Handle opacity.

src/Diagrams/Backend/Canvas.hs

@@ -61,6 +61,7 @@ canvasStyle s = foldr (>>) (return ())
                             , handle lWidth
                             , handle lJoin
                             , handle lCap
+                            , handle opacity_
                             ]
   where handle :: (AttributeClass a) => (a -> C.Render ()) -> Maybe (C.Render ())
         handle f = f `fmap` getAttr s
@@ -69,6 +70,7 @@ canvasStyle s = foldr (>>) (return ())
         lWidth = C.lineWidth . getLineWidth
         lCap = C.lineCap . getLineCap
         lJoin = C.lineJoin . getLineJoin
+        opacity_ = C.globalAlpha . getOpacity
 
 canvasTransf :: Transformation R2 -> C.Render ()
 canvasTransf t = C.transform a1 a2 b1 b2 c1 c2

src/Graphics/Rendering/Canvas.hs

@@ -23,6 +23,7 @@ module Graphics.Rendering.Canvas
   , lineWidth
   , lineCap
   , lineJoin
+  , globalAlpha
   , withStyle
   ) where
 
@@ -43,11 +44,12 @@ data DrawState = DS
                  , dsCap :: LineCap
                  , dsJoin :: LineJoin
                  , dsWidth :: Float
+                 , dsAlpha :: Float
                  , dsTransform :: (Float,Float,Float,Float,Float,Float)
                  } deriving (Eq)
 
 emptyDS :: DrawState
-emptyDS = DS 0 (0,0,0,1) 0 LineCapButt LineJoinMiter 0 (1,0,0,1,0,0)
+emptyDS = DS 0 (0,0,0,1) 0 LineCapButt LineJoinMiter 0 1 (1,0,0,1,0,0)
 
 data RenderState = RS
                    { drawState :: DrawState
@@ -202,6 +204,12 @@ fromLineJoin LineJoinRound = show "round"
 fromLineJoin LineJoinBevel = show "bevel"
 fromLineJoin _             = show "miter"
 
+globalAlpha :: Double -> Render ()
+globalAlpha a = setDSWhen
+                (\ds -> ds { dsAlpha = a' })
+                (canvas $ C.globalAlpha a')
+  where a' = realToFrac a
+
 -- TODO: update the transform's state for translate, scale, and rotate
 translate :: Double -> Double -> Render ()
 translate x y = canvas $ C.translate (realToFrac x,realToFrac y)