Help need for sensors and how to make it work

Hi, I'm currently working on a interactive project that basically involve with projectors, sensors, and a make controller. (maybe more, I just don't know)

The project concept is when people walk in front of the canvas(screen), the sensor will sense it and the projector from the back of the screen(the screen will be a vellum sheet) will project words that will form the person's body (or almost, don't have to be exactly follow the shape of the person's body but close), and hopefully the words will follow that person around.

I know I need some kind of the motion sensor, but I'm just not sure if i need a motion sensor or a proximity sensor for this project. I'm a new beginner with all these things so I have no clue what to buy and how to make it work. I could ask my instructor but he's out of town and the assignment is due next week. ANY INFORMATION WILL HELP!

Here is the image I created for top view of the project : http://farm4.static.flickr.com/3064/2572403335_d325b00bc8_o.png

and here is what I'm hoping it could work like: http://flickr.com/photos/42356257@N00/2573227008/

Anything will help, thank you very much!

The installation will be in a class room, no windows. Is there anything I need to be aware of? And do you know how many motion sensor I should be purchase? Sorry I'm very new to this, should I purchase one for each person or one detector will cover the entire room? The room is about 20 square feet. Thanks!