This is some cool shit that Graffiti Research Lab Vienna presented 24C3. As far as I understand, this is how to do it: You’re using a projector and a digital video camera, forming a closed loop system, where data is fed back into system. You project an image to a wall that you also film with the camera. After calibration you can point a laser point to the wall. The camera sees this point of light and draws light on the area of projection.