Traces are always wiped-out: marks of hands on a table, feet on the floor, on books, images, desks, screens. It seems to be a natural goal to give people the feeling of always entering a space in which nobody has lived. It is a way to always have a fresh start: people are free to seize their immediate past and build their story, and free to seize their future and build their project. Nothing contradicts the two since everything is erased. But what if traces were not erased but revealed? The installation displays foot prints under each user's steps. The marks are detected by intersecting visual hulls generated by multiple cameras. This technique from computer vision is implemented in a novel way on the graphic hardware for better than real-time performance. Based on this technique, we are developing a system that can easily turn any surface into an interactive display that can track many simultaneous human touches, from finger or foot to entire body. The system will allow media artists to easily develop new pieces that make use of physical surfaces for human interactions with visual displays.