[Tom Gerhardt] has made this very interesting mud interface for a computer.  Follow the link to see a video of it in action. It appears as though he’s using a laser grid of some kind to establish elevation. We might be way off on that though, there aren’t any details on the construction. He does mention that it is an open source hardware and software project, so maybe the details are available on request. In the video you can see it running as a projection surface where people are interacting with items directly on the mud. You can also see it being used as an external input device. People play Tetris using it in that example.

UPDATE: [Moon] reports from the ITP show that the tub has a 16×12 grid of generic pressurs sensors on the bottom. These feed into a MacBook Pro which is projecting on the surface. Despite the sparse grid, [Tom] says he gets good resolution by interpolating between sensors; it can detect a resting hand pivoting on the surface.