Implement color for quadratic bezier curves

src/Picture.hs

@@ -69,8 +69,20 @@ polygonCol :: [(Point2,RGBA)] -> Picture
 {-# INLINE polygonCol #-}
 polygonCol = PolygonCol 0
 
-bezierQuad :: Point2 -> Point2 -> Point2 -> Picture
-bezierQuad a b c = BezierQuad 0 [a,b,c]
+-- 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)
+                                                ,(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))
 
 color :: RGBA -> Picture -> Picture
 {-# INLINE color #-}