--- a/pkg/octree/tree.go	Wed Sep 26 12:43:44 2018 +0200
+++ b/pkg/octree/tree.go	Wed Sep 26 12:44:22 2018 +0200
@@ -15,91 +15,6 @@
 	Max Vertex
 }
 
-type box2d struct {
-	x1 float64
-	y1 float64
-	x2 float64
-	y2 float64
-}
-
-func (a box2d) intersects(b box2d) bool {
-	return !(a.x2 < a.x1 || a.x2 < b.x1 ||
-		a.y2 < a.y1 || a.y2 < b.y1)
-}
-
-func (a box2d) mid() (float64, float64) {
-	return (a.x2-a.x1)*0.5 + a.x1, (a.y2-a.y1)*0.5 + a.y1
-}
-
-func (a box2d) xi(i int) float64 {
-	if i == 0 {
-		return a.x1
-	}
-	return a.x2
-}
-
-func (a box2d) yi(i int) float64 {
-	if i == 0 {
-		return a.y1
-	}
-	return a.y2
-}
-
-type plane2d struct {
-	a float64
-	b float64
-	c float64
-}
-
-func newPlane2d(x1, y1, x2, y2 float64) plane2d {
-	a := x2 - x1
-	b := y2 - y1
-
-	l := math.Sqrt(a*a + b*b)
-	a /= l
-	b /= l
-
-	// a*x1 + b*y1 + c = 0
-	c := -(a*x1 + b*y1)
-	return plane2d{a, b, c}
-}
-
-func (p plane2d) eval(x, y float64) float64 {
-	return p.a*x + p.b*y + p.c
-}
-
-const epsPlane = 1e-5
-
-func sides(s int, x float64) int {
-	if math.Signbit(x) {
-		return s | 2
-	}
-	return s | 1
-}
-
-const eps = 1e-05
-
-func (a box2d) intersectsPlane(p plane2d) bool {
-	var s int
-	for i := 0; i < 2; i++ {
-		x := a.xi(i)
-		for j := 0; j < 2; j++ {
-			y := a.yi(j)
-			v := p.eval(x, y)
-			if math.Abs(v) < epsPlane {
-				//log.Printf("on line")
-				return true
-			}
-			if s = sides(s, v); s == 3 {
-				//log.Printf("... on both sides (djt)")
-				return true
-			}
-		}
-	}
-	//log.Printf("side: %d\n", s)
-	return false
-}
-
 var scale = [4][4]float64{
 	{0.0, 0.0, 0.5, 0.5},
 	{0.5, 0.0, 1.0, 0.5},
@@ -109,29 +24,29 @@
 
 func (ot *Tree) Vertical(x1, y1, x2, y2 float64, fn func(*Triangle)) {
 
-	box := box2d{
-		x1: math.Min(x1, x2),
-		y1: math.Min(y1, y2),
-		x2: math.Max(x1, x2),
-		y2: math.Max(y1, y2),
+	box := Box2D{
+		X1: math.Min(x1, x2),
+		Y1: math.Min(y1, y2),
+		X2: math.Max(x1, x2),
+		Y2: math.Max(y1, y2),
 	}
 
 	// out of bounding box
-	if box.x2 < ot.Min.X || ot.Max.X < box.x1 ||
-		box.y2 < ot.Min.Y || ot.Max.Y < box.y1 {
+	if box.X2 < ot.Min.X || ot.Max.X < box.X1 ||
+		box.Y2 < ot.Min.Y || ot.Max.Y < box.Y1 {
 		return
 	}
 
-	line := newPlane2d(x1, y1, x2, y2)
+	line := NewPlane2D(x1, y1, x2, y2)
 
 	type frame struct {
 		pos int32
-		box2d
+		Box2D
 	}
 
 	dupes := map[int32]struct{}{}
 
-	all := box2d{ot.Min.X, ot.Min.Y, ot.Max.X, ot.Max.Y}
+	all := Box2D{ot.Min.X, ot.Min.Y, ot.Max.X, ot.Max.Y}
 	//log.Printf("area: %f\n", (box.x2-box.x1)*(box.y2-box.y1))
 	//log.Printf("all: %f\n", (all.x2-all.x1)*(all.y2-all.y1))
 
@@ -148,15 +63,15 @@
 				if a == 0 && b == 0 {
 					continue
 				}
-				dx := top.x2 - top.x1
-				dy := top.y2 - top.y1
-				nbox := box2d{
-					dx*scale[i][0] + top.x1,
-					dy*scale[i][1] + top.y1,
-					dx*scale[i][2] + top.x1,
-					dy*scale[i][3] + top.y1,
+				dx := top.X2 - top.X1
+				dy := top.Y2 - top.Y1
+				nbox := Box2D{
+					dx*scale[i][0] + top.X1,
+					dy*scale[i][1] + top.Y1,
+					dx*scale[i][2] + top.X1,
+					dy*scale[i][3] + top.Y1,
 				}
-				if nbox.intersects(box) && nbox.intersectsPlane(line) {
+				if nbox.Intersects(box) && nbox.IntersectsPlane(line) {
 					if a != 0 {
 						stack = append(stack, frame{a, nbox})
 					}
@@ -181,9 +96,9 @@
 					ot.vertices[tri[2]],
 				}
 
-				v0 := line.eval(t[0].X, t[0].Y)
-				v1 := line.eval(t[1].X, t[1].Y)
-				v2 := line.eval(t[2].X, t[2].Y)
+				v0 := line.Eval(t[0].X, t[0].Y)
+				v1 := line.Eval(t[1].X, t[1].Y)
+				v2 := line.Eval(t[2].X, t[2].Y)
 
 				if math.Abs(v0) < eps || math.Abs(v1) < eps ||
 					math.Abs(v2) < eps ||

--- a/pkg/octree/vertex.go	Wed Sep 26 12:43:44 2018 +0200
+++ b/pkg/octree/vertex.go	Wed Sep 26 12:44:22 2018 +0200
@@ -22,6 +22,19 @@
 
 	LineStringZ      []Vertex
 	MultiLineStringZ []LineStringZ
+
+	Box2D struct {
+		X1 float64
+		Y1 float64
+		X2 float64
+		Y2 float64
+	}
+
+	Plane2D struct {
+		A float64
+		B float64
+		C float64
+	}
 )
 
 const (
@@ -297,3 +310,75 @@
 
 	return buf.Bytes()
 }
+
+func (a Box2D) Intersects(b Box2D) bool {
+	return !(a.X2 < a.X1 || a.X2 < b.X1 ||
+		a.Y2 < a.Y1 || a.Y2 < b.Y1)
+}
+
+func (a Box2D) Mid() (float64, float64) {
+	return (a.X2-a.X1)*0.5 + a.X1, (a.Y2-a.Y1)*0.5 + a.Y1
+}
+
+func (a Box2D) Xi(i int) float64 {
+	if i == 0 {
+		return a.X1
+	}
+	return a.X2
+}
+
+func (a Box2D) Yi(i int) float64 {
+	if i == 0 {
+		return a.Y1
+	}
+	return a.Y2
+}
+
+func NewPlane2D(x1, y1, x2, y2 float64) Plane2D {
+	a := x2 - x1
+	b := y2 - y1
+
+	l := math.Sqrt(a*a + b*b)
+	a /= l
+	b /= l
+
+	// a*x1 + b*y1 + c = 0
+	c := -(a*x1 + b*y1)
+	return Plane2D{a, b, c}
+}
+
+func (p Plane2D) Eval(x, y float64) float64 {
+	return p.A*x + p.B*y + p.C
+}
+
+const epsPlane = 1e-5
+
+func sides(s int, x float64) int {
+	if math.Signbit(x) {
+		return s | 2
+	}
+	return s | 1
+}
+
+const eps = 1e-05
+
+func (a Box2D) IntersectsPlane(p Plane2D) bool {
+	var s int
+	for i := 0; i < 2; i++ {
+		x := a.Xi(i)
+		for j := 0; j < 2; j++ {
+			y := a.Yi(j)
+			v := p.Eval(x, y)
+			if math.Abs(v) < epsPlane {
+				//log.Printf("on line")
+				return true
+			}
+			if s = sides(s, v); s == 3 {
+				//log.Printf("... on both sides (djt)")
+				return true
+			}
+		}
+	}
+	//log.Printf("side: %d\n", s)
+	return false
+}