diff -r 3089611c99d1 -r 5933250813a3 geometry.py
--- a/geometry.py	Tue Jan 23 12:29:30 2018 +0200
+++ b/geometry.py	Tue Jan 23 14:09:44 2018 +0200
@@ -410,3 +410,48 @@
     assert(len(polygon.vertices) == 3)
     a, b, c = polygon.vertices[:3]
     return cross_product(b - a, b - c)
+
+def line_intersection_xy(line_1, line_2):
+    '''
+        Computes 2D line intersection. Can return a point outside the given
+        segments. Z is ignored.
+    '''
+    a, b = line_1.vertices
+    c, d = line_2.vertices
+    denominator = (a.x - b.x) * (c.y - d.y) - (a.y - b.y) * (c.x - d.x)
+    x = (c.x - d.x) * (a.x * b.y - a.y * b.x)
+    x -= (a.x - b.x) * (c.x * d.y - c.y * d.x)
+    y = (c.y - d.y) * (a.x * b.y - a.y * b.x)
+    y -= (a.y - b.y) * (c.x * d.y - c.y * d.x)
+    try:
+        x /= denominator
+        y /= denominator
+    except ZeroDivisionError:
+        return None
+    else:
+        return Vertex(x, y, 0)
+
+def line_segment_intersection_xy(line_1, line_2):
+    '''
+        Computes 2D line intersection. Only returns a result if it falls
+        inside the line segments. Z is ignored.
+    '''
+    from math import inf
+    intersection = line_intersection_xy(line_1, line_2)
+    if intersection:
+        a, b = line_1.vertices
+        c, d = line_2.vertices
+        try:
+            t = (intersection.x - a.x) / (b.x - a.x)
+        except ZeroDivisionError:
+            t = inf
+        try:
+            u = (intersection.y - a.y) / (b.y - a.y)
+        except ZeroDivisionError:
+            u = inf
+        if 0 < t < 1 or 0 < u < 1:
+            return intersection
+        else:
+            return None
+    else:
+        return None