Simples Assim

http://www.k2.t.u-tokyo.ac.jp/vision/typing_system/index-e.html

http://en.wikipedia.org/wiki/Lucas–Kanade_Optical_Flow_Method

According to the paper, a high frame rate over 100 fps is required to recognize rapid typing gesture, which is 2.5x higher than in HTC EVO 4G and iPhone 4, for example.

With fewer frames, though, you can recognize simpler gestures:

http://www.eyesight-tech.com/