Looking at the photo prints from your Washington, D.C., vacation can prompt memories of being at real, three-dimensional places like the Lincoln Memorial. But what if you could actually walk into your photograph and stand at Lincoln's feet all over again--or at least zoom inside a 3-D version of your image on a computer screen? A new Web service called Fotowoosh promises to deliver such an experience, courtesy of computer-vision researchers at Carnegie Mellon University, in Pittsburgh.

Derek Hoiem, a doctoral candidate at Carnegie Mellon's Robotics Institute, has spent the past year and a half figuring out how to get software to convert flat images into 3-D virtual-reality models that can be manipulated on-screen. Working with faculty members Alexei Efros and Martial Hebert, Hoiem came up with a machine-learning system that identifies various surfaces and their orientations based on what it has learned from examining previous photos. In essence, Fotowoosh frees the person viewing a photograph from the photographer's point of view so that he or she can explore perspectives other than the one the camera actually captured.

Now Freewebs, a Silver Spring, MD, company that hosts 14 million personal websites, is about to launch a consumer version of Hoiem's software on the Web. Freewebs president Shervin Pishevar says that he hopes Web users will upload thousands of photographs to Fotowoosh and share the 3-D versions with other visitors, making the service into what he calls "a 3-D Flickr." Flickr is, of course, Yahoo's highly popular photo-sharing and social-networking site.

A test version of the Fotowoosh system will be launched in May, Pishevar says. The system works best on outdoor images. Converted photos look a bit like the illustrations in children's pop-up books: there's an obvious "ground" corresponding to the flat page in a pop-up book, and vertical surfaces stand at right angles to the ground, representing objects such as walls, trees, and vehicles. The images appear inside a Web page loaded with a special viewer with controls for zooming, panning, and rotating the 3-D model. While the software literally adds a new dimension to old tourist photos, in the future it could also be applied for purposes such as robot navigation or building photorealistic 3-D virtual worlds.

Hoiem says the software mimics some of the tricks our brains use to give depth to the two-dimensional images constantly landing on our retinas. Traditional (nonstereoscopic) cameras only have one "eye" compared with our two. That means they can't take advantage of parallax--the phenomenon in which our right and left eyes see nearby objects in slightly different positions relative to objects farther away--to get a stereo image. In fact, it's mathematically impossible for software to compute the shape of a 3-D scene from a single two-dimensional image with 100 percent confidence, since the objects in the scene could theoretically be any distance away. "But people can do it," notes Hoiem. "It's just that there is not a simple algebraic solution."