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Today's most advanced graphics systems take Evans and Sutherland's procedure one step further. Using a programming technique known as ray tracing, they follow the path of each ray of light as it travels from its source, say the sun, to the viewer's eye. Upon striking a surface, each ray will be absorbed, reflected or transmitted in accordance with the laws of optics. Programmed with a mathematical model of the behavior of light rays, the machines can re-create lighting effects of dizzying complexity. Caltech's Jim Kajiya, for example, has used ray tracing to show how ripples travel through a reflecting pool.
Once the color and intensity of each point of light have been calculated, those data are converted directly into the pixels, or picture elements, that make up the images on the computer's screen. Each pixel is either red, green or blue. When viewed from a distance, however, they coalesce like the dots in a pointillist painting. Says Lucasfilm's Cook: "It's like mixing paint. If you stand back, they all blend together."
The whole process consumes vast quantities of computer time. One minute of film may involve as many as 100 billion calculations, driving the costs of TV commercials as high as $4,000 per sec. But conventional filmmaking techniques can be even more expensive. Using a Cray X-MP supercomputer and the latest graphics technology, the special-effects team at Digital Productions was able to create the battling spaceships in the film The Last Starfighter for $4 million. To produce the same scenes with scale-model miniatures would have cost $12 million to $24 million.
With bit parts in such movie hits as Ghostbusters and Amadeus, computer graphics has become part of the standard repertoire of filmmakers and art directors. But the technology is not restricted to Hollywood and Madison Avenue. The Defense Department has invested heavily in complex simulators that use computer graphics for training personnel in the use of tanks, jets and submarines. In the simulators that help teach pilots to fly the CH-53 Sea Stallion helicopter, the computer graphics alone can cost $3 million.
The technology is fast spreading to other professions as well. By mathematically building their scale models within a computer, architects can see what large buildings will look like from the ground, the air or the window of a high-rise across the street. Petroleum engineers can explore graphic versions of geological formations thousands of feet below the ocean floor without drilling. Physicians, manipulating the images produced by CAT scanners, can visually probe the brains of patients without having to perform exploratory surgery. Says Don Greenberg, director of computer graphics at Cornell University: "It's like having a doctor walk on the inside of the skull."
Perhaps the most dramatic use of the technology is in the design of new chemicals and drugs. By showing how the molecules of the active ingredients in a drug attach themselves, like keys in locks, to target molecules in the body, computer models can help researchers see how a change in molecular structure will affect a drug's behavior. Says Robert Langridge, who heads the computer-graphics laboratory at the University of California, San Francisco: "I call it computer-assisted insight."