--- a/pkg/octree/tree.go	Sun Dec 30 16:24:51 2018 +0100
+++ b/pkg/octree/tree.go	Mon Dec 31 11:13:49 2018 +0100
@@ -17,14 +17,18 @@
 	"math"
 )
 
+// Tree is an Octree holding triangles.
 type Tree struct {
+	// EPSG is the projection.
 	EPSG uint32
 
 	vertices  []Vertex
 	triangles [][]int32
 	index     []int32
 
+	// Min is the lower left corner of the bbox.
 	Min Vertex
+	// Max is the upper right corner of the bbox.
 	Max Vertex
 }
 
@@ -35,6 +39,7 @@
 	{0.5, 0.5, 1.0, 1.0},
 }
 
+// Vertical does a vertical cross cut from (x1, y1) to (x2, y2).
 func (ot *Tree) Vertical(x1, y1, x2, y2 float64, fn func(*Triangle)) {
 
 	box := Box2D{
@@ -123,6 +128,7 @@
 	}
 }
 
+// Horizontal does a horizontal cross cut.
 func (ot *Tree) Horizontal(h float64, fn func(*Triangle)) {
 
 	if h < ot.Min.Z || ot.Max.Z < h {