Bullet Time

The distinctive special effect where, moving in a weird, almost dream-like slow-motion, Keanu limbo-dances beneath the line of fire of someone shooting at him. This fancy camera trick is in fact a variation on the fairly well-known special effect known as time-slicing - a technique popularised in adverts for Capital Radio and The Gap, and employed to great effect in Vincent Gallo's otherwise ultra-low-tech Buffalo 66. In each case, an object appears frozen in time, and the camera circles around to show it in three dimensions - whether it's a head in mid-explosion or a casually-trousered swing dancer in mid-air. Basically, time-slice works like this. All moving-image photography relies on the phenomenon known as "persistence of vision": a film camera pointed at its subject takes a rapid succession of still images, 24 of them per second, and on the cinema screen they blur into the impression of constant motion. More sophisticated cinematography is conjured up with tracking shots - the camera can jolt around like the Shaky-Cam employed in The Evil Dead films, or just lazily drift past, like every single shot in Armageddon. Again, the movie camera is actually taking a long strip of still images in rapid sequence, all from slightly different vantage points. Then, when they're played back, your brain naturally interprets them as the camera moving past the scene. The essence of time-slice photography is that, instead of using a single moving motion-picture camera, it uses several still cameras, all of which take pictures simultaneously. If you take those pictures, and show them in sequence, from left to right, as if they were the individual frames of a movie, it creates the illusion of "virtual camera movement" - as if your viewpoint was physically roving around the object. Obviously you're accustomed to seeing single, captured moments in normal still photographs, as well as the simple optical cinematic effect of a freeze-frame; but here the camera appears to be moving around, so clearly time must be passing. Yet the subject remains hanging there in space - oddly, inexplicably frozen in time. The effect used in The Matrix is similar, but a bit more complicated. To keep the action going, the "bullet time" team didn't actually fire their 120 cameras simultaneously, but fractions of a second after each other, creating super slow-motion instead. A 3D simulation was used to position the cameras and trigger their exposures - and because, in most of the sequences, the cameras circle the subject almost completely, computer technology was used to "paint out" the cameras that appeared in shot on the other side. The cameras at each end of the row were standard movie cameras, to pick up the normal speed action before and after.