The Silent Drum Controller is a transparent drum shell with an elastic head. As one presses it, the head deforms and a variety of shapes with peaks are created reflecting the shape of the mallet or hand. These shapes are captured by a video camera that sends these images to the computer, which analyzes them and outputs the tracked parameters.
Oliver, Jaime, and Jenkins, Mathew. 2008. The Silent Drum Controller: A New Percussive Gestural Interface. In Proceedings of the International Computer Music Conference, Belfast, UK, 2008. [pdf]