A card trick to flummox even the most expert of conjurers could hold the key to unparalleled trick photography and special effects.
Researchers from Stanford University in California and Cornell University, New York, both in the US, have demonstrated a way to identify a playing card whose face is turned away from the camera. They did it by measuring projected light as it bounced off the face of the card and one other surface before reaching a camera.
Key to the conjuring trick is a technique known as “dual photography”. This involves retracing the path light takes as it bounces off different surfaces, pixel by pixel. A light projector – with millions of controllable pixels which are switched on in turn – illuminates the scene. The reflected light from each pixel is recorded sequentially by a digital camera.
It is then possible to retrace the path of each pixel’s light and reconstruct the image, as it would appear at the projector, using a principle known as Helmholtz reciprocity, which states that light flow can effectively be reversed without altering its transport properties. The entire process can also be accelerated by illuminating several pixels at once, after calculating which ones will not interfere with each other.
Dual photography faithfully records even complex light effects, and this is what makes it possible to read the playing card in the researchers’ trick. Illuminating individual pixels in turn allows reflected light to reveal the image of the card after bouncing off the wall behind the card. It is then possible to reconstruct the face of the card on a computer.
A video (64MB, Mpeg4 format) explaining dual photography can be downloaded from the homepage of one of the Stanford researchers.
But it is more than just a way for card sharps to outwit their opponents. Eventually, dual photography could be used alter lighting after a movie scene has been filmed or to add photorealistic virtual objects and characters that cast realistic shadows in post-production.
“The card trick is just there as a demonstration of the principle,” says Steve Marschner at Cornell University. “Certainly film companies are interested in capturing a scene in such a way that you could then relight it afterwards.”
But Marschner adds that the technique could find use in the computer games industry, or perhaps in scientific experiments that require very high quality imaging.
“It’s definitely a very interesting way to go for film effects,” says Paul Debevec, a researcher at the University of Southern California who has worked as a consultant on Hollywood films including Spiderman 2 and The Matrix. “Having aesthetic [virtual] objects reflect light in a way that’s identical to the way a real object reflects light is very important.”
“It’s very clever,” notes Mark Nixon, a computer imaging expert at Southampton University in the UK. “And in graphics, it is vitally important to keep the resolution high.”
The research will be presented at the 32nd International Conference on Computer Graphics and Interactive Techniques, which is held between 3 and 5 November, 2005, in California.