Finding the touching angle on a touch-screen?
Four days ago, I received a news email from a technology site I follow. One of the news was that Microsoft registered a patent to do what you're asking.

Here is the link for the patent.

The news site is "Clube do Hardware" and the news about this theme was in "Boletim 1440". You can see the (old) news bulletin in this page. Although this specific edition is not yet on the page, it may appear there soon. (Sorry, due to the copyright terms, I can't post the news here)