Wil McCarthy's written a fascinating piece for Wired on "invisibility cloaks" — micro-video camouflage systems that reproduce what's behind you on pixels in draped in front of you.
A real invisibility cloak, if it's going to dupe anyone who might see it, needs to represent the scene behind its wearer accurately from any angle. Moreover, since any number of people might be looking through it at any given moment, it has to reproduce the background from all angles at once. That is, it has to project a separate image of its surroundings for every possible perspective.
Impossible? No, just difficult. Rather than one video camera, we'll need at least six stereoscopic pairs (facing forward, backward, right, left, upward, and downward) – enough to capture the surroundings in all directions. The cameras will transmit images to a dense array of display elements, each capable of aiming thousands of light beams on their own individual trajectories. And what imagery will these elements project? A virtual scene derived from the cameras' views, making it possible to synthesize various perspectives. Of course, keeping this scene updated and projected realistically onto the cloak's display fabric will require fancy software and a serious wearable computer.
