I think that its not as simple as shown in the picture, but maybe tracking finger shadows in an infrared backgorund may be factible.
I did take some photos with my web cam of the shadows produced by my own fingers over a paper sheet attached to a glass and I achieved very recognigzable patterns of dark circles. However it may be difficult to create an algorithm able to detect them. I´m quite interested in this issue, so I will take a look to those libraries for processing you say.
It would be nice having an "open" version of this technology ¿isn't it?