Really interesting video explaining an interactive system with handheld projectors
"SideBySide is a novel interactive system that allows multiple people to play and work together using handheld projectors at anytime and anyplace. The system is immediate and simple: users simply project onto a surface and their projection becomes aware and responsive to other projections nearby. Interaction can range from projector-based games, such as boxing with projected characters, to everyday tasks such as exchanging contact information by 'dragging and dropping' onto another user's projection. Importantly, SideBySide does not require any fixed sensing in the environment and can be used anywhere: at home, at the office, or even inside the car during long road trips. The system consists of a hybrid mobile projector that outputs both visible and invisible projections at the same time. The invisible projection contains tracking data that can be recognized by the device camera, allowing accurate location tracking of multiple projections and lightweight communication between devices."