Last year, we featured artist Chris O'Shea's giant human-crushing hand projected onto a public screen. One of his more recent projects was to hack the Xbox Kinect gaming platform so that it can play an air guitar:
First it thresholds the scene to find a person, then uses a histogram to get the most likely depth of a person in the scene. Then any pixels closer than the person to the camera are possible hands. It also uses contour extremity finding on the person blob to look for hands in situations where your hand is at the same depth as your body. It only works if you are facing the camera front on. Then it uses one hand as the neck of the guitar, drawing a virtual line from the neck through the person centroid to create the guitar line. The other hand is tracked to see if it passes through this line, strumming the guitar. The neck hand position controls the chord.
http://www.chrisoshea.org/lab/air-guitar-prototype via CrunchGear