Interactive projections are, as the name suggests projections where a persons motion is defected and the image is manipulated by it. It is often used for games and advertising and shown on the floor by projecting vertically down.
A simple example could be a projection of fallen leaves. When a person walks through them and sweeps their feet the leaves move to repeal a picture below. The technology used is commonly an infra-red camera and infra-red light source. The camera and projector are connected to a computer and both focused onto the area where the interactive projection is to be shown. Within the computer software, the projection area is applied to the view of the camera and calibrated. Because the camera is only picking up invisible infra-red light, it only detects the motion. This motion is then processed by the computer so that it triggers the video effect of the leaves moving in the correct place on the screen.
The technology is very popular in retail environments as it always attract potential customers to their campaign message.