Adding effects on image in iPad application

I am playing with images in my application and I wonder if it is possible to make something like inner shadow/drop shadow on my images in the application? The reason why I am asking is where/what should I start reading 开发者_开发百科if I want something like that if it is even possible?

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There is a library called Core Graphics. Check this article. Then of course, there is my favorite information source apart from SO. Just for kicks, there is this article to be read, but since it was a private API (and I'm not sure it's still doable), it's only for reading pleasure.

By the way, have you looked at this question? It's not exactly your problem, but does bring some information you might want to read.