Drawing circle bitmap clearing it's inside content

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http://stackoverflow.com – Following picture shows what i need: So I have a bitmap or circle which have shadowing in it. My goal is to clear the inside of the circle and later on apply animation to scale the circle up and down. PorterDuffXfermode.CLEAR with Paint and canvas.drawCircle is not acceptable because first of all it draws black circle and second there is no safe way to calculate the radius not intercepting the circle bitmap itself. I am drawing background color and the circle in same layout. Bottom view is another layout under the circle layout. (HowTos)