Add closed param to kerf()

Allow for non close line segments and handle ends correctly

boxes/vectors.py

@@ -83,7 +83,7 @@ def mmul(m0, m1):
     return result
 
 
-def kerf(points, k):
+def kerf(points, k, closed=True):
     """Outset points by k
     Assumes a closed loop of points
     """
@@ -94,6 +94,12 @@ def kerf(points, k):
         # get normalized orthogonals of both segments
         v1 = vorthogonal(normalize(vdiff(points[i - 1], points[i])))
         v2 = vorthogonal(normalize(vdiff(points[i], points[(i + 1) % lp])))
+
+        if not closed:
+            if i == 0:
+                v1 = v2
+            if i == lp-1:
+                v2 = v1
         # direction the point has to move
         d = normalize(vadd(v1, v2))
         # cos of the half the angle between the segments