Many optical effects are produced in camera, among them irising in and irising out (an effect that relies on literally manipulating the camera's iris, a technique already well established when Billy Bitzer (1872-1944) shot Broken Blossoms for Griffith in 1919 and blanking out areas of the field of view to emulate binoculars, telescopes, keyholes, gun sights, and similar shapes. Double exposure can be achieved in camera as well as in postproduction, by the simple expedient of rewinding the film and shooting over it again.
Many more effects relied on the optical printer, a device used to print from the master negative to the positive for editing. Dissolves from one shot to another and fades to black, for example, could be achieved by running two strips of negative through the printer simultaneously. Passing a matte (in this case a thin sheet of opaque material) across the interface of the two filmstrips, exposing first one area and then the area previously masked by the matte, produced wipes, whose variety can be best seen displayed in RKO's Flying Down to Rio (1933). Different areas of the filmstrip can be printed with different images, a technique used extensively in the documentary Woodstock (1970). Crucially, optical printing can be used to match shots from disparate sources: for example, a landscape with characters reacting matched with a sky filled with billowing clouds (produced by spilling specially mixed pigments into a tank of translucent oil) for the arrival of the aliens in Independence Day (1996). The optical printer was also a crucial device in titling, where the lettering was filmed separately on a rostrum, and then printed over the photographic plate. Likewise, optical printing provided the base for such innovations as the mixture of cartoon with rotoscoped live action in Ub Iwerks's (1901-1971) early Alice animations, such as Alice the Toreador (1925), Alice Rattled by Rats (1925), and Alice the Whaler (1927).
Indeed, animation has remained a consistent source of effects within live action cinema, including such landmarks of animation as the city of the Krell in Forbidden Planet (1956) and the painterly effects of Waking Life (2001). The full integration of animation techniques into features had to wait, however, for the development of three-dimensional digital animation. Pioneer attempts like Disney's Tron (1982) and the genesis effect in Star Trek: The Wrath of Khan (1982) intimated what might be possible. The financial success of the first Star Wars (1977) indicated what could be achieved with almost exclusively analogue effects. By 1988, Industrial Light and Magic, the effects shop established by George Lucas to work on Willow (released that year, the film in which he pioneered the digital morph), would provide over a thousand shots for Robert Zemeckis's Who Framed Roger Rabbit? (also released that year). Certain techniques have remained fairly constant, notably the use of key frame animation to establish the most important moments (frequently the beginning and end) of an animated gesture. Others were the fruit of laborious research, such as the problem of soft objects (which explains the preponderance of billiard balls in early digital animation) and z-buffering (getting objects to touch without penetrating each other on the z or depth axis of the image, as opposed to the x and y axes of two-dimensional images). Celebrated in early examples such as the watery pseudopod in James Cameron's (b. 1954) The Abyss (1989), digital animation swiftly reached for less self-conscious, more embedded functions in movies, achieving a notable success in Cameron's Titanic in 1997, where the distinctions between set, model, and animation were all but invisible to contemporary audiences.
Early vector animation composed creations out of algebraic descriptions of curves. The popular NURBS (Non-Uniform Rational B-Splines) uses such vectors to define sections of the surface of a creature rendered initially in wire frame view, a lattice of interconnecting lines. The areas bounded by these lines (polygons) can be programmed to relate to neighboring polygons, so that if one stretches, another may contract to make up for the move. More recently, animators have moved toward subdivision modeling, in which a crude figure is gradually refined by adding and subtracting polygons to provide detail. Industry wisdom has it that ''reality begins at 1 million polygons,'' a mathematical response to the idea that a typical frame of 35mm film has approximately that many grains of silver compounds. Wire frame was for some years the basic view designers had during production, since the frames required relatively little processing time. Once the movements were approved, the frames would have surfaces applied to them. These may be generated digitally, typically by the process of ray-tracing, which allows for both surface color and texture and for different lighting conditions. Alternatively, they may have a ''skin'' applied, a surface texture derived from photography, as in the case of the digital Harrier jumpjet in True Lies (1994). Especially for close-up shots, animators will frequently add bitmap effects, such as the paint effects available in Adobe Photoshop, to add extra detail or to provide digital ''dirt.'' One attraction of three-dimensional modeling is that once built, a creature can be reused numerous times. A three-dimensional model is a dataset, and can be recycled not only in films but, for example, as a Computer-Aided Design and Manufacture (CADCAM) file, as was the case with the Buzz Lightyear character in Toy Story (1995), subsequently mass produced as a toy.
Individually handcrafted creatures may be too time-consuming, expensive, or processor-heavy for larger scale projects. Disney's The Lion King (1994) used a technique developed in scientific computing to analyze flocking behavior in order to animate the wildebeest stampede. Each wildebeest was given a small list of behaviors that it applied repeatedly, such as ''run in the same direction as the others'' and ''always try to get to the inside of the group.'' Referred to as recursive (to describe the complex behavior emerging from the repeated application of a small rule set), this basic artificial life technology allowed the wildebeest effectively to animate themselves. Similar techniques have been used with larger numbers of ''agents'' with a broader range of behaviors in Disney's follow-up The Hunchback of Notre Dame (1996) for carnival crowds including a hundred or so different characters, each with a special attribute such as juggling, dancing, or carousing. Massive (Multiple Agent Simulation System in Virtual Environment), developed
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