iOS: How do I create irregularly shaped / partially transparent & click-thru buttons

How would I create these buttons, so that clicking handles correctl开发者_如何转开发y?

Each button is a .png, transparent outside of the coloured square. I need a transparent areas to be click-thru.

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I have posted an article on how to do it here.

If you like it, please mark me up -- I currently don't have enough reputation points to do very much on this site.

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On clicking the button, check the location where it is clicked. Detect the pixel alpha at that location. If alpha is 0 the disable the click else enable it. This would definitly help.