Let's say you want to create a diagram / doodle / sketch as a computer file, but are inexpert at drawing with your mouse or trackball, and do not have a tablet PC or a convenient tablet.
Solution: Draw on a sheet of paper (or, , in a pinch, pretend to draw, on a tabletop or other surface) with a
reflective stylus, which could be a pencil wrapped with a few strips of 3M tape, such that a camera (built in, or attached by USB or otherwise) observes and interprets the motion.
This is perhaps best explained through a use scenario:
1) Fire up the controlling application (which is the tricky part, but not exactly "... and then a miracle occurs." Motion discernment of a limited universe of objects in a small area at least seems to oldish hat, certainly in places like the MIT Media Lab. Ensure that the camera feeding the app is pointing where you'd like it to, such that it can see your hand and chosen implement of doodle-struction.
2) Callibrate the display: depending on how large an area you'll be using, the controlling software would need to adjust itself, so as to know how large an area to focus on. Callibration would consist of tapping out the corners of your "canvas" -- the order ought not matter. (Tapping here could mean a) actual tapping, if your stylus has a light turned on my pressure, so the controller app would know when you are indicating HERE! or b) hitting a key or key combo on the keyboard of the attached computer when stylus is in place. More annoying, but not so very terrible. Then calibrate angle, with a "close to straight up" and a "just about 45-degrees" pose for the camera.
3) Once a "Callibration successful!" message is obtained, that area you've outlined is watched for the movement of your stylus. Here, too, a stylus with a built-in light activated by pressure (where a pencil's erasor would be, at the top of the pen) would be helpful not only to indicate contact with the surface, but through variation in intensity to indicate degree degree of pressure.
4) when drawing is detected, the interpreted movements are fed as mouse input to the current application (for instance, Inkscape or The Gimp). Having been calibrated for angle, the reflective stylus can be used to convey angle to take advantage of asymmetrical pen shapes in such programs.
Voila! Pen-input, sansatablet.
(The closest I've seen to this -- to me -- very plausible system are drawing-interpreting devices for Palm OS machines which rely on, as I understand them, two small cameras mounted at top and to one side of a portfolio folder which holds a legal pad. However, the laser-projected phantom keyboard input prototypes shown at computer shows also do something very similar.)