--- a/pkg/mesh/vertex.go	Wed Jul 07 10:58:14 2021 +0200
+++ b/pkg/mesh/vertex.go	Wed Jul 07 11:42:25 2021 +0200
@@ -1134,6 +1134,36 @@
 	return out
 }
 
+// FilterRemoved returns an iterator that only delivers the vertices
+// which indices are not marked as removed.
+func (mpz MultiPointZ) FilterRemoved(removed map[int]struct{}) func() (Vertex, bool) {
+	var idx int
+	return func() (Vertex, bool) {
+		for {
+			if idx >= len(mpz) {
+				return Vertex{}, false
+			}
+			if _, rm := removed[idx]; rm {
+				idx++
+				continue
+			}
+			break
+		}
+		return mpz[idx], true
+	}
+}
+
+// MinMaxVertex runs over a point iterator and figures out its boundary.
+func MinMaxVertex(points func() (Vertex, bool)) (Vertex, Vertex) {
+	min := Vertex{math.MaxFloat64, math.MaxFloat64, math.MaxFloat64}
+	max := Vertex{-math.MaxFloat64, -math.MaxFloat64, -math.MaxFloat64}
+	for v, ok := points(); ok; v, ok = points() {
+		min.Minimize(v)
+		max.Maximize(v)
+	}
+	return min, max
+}
+
 // 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)