Bullet time (also known as frozen time, the big freeze, dead time, flow motion, or time slice) is a special and visual effect that refers to a digitally enhanced simulation of variable-speed (i.e. slow motion, time-lapse, etc.) photography used in films, broadcast advertisements, and video games. It is characterized both by its extreme transformation of time (slow enough to show normally imperceptible and unfilmable events, such as flying bullets) and space (by way of the ability of the camera angle—the audience's point-of-view—to move around the scene at a normal speed while events are slowed). This is almost impossible with conventional slow-motion, as the physical camera would have to move impossibly fast; the concept implies that only a "virtual camera", often illustrated within the confines of a computer-generated environment such as a virtual world or virtual reality, would be capable of "filming" bullet-time types of moments. Technical and historical variations of this effect have been referred to as time slicing, view morphing, slow-mo, temps mort, and virtual cinematography.
The term "bullet time" is a registered trademark of Warner Bros., who first used it in March 2005, in connection with the video game The Matrix Online. The term had previously been used in the promotion of the 1999 film The Matrix, and in reference to the slow motion effects in the 2001 video game Max Payne, although the actual technique predates both by 20 years.
The bullet time effect was originally achieved photographically by a set of still cameras surrounding the subject. The cameras are fired sequentially, or all at the same time, depending on the desired effect. Single frames from each camera are then arranged and displayed consecutively to produce an orbiting viewpoint of an action frozen in time or as hyper-slow-motion. This technique suggests the limitless perspectives and variable frame rates possible with a virtual camera. However, if the still array process is done with real cameras, it is often limited to assigned paths.
For many years, it has been possible to use computer vision techniques to capture scenes and render images of novel viewpoints sufficient for bullet time type effects. More recently, these have been formalized into what is becoming known as free viewpoint television (FTV). At the time of The Matrix, FTV was not a fully mature technology. FTV is effectively the live action version of bullet time, without the slow motion.
In The Matrix, the camera path was pre-designed using computer-generated visualizations as a guide. Cameras were arranged, behind a green or blue screen, on a track and aligned through a laser targeting system, forming a complex curve through space. The cameras were then triggered at extremely close intervals, so the action continued to unfold, in extreme slow-motion, while the viewpoint moved. Additionally, the individual frames were scanned for computer processing. Using sophisticated interpolation software, extra frames could be inserted to slow down the action further and improve the fluidity of the movement (especially the frame rate of the images); frames could also be dropped to speed up the action. This approach provides greater flexibility than a purely photographic one. The same effect can also be produced using pure CGI, motion capture and universal capture.
The technique of using a group of still cameras to freeze motion occurred before the invention of cinema itself. It dates back to the 19th-century experiments by Eadweard Muybridge, who analyzed the motion of a galloping horse by using a line of cameras to photograph the animal as it ran past. Eadweard Muybridge used still cameras placed along a racetrack, and each camera was actuated by a taut string stretched across the track; as the horse galloped past, the camera shutters snapped, taking one frame at a time. (The original intent was to settle a debate the governor of California had engaged in, as to whether all four of the animal's legs would leave the ground when galloping.) Muybridge later assembled the pictures into a rudimentary animation, by placing them on a glass disk which he spun in front of a light source. His zoopraxiscope may have been an inspiration for Thomas Edison to explore the idea of motion pictures.
The first widely noticed predecessor of bullet time were Leni Riefenstahl's famous diving sequences from her documentary Olympia of the Olympic Games in 1936. Riefenstahl used a single camera slow motion tracking shot technique to achieve a similar effect.
Muybridge also took photos of actions from many angles at the same instant in time, to study how the human body went up stairs, for example. In effect, however, Muybridge had achieved the aesthetic opposite to modern bullet-time sequences, since his studies lacked the dimensionality of the later developments. A debt may also be owed to MIT professor Doc Edgerton, who, in the 1940s, captured now-iconic photos of bullets using xenon strobe lights to "freeze" motion.
Long before the emergence of a technology permitting a live-action application, bullet-time as a concept was frequently developed in cel animation. One of the earliest examples is the shot at the end of the title sequence for the 1966 Japanese anime series Speed Racer: as Speed leaps from the Mach Five, he freezes in mid-jump, and then the camera does an arc shot from front to sideways.
In 1980, Tim Macmillan started producing pioneering video work in this field while studying for a BA at the (then named) Bath Academy of Art using 16mm film arranged in a progressing sequence of pinhole cameras.
The first music video to use bullet-time was "Midnight Mover", a 1985 Accept video. In the 1990s, a morphing-based variation on time-slicing was employed by director Michel Gondry and the visual effects company BUF Compagnie in the music video for The Rolling Stones' "Like A Rolling Stone", and in a 1996 Smirnoff commercial the effect was used to depict slow-motion bullets being dodged. Similar time-slice effects were also featured in commercials for The Gap (which was directed by M.Rolston and again produced by BUF), and in feature films such as Lost in Space (1998) and Buffalo '66 (1998).
It is well-established for feature films' action scenes to be depicted using slow-motion footage, for example the gunfights in The Wild Bunch (directed by Sam Peckinpah) and the heroic bloodshed films of John Woo. Subsequently, the 1998 film Blade featured a scene that used computer generated bullets and slow-motion footage to illustrate characters' superhuman bullet-dodging reflexes. The 1999 film The Matrix combined these elements (gunfight action scenes, superhuman bullet-dodging, and time-slice effects), popularizing both the effect and the term "bullet-time". The Matrix's version of the effect was created by John Gaeta and Manex Visual Effects. Rigs of still cameras were set up in patterns determined by simulations, and then shot either simultaneously (producing an effect similar to previous time-slice scenes) or sequentially (which added a temporal element to the effect). Interpolation effects, digital compositing, and computer generated "virtual" scenery were used to improve the fluidity of the apparent camera motion. Gaeta said of The Matrix's use of the effect:
For artistic inspiration for bullet time, I would credit Otomo Katsuhiro, who co-wrote and directed Akira, which definitely blew me away, along with director Michel Gondry. His music videos experimented with a different type of technique called view-morphing and it was just part of the beginning of uncovering the creative approaches toward using still cameras for special effects. Our technique was significantly different because we built it to move around objects that were themselves in motion, and we were also able to create slow-motion events that 'virtual cameras' could move around – rather than the static action in Gondry's music videos with limited camera moves.
Bullet time evolved further through The Matrix series (1999-2003) with the introduction of high-definition computer-generated approaches like virtual cinematography and universal capture. Virtual elements within the Matrix Trilogy utilized state-of-the-art image-based computer rendering techniques pioneered in Paul Debevec's 1997 film  and custom evolved for the Matrix by George Borshukov, an early collaborator of Debevec.
Following The Matrix, bullet time and other slow-motion effects were featured as key gameplay mechanics in various video games. Cyclone Studios' Requiem: Avenging Angel, released in March 1999, features slow-motion effects. Remedy Entertainment's 2001 video game Max Payne contains a slow-motion mechanic that allows players to view the paths of bullets, an effect explicitly referred to as "Bullet Time". Max Payne was actually in development long before the release of the The Matrix (1999), and slow-motion was a major gameplay element from the beginning, nonetheless the game has been perceived to have been greatly influenced by film as it adopted the bullet time effect for that gameplay mechanic. As a result of the inevitable comparisons to The Matrix, the designers have included several homages to the film in order to capitalize on the hype (for instance, the detonation of the subway tunnel door to gain access to the bank vault is similar to the cartwheeling elevator door in the film, while the introduction "Nothing to Lose" level is similar to the lobby shootout scene in the film).
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