diff --git a/scenes/extrudes.fart b/scenes/extrudes.fart
index 4c977a4..45f9e59 100644
--- a/scenes/extrudes.fart
+++ b/scenes/extrudes.fart
@@ -3,14 +3,15 @@ scene
 {
     options
     {
-        multisample 3
+        ambient_occlusion 2
+        multisample 2
         exposure 1.2
     }
 
     camera
     {
         position <0, -10, 10>
-        look_at <0, 0, 1>
+        look_at <0, 0, 2>
     }
 
     light { position <10, -12, 8> }
@@ -24,30 +25,29 @@ scene
         }
     }
 
-    extrude
+    subtract
     {
-        polygon
-        {
-            <3, 0>
-            <1, 1>
-            <2, 3>
-            <-1, 1>
-            <-3, 2>
-            <-2, -1>
-            <-3, -3>
-            <-1, -2>
-            <1, -4>
-            <1, -1>
-        }
-        offset 3
+        extrude { ngon 5, 6 offset 2 }
+        extrude { ngon 5, 5 offset 3 translate <0, 0, -0.5> }
         material { color <1, 0.7, 0> }
-        translate <-3, 0, 0>
     }
 
-    extrude
+    union
     {
-        ngon 5, 2
-        offset 1
-        translate <4, 0, 0>
+        extrude
+        {
+            ngon 6, 2
+            offset 3 { scale 0.5 position <4, 0> }
+            offset 3
+            offset 1 { scale 0 }
+        }
+        extrude
+        {
+            ngon 6, 2
+            offset 3 { scale 0.5 position <-4, 0> }
+            offset 3
+            offset 1 { scale 0 }
+        }
+        material { color <0, 0.9, 0.9> }
     }
 }
diff --git a/shapes/Extrude.cc b/shapes/Extrude.cc
index b42cdbb..a0876ee 100644
--- a/shapes/Extrude.cc
+++ b/shapes/Extrude.cc
@@ -66,29 +66,26 @@ Shape::IntersectionList Extrude::intersect(refptr<Shape> _this, const Ray & ray)
                 Vector e1 = p2 - p1;
                 Vector e2 = p3 - p2;
                 Vector normal = (e1 * e2).normalize();
-                if (ray_inv.getDirection() % normal < 0) /* skip backfaces */
+                double a = normal[0], b = normal[1], c = normal[2];
+                double d = -(a * p1[0] + b * p1[1] + c * p1[2]);
+                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 && solutions.results[0] > 0.0)
                 {
-                    double a = normal[0], b = normal[1], c = normal[2];
-                    double d = -(a * p1[0] + b * p1[1] + c * p1[2]);
-                    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 && 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.containsPointConvex(ipoint))
                     {
-                        Vector ipoint = ray_inv[solutions.results[0]];
-                        Polygon quad;
-                        quad.add(p1).add(p2).add(p3).add(p4);
-                        if (quad.containsPointConvex(ipoint))
-                        {
-                            res.add(Intersection(_this,
-                                        m_transform.transform_point(ipoint),
-                                        m_transform.transform_normal(normal)));
-                        }
+                        res.add(Intersection(_this,
+                                    m_transform.transform_point(ipoint),
+                                    m_transform.transform_normal(normal)));
                     }
                 }
             }
@@ -99,8 +96,31 @@ Shape::IntersectionList Extrude::intersect(refptr<Shape> _this, const Ray & ray)
     }
 
     double a = 0, b = 0, c = -1.0, d = 0.0;
-    if (ray_inv.getDirection() % Vector(a, b, c) < 0)
+    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 && solutions.results[0] > 0.0)
     {
+        Vector ipoint = ray_inv[solutions.results[0]];
+        for (int p = 0; p < n_polygons; p++)
+        {
+            refptr<Polygon> polygon = m_polygons[p];
+            if (polygon->containsPoint2D(ipoint))
+            {
+                res.add(Intersection(_this,
+                            m_transform.transform_point(ipoint),
+                            m_transform.transform_normal(Vector(a, b, c))));
+            }
+        }
+    }
+    if (scale[0] > 0.0 && scale[1] > 0.0)
+    {
+        a = 0, b = 0, c = 1.0, d = -distance;
         LinearSolver solver(  a * ray_inv.getDirection()[0]
                             + b * ray_inv.getDirection()[1]
                             + c * ray_inv.getDirection()[2],
@@ -114,47 +134,18 @@ Shape::IntersectionList Extrude::intersect(refptr<Shape> _this, const Ray & ray)
             Vector ipoint = ray_inv[solutions.results[0]];
             for (int p = 0; p < n_polygons; p++)
             {
-                refptr<Polygon> polygon = m_polygons[p];
-                if (polygon->containsPoint2D(ipoint))
+                Polygon tp = *m_polygons[p];
+                for (int i = 0, sz = tp.size(); i < sz; i++)
+                {
+                    tp[i] = new Vector(tp[i]->mult(scale) + shift);
+                    (*tp[i])[2] += distance;
+                }
+                if (tp.containsPoint2D(ipoint))
                 {
                     res.add(Intersection(_this,
                                 m_transform.transform_point(ipoint),
                                 m_transform.transform_normal(Vector(a, b, c))));
-                }
-            }
-        }
-    }
-    if (scale[0] > 0.0 && scale[1] > 0.0)
-    {
-        a = 0, b = 0, c = 1.0, d = -distance;
-        if (ray_inv.getDirection() % Vector(a, b, c) < 0)
-        {
-            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 && solutions.results[0] > 0.0)
-            {
-                Vector ipoint = ray_inv[solutions.results[0]];
-                for (int p = 0; p < n_polygons; p++)
-                {
-                    Polygon tp = *m_polygons[p];
-                    for (int i = 0, sz = tp.size(); i < sz; i++)
-                    {
-                        tp[i] = new Vector(tp[i]->mult(scale) + shift);
-                        (*tp[i])[2] += distance;
-                    }
-                    if (tp.containsPoint2D(ipoint))
-                    {
-                        res.add(Intersection(_this,
-                                    m_transform.transform_point(ipoint),
-                                    m_transform.transform_normal(Vector(a, b, c))));
-                        break;
-                    }
+                    break;
                 }
             }
         }
diff --git a/util/Polygon.cc b/util/Polygon.cc
index 6524fb9..a187cb5 100644
--- a/util/Polygon.cc
+++ b/util/Polygon.cc
@@ -3,7 +3,7 @@
 
 #include "Polygon.h"
 
-#define FP_EQUAL(x,y) (fabs((x)-(y)) < 1E-6)
+#define FP_EQUAL(x,y) (fabs((x)-(y)) < 1E-3)
 
 /*
  * from http://local.wasp.uwa.edu.au/~pbourke/geometry/insidepoly/
diff --git a/vim/syntax/fart.vim b/vim/syntax/fart.vim
index 85ca63e..8741afa 100644
--- a/vim/syntax/fart.vim
+++ b/vim/syntax/fart.vim
@@ -10,8 +10,8 @@ endif
 
 syn case match
 
-syn keyword fartKeywords ambient ambient_occlusion color define diffuse exposure extrude height jitter look_at material max_depth multisample ngon offset polygon position radius reflectance rotate scale shininess size specular translate transparency union up vfov width
-syn keyword fartObjects box camera cyl intersect light options plane scene sphere subtract union
+syn keyword fartKeywords ambient ambient_occlusion color define diffuse exposure height jitter look_at material max_depth multisample ngon offset polygon position radius reflectance rotate scale shininess size specular translate transparency union up vfov width
+syn keyword fartObjects box camera cyl extrude intersect light options plane scene sphere subtract union
 syn match fartNumber "\(^\|\W\)\@<=[+-]\=\(\d\+\)\=\.\=\d\+\([eE][+-]\=\d\+\)\="
 
 hi def link fartKeywords    Operator