inv.getDirection() % normal < 0) /* skip backfaces */
+                {
+                    LinearSolver solver(  a * ray_inv.getDirection()[0]
+                                        + b * ray_inv.getDirection()[1]
+                                        + c * ray_inv.getDirection()[2],
+                                          a * ray_inv.getOrigin()[0]
+                                        + b * ray_inv.getOrigin()[1]
+                                        + c * ray_inv.getOrigin()[2]
+                                        + d);
+                    Solver::Result solutions = solver.solve();
+                    if (solutions.numResults > 0)
+                    {
+                        if (solutions.results[0] > 0.0)
+                        {
+                            Vector ipoint = ray_inv[solutions.results[0]];
+                            Polygon quad;
+                            quad.add(p1).add(p2).add(p3).add(p4);
+                            if (quad.containsPoint(ipoint))
+                            {
+                                res.add(Intersection(_this,
+                                            m_transform.transform_point(ipoint),
+                                            m_transform.transform_normal(normal)));
+                            }
+                        }
+                    }
+                }
+            }
+            distance += offset.distance;
+            scale = offset.scale;
+            shift = offset.shift;
+        }
+    }
+
     return res;
 }
 
diff --git a/util/Polygon.cc b/util/Polygon.cc
index ccf9415..b57ac67 100644
--- a/util/Polygon.cc
+++ b/util/Polygon.cc
@@ -1,2 +1,20 @@
 
+#include <math.h>                   /* acos(), M_PI */
+
 #include "Polygon.h"
+
+#define FP_EQUAL(x,y) (fabs((x)-(y)) < 1E-6)
+
+bool Polygon::containsPoint(const Vector & v)
+{
+    double angle_sum = 0.0;
+    for (int i = 0, sz = size(); i < sz; i++)
+    {
+        Vector v1 = *(*this)[i] - v;
+        Vector v2 = *(*this)[(i+1) % sz] - v;
+        double cosine = (v1 % v2) / (v1.mag() * v2.mag());
+        double angle = acos(cosine);
+        angle_sum += angle;
+    }
+    return FP_EQUAL(angle_sum, 2.0 * M_PI);
+}
diff --git a/util/Polygon.h b/util/Polygon.h
index 3226d99..ffb00fd 100644
--- a/util/Polygon.h
+++ b/util/Polygon.h
@@ -9,6 +9,18 @@
 
 class Polygon : public std::vector< refptr<Vector> >
 {
+    public:
+        Polygon & add(const Vector & v)
+        {
+            push_back(new Vector(v));
+            return *this;
+        }
+        Polygon & add(refptr<Vector> v)
+        {
+            push_back(v);
+            return *this;
+        }
+        bool containsPoint(const Vector & v);
 };
 
 #endif