Finger Synth reads the position of each fingertip through the camera and maps it to pitch, volume and timbre. There is no keyboard and no controller — the space in front of the screen becomes the instrument, and the distance between your fingers becomes the note.
The interesting part was not the tracking itself but the feel of it: how much smoothing to apply before a gesture reads as musical rather than twitchy, and how fast sound has to respond before the link between hand and note breaks.