Multi-touch and Tangible Interface: Two Different Interaction Modes in the Same System

We present here a system built around the idea of giving to several users the possibility to interact with a cheap and solid hardware, at the same time, using natural gestures supported by an intuitive user interface. A projector and a camera are placed underneath a Plexiglas sheet, framed with an array of infrared LEDs, all set into a wooden table box. This allows for multiple users (up to four or ve) to freely move around the box and manipulate the objects retro-projected on the screen, using a tangible interface designed aiming at offering few simple operations: geometric transforms (rotations, translations and scales), drawing, erasing and color se-lections. All of these are executed through the use of either a custom built IR LED pen and/or directly the fingers. The main purpose of the project is to offer an instrument of tangible interaction to classrooms of naive users (i.e.: neither technology nor science professors and students) in a university environment.