--- a/pkg/octree/vertex.go	Mon Jul 15 17:09:59 2019 +0200
+++ b/pkg/octree/vertex.go	Mon Jul 15 17:49:50 2019 +0200
@@ -45,6 +45,9 @@
 	// and the second being the direction.
 	Line [2]Vertex
 
+	// Box is a 3D box.
+	Box [2]Vertex
+
 	// MultiPointZ is a set of vertices.
 	MultiPointZ []Vertex
 
@@ -288,9 +291,10 @@
 	}
 }
 
-// Interpolate returns a function that return s*v2 + v1
+// Interpolate returns a function that return s*b[1] + b[0]
 // component-wise.
-func Interpolate(v1, v2 Vertex) func(Vertex) Vertex {
+func (b Box) Interpolate() func(Vertex) Vertex {
+	v1, v2 := b[0], b[1]
 	v2 = v2.Sub(v1)
 	return func(s Vertex) Vertex {
 		return Vertex{
@@ -301,6 +305,10 @@
 	}
 }
 
+func (b Box) HasX() bool { return math.Abs(b[0].X-b[1].X) > epsPlane }
+func (b Box) HasY() bool { return math.Abs(b[0].Y-b[1].Y) > epsPlane }
+func (b Box) HasZ() bool { return math.Abs(b[0].Z-b[1].Z) > epsPlane }
+
 // Less returns if one of v component is less than the
 // corresponing component in w.
 func (v Vertex) Less(w Vertex) bool {