How do you do it? (part 2)

Your brain knows that your eyes are about two inches apart. But when Photosynth does its magic, it doesn't know where the cameras were, or which way they were pointing. Fortunately, when there are many cameras, and many features in common, the algorithms behind Photosynth can figure out not only where the features are in 3D, but where all of the cameras would have to have been, and which way they were aimed, consistent with the features they "saw".

The Photosynth client shows you the 3D point cloud, but more importantly, it also shows you the original pictures overlaid on the model. Imagine a slide projector placed at each original camera position, aimed how the camera was, and projecting the picture that camera took. A screen is placed in the 3D environment at an appropriate distance from the projector. As you move around in the Photosynth environment, projectors turn on and off, giving you a changing perspective on a world built entirely out of the original photos.