diff --git a/lib/stroke_extractor.js b/lib/stroke_extractor.js
index 09f41c29..9b0a48d5 100644
--- a/lib/stroke_extractor.js
+++ b/lib/stroke_extractor.js
@@ -306,8 +306,8 @@ function extract_stroke(paths, endpoint_map, bridge_adjacency,
     var diff1 = Point.subtract(segment1[1], segment1[0]);
     var diff2 = Point.subtract(segment2[1], segment2[0]);
 
-    var s = (-diff1[1]*(segment1[0][0] - segment2[0][0]) + diff1[0]*(segment1[0][1] - segment2[0][1]))/(-diff2[0]*diff1[1] + diff1[0]*diff2[1]);
-    var t = ( diff2[0]*(segment1[0][0] - segment2[0][0]) - diff2[1]*(segment1[0][1] - segment2[0][1]))/(-diff2[0]*diff1[1] + diff1[0]*diff2[1]);
+    var s = (diff1[0]*(segment1[0][1] - segment2[0][1]) - diff1[1]*(segment1[0][0] - segment2[0][0]))/(diff1[0]*diff2[1] - diff1[1]*diff2[0]);
+    var t = (diff2[0]*(segment1[0][1] - segment2[0][1]) - diff2[1]*(segment1[0][0] - segment2[0][0]))/(diff1[0]*diff2[1] - diff1[1]*diff2[0]);
 
     if (0 < s && s < 1 && 0 < t && t < 1) {
       return [segment1[0][0] + t*diff1[0], segment1[0][1] + t*diff1[1]];