--- a/pkg/octree/vertex.go	Thu Sep 27 08:52:17 2018 +0200
+++ b/pkg/octree/vertex.go	Thu Sep 27 08:52:44 2018 +0200
@@ -498,9 +498,27 @@
 				out = append(out, t[i], t[j])
 				return out
 			case !r1:
-				// TODO
+				if t1 < 0 {
+					// below first -> clip by first
+					lin := linear(t[i], t[j])
+					out = append(out, t[i], lin(t2))
+				} else {
+					// above second -> clip by second
+					lin := linear(t[i], t[j])
+					out = append(out, lin(t2), t[j])
+				}
+				return out
 			case !r2:
-				// TODO
+				if t2 < 0 {
+					// below first -> clip by first
+					lin := linear(t[i], t[j])
+					out = append(out, t[i], lin(t1))
+				} else {
+					// above second -> clip by second
+					lin := linear(t[i], t[j])
+					out = append(out, lin(t1), t[j])
+				}
+				return out
 			}
 
 		case o1: