diff --git a/core/vip.c b/core/vip.c
index 85308bf..ce94d22 100644
--- a/core/vip.c
+++ b/core/vip.c
@@ -1065,8 +1065,13 @@ static void vipDrawAffine(VB *sim, World *world) {
 
             /* Adjust left-edge parameters */
             if ((mp < 0) ^ i) {
-                mx += dx * (wx + mp);
-                my += dy * (wx + mp);
+                if (mp < 0) {
+                    mx += dx * (wx - mp);
+                    my += dy * (wx - mp);
+                } else {
+                    mx += dx * (wx + mp);
+                    my += dy * (wx + mp);
+                }
             } else {
                 mx += dx * wx;
                 my += dy * wx;