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I want to make an app for children that when they start touching the screen, the background image appears under their finger. Means lets suppose we have a white screen, then you start touching the screen, wherever you touch the background image appears only on that areas. just like layer masking.

Sincerely yours, Peyman Mortazavi

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There are many way to achieve this. Since you tagged mono on your question I'll assume you're interested in a way that's easy to port to other graphical toolkits.

  1. Create and show your background image;

  2. Create and show (over the background) a second image (identical size) that is pure white with an alpha channel (opaque);

  3. When you get touch events simply set the second image alpha to transparent (e.g. a circle at the touched position with a radius roughly the size of child finger) and the background image will appear behind it;

As an alternative you could gradually reduce from opaque to transparent (in #3) so it would take a few swipes to get a totally clear image.

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This approach seems to work, but I could not implement it, would you please share a sample project. thanks you –  Peyman Jul 19 '12 at 17:34
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