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AlphaMask and keep some points


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i have to draw full circle (color can change at any time) on on image BUT i need to keep black points (other colors can be redraw with circle color). 


In the first version, i use a bitmapdata to store image and another bitmapdata for circle. To draw circle and keep black points, i iterate through circle points coordinates and check if point on image is black or not. 

It works pretty fine but is a bit (to be gentle) slow on tablet. After looking in examples, i discovered "alphaMask" and it seems very quick.


So i'm wondering: is there a simple way to achieve my goal using alphaMask? 


Thx for your help and advices.




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