iPhone之Quartz 2D系列--阴影和透明层（5）Shadows&Transparency Layers

以下几遍关于Quartz 2D博文都是转载自：http://www.cocoachina.com/bbs/u.php?action=topic&uid=38018 苹果官网英语介绍：https://developer.apple.com/library/mac/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_overv

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