Michael Egger's VideoBass is an instrument that plays moving images instead of sound. With your left hand you chose a video clip on the strings and you trigger it in rhythm with your right hand. The four strings can play up to four different clips at the same time.
The four machine heads are really rotary encoders for selecting video clips. The four strings are wound with resistance wire and the fretless fingerboard is electrically conductive. This means that each string is effectively a very long linear potentiometer. The first version used a spherical tactile sensor on the body as a multipurpose sensor for triggering and controlling video effects and speed. Later versions used a plexiglass keyboard and finally playstation knobs (which were more reliable). The electronics for converting analog sensor data are located inside the body, a small microprocessor generates MIDI-data which is sent to the computer. Specially written software then decodes the VideoBass signals and generates corresponding video images.
Link: VideoBass project