Harry Potter and the Order of the Phoenix opened this week on 100 IMAX theater screens nationwide with an apparently fantastic 3-D climax in the final 20 minutes.

While old-fashioned 3-D movies like the classic 1953 horror flick House of Wax required filmmakers to shoot films with two separate camera lenses to create left-and-right view films, IMAX producers now use computer algorithms to turn the original 2-D film into a 3-D version. Hundreds of computers ground away for two months on this film's translation to create a virtual second perspective, which serves as the film's offset lens view.

Three-dimensional film effects rely on a basic trick of neurobiology. Our eyes each have a slightly different view of the world, and it is only in the brain that the two images are welded to create depth perception. The trick to turning a two-dimensional screen image into a three-dimensional one is to simultaneously show viewers two slightly offset views of the same scene and let the brain do the rest. This is why 3-D movie viewers need 3-D glasses — which are either polarized or electronically controlled to allow light from one view into your left eye and light from the other view into the right eye. Scientists call this trick of depth perception "stereopsis," first described by the British researcher Charles Wheatstone in 1838.