A project at Cornell and Stanford can "see through playing cards" by measuring the light on objects behind the cards, such as books. The light bouncing off the card casts a subtle reflection on the surfaces behind it, which can be measured and analyzed with a high-resolution camera and a smart algorithm.
The trick to reading a playing card that is facing away from the camera is picking up light that is reflected off of a surface behind the card. "In the card experiment, the camera cannot see the card directly, but it can see the surface of the book [behind the card]; the light from the projector bounces off the card, then bounces off the book and hits the camera," said Sen.
When the projector shines on a red part of the card, like the heart of the suit, the light gets a red tint. "The camera observes it and our algorithm determines that the projector saw something red at that position," said Sen. When the camera shines on a blue part of the card, the light is blue. "In this manner, we put together the projector image pixel-by-pixel and can see the card," he said.
(via We Make Money Not Art)
Update: Keith sez, "this link goes to the researchers' website, where they have a video demonstration of their technique (including the card trick at the end)."
