“Researchers from Stanford University and Cornell University have put together a projector-camera system that can pull off a classic magic trick: it can read a playing card that is facing away from the camera.
The dual-photography system gains information from a subject by analyzing the way projected patterns of light bounce off it.
The system can show a scene from the point of view of the projector as well as that of the camera. It could eventually be used to quickly add lighting effects in movie scenes, including the ability to realistically integrate actors who are shot separately and computer graphics into previously shot scenes.
The work also advances efforts aimed at collecting all of the visual information about a scene by sensing light scattered off objects within it and using the information to create views of the scene from any angle under any lighting condition. The ultimate goal of this area of imaging research is photorealistic virtual reality -- the visual component of the Star Trek holodeck.
The system consists of a digital camera and digital projector. The projector beams a series of black and white pixels at a scene and the camera captures the way the light bounces off objects in the scene. The heart of the system is a computer algorithm that continually monitors the data and changes the patterns in order to gain the needed information. [...]
This allows the researchers to measure the light changes from the projector to the camera, then reverse the light to provide a picture from the point of view of the projector. The method works because the properties of a ray of light are unchanged when the ray is reversed, a characteristic of light termed Helmholtz reciprocity. 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.”