Determine bug in bezier curve rendering

src/Picture.hs

@@ -70,19 +70,22 @@ polygonCol :: [(Point2,RGBA)] -> Picture
 polygonCol = PolygonCol 0
 
 -- note that much of work computing the width of the bezier curve is done here
--- not sure that moving the control point does much...
 bezierQuad :: Color -> Color -> Float -> Float -> Point2 -> Point2 -> Point2 -> Picture
 bezierQuad cola colc ra rc a b c = BezierQuad 0 [(aX,cola                 ,raX)
-                                                ,(bx,mixColors 0.5 0.5 cola colc,1)
+                                                ,(bX,mixColors 0.5 0.5 cola colc,1)
                                                 ,(cX,colc                 ,rcX)]
-    where v = a -.- c
-          nv = normalizeV v
-          aX = a +.+ ra * 0.5 *.* nv
-          cX = c -.- rc * 0.5 *.* nv
-          raX = ra / magV (aX -.- cX)
-          rcX = rc / magV (aX -.- cX)
-          bx = b +.+ (0.5 * ra *.* normalizeV (b -.- a))
-                 +.+ (0.5 * rc *.* normalizeV (b -.- c))
+    where b2a | isLHS a b c = b -.- a
+              | otherwise   = a -.- b
+          nb2a = normalizeV $ vNormal b2a
+          aX = a +.+ ra * 0.5 *.* nb2a
+          b2c | isLHS a b c = c -.- b
+              | otherwise   = b -.- c
+          nb2c = normalizeV $ vNormal b2c
+          cX = c +.+ rc * 0.5 *.* nb2c
+          raX = ra / magV (aX -.- bX)
+          rcX = rc / magV (cX -.- bX)
+          bX = b +.+ (0.5 * ra *.* nb2a)
+                 +.+ (0.5 * rc *.* nb2c)
 
 color :: RGBA -> Picture -> Picture
 {-# INLINE color #-}