diff --git a/corners.c b/corners.c
index 22b70e5..37dddea 100644
--- a/corners.c
+++ b/corners.c
@@ -17,177 +17,8 @@ static int debug = 0;
 static int draw_labels = 0;
 
 
-#if 0
-/* Returns 1 for ever edge in f1 that is shared with f2.
- */
 int
-coplanar_check(
-	const stl_face_t * const f1,
-	const stl_face_t * const f2
-)
-{
-	// Verify that there are three matching points
-	int match[3] = {0,0,0};
-	for (int i = 0 ; i < 3 ; i++)
-	{
-		for (int j = 0 ; j < 3 ; j++)
-			if (v3_eq(&f1->p[i], &f2->p[j]))
-				match[i] = 1;
-	}
-
-	uint8_t mask = 0;
-
-	if (match[0] && match[1])
-		mask = 1;
-	if (match[1] && match[2])
-		mask = 2;
-	if (match[2] && match[0])
-		mask = 4;
-
-	if (debug)
-		fprintf(stderr, "%p %p: %d %d %d\n",
-			f1,
-			f2,
-			match[0], match[1], match[2]
-		);
-
-	// otherwise they do not share enough points
-	if (mask == 0)
-		return 0;
-
-#if 0
-	// find the four distinct points
-	v3_t x1 = f1->p[0];
-	v3_t x2 = f1->p[1];
-	v3_t x3 = f1->p[2];
-	v3_t x4;
-
-	for (int i = 0 ; i < 3 ; i++)
-	{
-		x4 = f2->p[i];
-		if (v3_eq(&x1, &x4))
-			continue;
-		if (v3_eq(&x2, &x4))
-			continue;
-		if (v3_eq(&x3, &x4))
-			continue;
-		break;
-	}
-
-	// (x3-x1) . ((x2-x1) X (x4-x3)) == 0
-	v3_t dx31 = v3_sub(x3, x1);
-	v3_t dx21 = v3_sub(x2, x1);
-	v3_t dx43 = v3_sub(x4, x3);
-	v3_t cross = v3_cross(dx21, dx43);
-	float dot = v3_dot(dx31, cross);
-
-	if (debug)
-	fprintf(stderr, "dot %f:\n %f,%f,%f\n %f,%f,%f\n %f,%f,%f\n %f,%f,%f\n",
-		dot,
-		x1.p[0], x1.p[1], x1.p[2],
-		x2.p[0], x2.p[1], x2.p[2],
-		x3.p[0], x3.p[1], x3.p[2],
-		x4.p[0], x4.p[1], x4.p[2]
-	);
-	
-	int check = -EPS < dot && dot < +EPS;
-
-	// if the dot product is not close enough to zero, they
-	// are not coplanar.
-	if (!check)
-		return 0;
-
-	// coplanar! return the shared edge mask
-	return mask;
-#else
-	// if the normals are close enough, then it is coplanner
-	if (v3_eq(&f1->normal, &f2->normal))
-		return mask;
-	else
-		return 0;
-#endif
-}
-
-
-static inline float
-sign(
-	const float x
-)
-{
-	if (x < 0)
-		return -1;
-	if (x > 0)
-		return +1;
-	return 0;
-}
-
-
-
-/**
- * Add a vector to the list of edges if it is not already present
- * and if it is not coplanar with other ones.
- * Note that if it is coplanar, but "outside" the other edges then it
- * will replace the inside one.
- */
-void
-stl_edge_insert(
-	stl_vertex_t * const v1,
-	stl_vertex_t * const v2
-)
-{
-	for (int i = 0 ; i < v1->num_edges ; i++)
-	{
-		// if v2 already exists in the edges, discard it
-		if (v1->edges[i] == v2)
-			return;
-	}
-
-	// if we reach this point, we need to insert the edge
-	if (debug)
-	fprintf(stderr, "%p: edge %d -> %p\n",
-		v1,
-		v1->num_edges,
-		v2
-	);
-	v1->edges[v1->num_edges++] = v2;
-}
-
-
-/** Find or create a vertex */
-stl_vertex_t *
-stl_vertex_find(
-	stl_vertex_t ** const vertices,
-	int * num_vertex_ptr,
-	const v3_t * const p
-)
-{
-	const int num_vertex = *num_vertex_ptr;
-
-	for (int x = 0 ; x < num_vertex ; x++)
-	{
-		stl_vertex_t * const v = vertices[x];
-
-		if (v3_eq(&v->p, p))
-			return v;
-	}
-
-	if (debug)
-	fprintf(stderr, "%d: %f,%f,%f\n",
-		num_vertex,
-		p->p[0],
-		p->p[1],
-		p->p[2]
-	);
-
-	stl_vertex_t * const v = vertices[(*num_vertex_ptr)++] = calloc(1, sizeof(*v));
-	v->p = *p;
-	return v;
-}
-#endif
-
-
-
-int main(void)
+main(void)
 {
 	stl_3d_t * const stl = stl_3d_parse(STDIN_FILENO);
 	if (!stl)
@@ -206,11 +37,11 @@ int main(void)
 		{
 			const stl_face_t * const f = v->face[j];
 
-			fprintf(stderr, "\t%d: %d,%d,%d\n",
+			fprintf(stderr, "\t%d: %d:%f,%d:%f,%d:%f\n",
 				f - stl->face,
-				f->face[0] ? f->face[0] - stl->face : -1,
-				f->face[1] ? f->face[1] - stl->face : -1,
-				f->face[2] ? f->face[2] - stl->face : -1
+				f->face[0] ? f->face[0] - stl->face : -1, f->angle[0],
+				f->face[1] ? f->face[1] - stl->face : -1, f->angle[1],
+				f->face[2] ? f->face[2] - stl->face : -1, f->angle[2]
 			);
 		}
 	}
diff --git a/stl_3d.c b/stl_3d.c
index b62d68d..6dd3e52 100644
--- a/stl_3d.c
+++ b/stl_3d.c
@@ -57,6 +57,9 @@ stl_vertex_find(
 }
 
 
+/** Check to see if the two faces share an edge.
+ * \return 0 if no common edge, 1 if there is a shared link
+ */
 static int
 stl_has_edge(
 	const stl_face_t * const f,
@@ -73,13 +76,61 @@ stl_has_edge(
 }
 
 
+/** Compute the angle between the two planes.
+ * This is an approximation:
+ * \return 0 == coplanar, negative == valley, positive == mountain.
+ */
 static double
 stl_angle(
 	const stl_face_t * const f1,
 	const stl_face_t * const f2
 )
 {
-	return 0;
+	// find the four distinct points
+	v3_t x1 = f1->vertex[0]->p;
+	v3_t x2 = f1->vertex[1]->p;
+	v3_t x3 = f1->vertex[2]->p;
+	v3_t x4;
+
+	for (int i = 0 ; i < 3 ; i++)
+	{
+		x4 = f2->vertex[i]->p;
+		if (v3_eq(&x1, &x4))
+			continue;
+		if (v3_eq(&x2, &x4))
+			continue;
+		if (v3_eq(&x3, &x4))
+			continue;
+		break;
+	}
+
+	// (x3-x1) . ((x2-x1) X (x4-x3)) == 0
+	v3_t dx31 = v3_sub(x3, x1);
+	v3_t dx21 = v3_sub(x2, x1);
+	v3_t dx43 = v3_sub(x4, x3);
+	v3_t cross = v3_cross(dx21, dx43);
+	float dot = v3_dot(dx31, cross);
+
+	if (debug)
+	fprintf(stderr, "dot %f:\n %f,%f,%f\n %f,%f,%f\n %f,%f,%f\n %f,%f,%f\n",
+		dot,
+		x1.p[0], x1.p[1], x1.p[2],
+		x2.p[0], x2.p[1], x2.p[2],
+		x3.p[0], x3.p[1], x3.p[2],
+		x4.p[0], x4.p[1], x4.p[2]
+	);
+	
+	int check = -EPS < dot && dot < +EPS;
+
+	// if the dot product is not close enough to zero, they
+	// are not coplanar.
+	if (check)
+		return 0;
+
+	if (dot < 0)
+		return -1;
+	else
+		return +1;
 }