--- a/pkg/octree/vertex.go	Sun Dec 30 16:24:51 2018 +0100
+++ b/pkg/octree/vertex.go	Mon Dec 31 11:13:49 2018 +0100
@@ -23,20 +23,30 @@
 )
 
 type (
+	// Vertex is a 3D vertex.
 	Vertex struct {
 		X float64
 		Y float64
 		Z float64
 	}
 
+	// Triangle is a triangle consisting of three vertices.
 	Triangle [3]Vertex
 
+	// Line is a line defined by first vertex on that line
+	// and the second being the direction.
 	Line [2]Vertex
 
-	MultiPointZ      []Vertex
-	LineStringZ      []Vertex
+	// MultiPointZ is a set of vertices.
+	MultiPointZ []Vertex
+
+	// LineStringZ is a line string formed of vertices.
+	LineStringZ []Vertex
+
+	// MultiLineStringZ is a set of line strings.
 	MultiLineStringZ []LineStringZ
 
+	// Box2D is 2D area from (X1, Y1) to (X2, Y2).
 	Box2D struct {
 		X1 float64
 		Y1 float64
@@ -44,6 +54,7 @@
 		Y2 float64
 	}
 
+	// Plane2D is a 2D plane (a line in 2D space).
 	Plane2D struct {
 		A float64
 		B float64
@@ -51,6 +62,8 @@
 	}
 )
 
+// Minimize adjust this vertex v to hold the minimum
+// values component-wise of v and w.
 func (v *Vertex) Minimize(w Vertex) {
 	if w.X < v.X {
 		v.X = w.X
@@ -63,6 +76,8 @@
 	}
 }
 
+// Maximize adjust this vertex v to hold the maximum
+// values component-wise of v and w.
 func (v *Vertex) Maximize(w Vertex) {
 	if w.X > v.X {
 		v.X = w.X
@@ -75,6 +90,7 @@
 	}
 }
 
+// Sub returns (v - w) component-wise.
 func (v Vertex) Sub(w Vertex) Vertex {
 	return Vertex{
 		v.X - w.X,
@@ -83,6 +99,7 @@
 	}
 }
 
+// Add returns (v + w) component-wise.
 func (v Vertex) Add(w Vertex) Vertex {
 	return Vertex{
 		v.X + w.X,
@@ -91,6 +108,7 @@
 	}
 }
 
+// Scale returns s*v component-wise.
 func (v Vertex) Scale(s float64) Vertex {
 	return Vertex{
 		s * v.X,
@@ -99,6 +117,8 @@
 	}
 }
 
+// Interpolate returns a function that return s*v2 + v1
+// component-wise.
 func Interpolate(v1, v2 Vertex) func(Vertex) Vertex {
 	v2 = v2.Sub(v1)
 	return func(s Vertex) Vertex {
@@ -110,10 +130,13 @@
 	}
 }
 
+// Less returns if one of v component is less than the
+// corresponing component in w.
 func (v Vertex) Less(w Vertex) bool {
 	return v.X < w.X || v.Y < w.Y || v.Z < w.Z
 }
 
+// NewLine return a line of point/direction.
 func NewLine(p1, p2 Vertex) Line {
 	return Line{
 		p2.Sub(p1),
@@ -121,10 +144,13 @@
 	}
 }
 
+// Eval returns the vertex for t*l[0] + l[1].
 func (l Line) Eval(t float64) Vertex {
 	return l[0].Scale(t).Add(l[1])
 }
 
+// IntersectHorizontal returns the intersection point
+// for a given z value.
 func (l Line) IntersectHorizontal(h float64) Vertex {
 	t := (h - l[1].Z) / l[0].Z
 	return l.Eval(t)
@@ -140,6 +166,11 @@
 	return 0
 }
 
+// IntersectHorizontal calculates the line string that
+// results when cutting a triangle a a certain height.
+// Can be empty (on intersection),
+// one vertex (only touching an vertex) or
+// two vertices (real intersection).
 func (t *Triangle) IntersectHorizontal(h float64) LineStringZ {
 	sides := [3]int{
 		side(t[0].Z, h),
@@ -213,12 +244,17 @@
 	}
 }
 
+// EpsEquals returns true if v and w are equal component-wise
+// with the values within a epsilon range.
 func (v Vertex) EpsEquals(w Vertex) bool {
 	const eps = 1e-5
 	return math.Abs(v.X-w.X) < eps &&
 		math.Abs(v.Y-w.Y) < eps && math.Abs(v.Z-w.Z) < eps
 }
 
+// JoinOnLine joins the the elements of a given multi line string
+// under the assumption that the segments are all on the line segment
+// from (x1, y1) to (x2, y2).
 func (mls MultiLineStringZ) JoinOnLine(x1, y1, x2, y2 float64) MultiLineStringZ {
 
 	position := linearScale(x1, y1, x2, y2)
@@ -269,6 +305,8 @@
 	return out
 }
 
+// Write writes a Vertex as three 64 bit values in little endian order
+// to the given writer.
 func (v *Vertex) Write(w io.Writer) error {
 	if err := binary.Write(
 		w, binary.LittleEndian, math.Float64bits(v.X)); err != nil {
@@ -282,6 +320,8 @@
 		w, binary.LittleEndian, math.Float64bits(v.Z))
 }
 
+// Read fills this vertex with three 64 bit values stored as
+// little endian from the given reader.
 func (v *Vertex) Read(r io.Reader) error {
 	var buf [8]byte
 	b := buf[:]
@@ -300,6 +340,7 @@
 	return nil
 }
 
+// AsWKB returns the WKB representation of the given multi line string.
 func (mls MultiLineStringZ) AsWKB() []byte {
 
 	// pre-calculate size to avoid reallocations.
@@ -328,6 +369,8 @@
 	return buf.Bytes()
 }
 
+// AsWKB2D returns the WKB representation of the given multi line string
+// leaving the z component out.
 func (mls MultiLineStringZ) AsWKB2D() []byte {
 
 	// pre-calculate size to avoid reallocations.
@@ -355,7 +398,7 @@
 	return buf.Bytes()
 }
 
-// Join joins two lines leaving the first of the secoung out.
+// Join joins two lines leaving the first of the second out.
 func (ls LineStringZ) Join(other LineStringZ) LineStringZ {
 	nline := make(LineStringZ, len(ls)+len(other)-1)
 	copy(nline, ls)
@@ -363,6 +406,8 @@
 	return nline
 }
 
+// Merge merges line segments of a given multi line string
+// by finding common start and end vertices.
 func (mls MultiLineStringZ) Merge() MultiLineStringZ {
 
 	var out MultiLineStringZ
@@ -467,15 +512,13 @@
 	return out
 }
 
+// Intersects checks if two Box2Ds intersect.
 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
-}
-
+// Xi returns the i-th x component.
 func (a Box2D) Xi(i int) float64 {
 	if i == 0 {
 		return a.X1
@@ -483,6 +526,7 @@
 	return a.X2
 }
 
+// Yi returns the i-th y component.
 func (a Box2D) Yi(i int) float64 {
 	if i == 0 {
 		return a.Y1
@@ -490,6 +534,7 @@
 	return a.Y2
 }
 
+// NewPlane2D creates a new Plane2D from two given points.
 func NewPlane2D(x1, y1, x2, y2 float64) Plane2D {
 	b := x2 - x1
 	a := -(y2 - y1)
@@ -503,6 +548,9 @@
 	return Plane2D{a, b, c}
 }
 
+// Eval determines the distance of a given point
+// from the plane. The sign of the result indicates
+// the sideness.
 func (p Plane2D) Eval(x, y float64) float64 {
 	return p.A*x + p.B*y + p.C
 }
@@ -516,6 +564,8 @@
 	return s | 1
 }
 
+// IntersectsPlane checks if a Box2D intersects with
+// a given Plane2D.
 func (a Box2D) IntersectsPlane(p Plane2D) bool {
 	var s int
 	for i := 0; i < 2; i++ {
@@ -537,6 +587,7 @@
 	return false
 }
 
+// Cross calculates the cross product of two vertices.
 func (v Vertex) Cross(w Vertex) Vertex {
 	return Vertex{
 		v.Y*w.Z - v.Z*w.Y,
@@ -545,6 +596,9 @@
 	}
 }
 
+// Intersection calcultes the 2D intersection point of
+// two Plane2Ds. If they do not intersect the returned
+// bool flags is set to false.
 func (p Plane2D) Intersection(o Plane2D) (float64, float64, bool) {
 
 	u1 := Vertex{p.A, p.B, p.C}
@@ -559,6 +613,7 @@
 	return plane.X / plane.Z, plane.Y / plane.Z, true
 }
 
+// VerticalLine is a 2D line segment.
 type VerticalLine struct {
 	x1 float64
 	y1 float64
@@ -568,6 +623,7 @@
 	line Plane2D
 }
 
+// NewVerticalLine creates a new 2D line segment.
 func NewVerticalLine(x1, y1, x2, y2 float64) *VerticalLine {
 	return &VerticalLine{
 		x1:   x1,
@@ -605,6 +661,7 @@
 
 func inRange(a float64) bool { return 0 <= a && a <= 1 }
 
+// Intersection intersects a line segment with a triangle.
 func (vl *VerticalLine) Intersection(t *Triangle) LineStringZ {
 
 	var out LineStringZ
@@ -780,6 +837,7 @@
 	return out
 }
 
+// AsWKB returns a WKB representation of the given point cloud.
 func (mpz MultiPointZ) AsWKB() []byte {
 	size := 1 + 4 + 4 + len(mpz)*(1+4+3*8)