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Ah-nuld gets top billing and Linda Hamilton's veins bulge off the screen, but the real stars of "Terminator 2: Judgment Day" aren't in front of the camera. Crowds sure go for that Shakespearean dialogue--"Hasta la vista, baby," Schwarzenegger tells the bad guy--but it seems to be the special effects that are making T2 this summer's around-the-blockbuster.

How do they ever get the evil T-1000 (Robert Patrick), a blob of gleaming liquid chrome, to take on so many different forms, including any human it touches? How about when Schwarzenegger blasts the cyborg full of holes--in one scene even slicing its head in two, only to see it fuse back together? And how about when the T-1000 fluidly emerges from a hospital tile floor?

Well, don't ask the director-producer-writer behind the $94 million movie. The obsessively possessive James Cameron doesn't want anyone to know just yet how he's made such cinematic magic. "We want to keep the mystery alive," says Larry Kasanoff, his partner at Lightstorm Entertainment. "They don't ask Siegfried and Roy how they make the lion disappear." None of this, of course, has anything to do with selling tickets or manipulating continued public fascination with T2. Anyway, if you don't want to learn the secrets, stop reading right now and skip back to NEWSMAKERS. Otherwise, here's How They Do It:

Digital Compositing. It is the state-of-the-art marriage of filmmaking and computer technology--courtesy of the wizards at Industrial Light & Magic in Marin County, Calif., the special-effects empire founded by George Lucas. ILM general manager Scott Ross says Lightstorm has forbidden the company from talking about its work on T2. But the process isn't entirely unknown in the industry. It begins by scanning a film image (like the T-1000 in human form) and converting it into a language that a computer can understand. That technique, called digitizing, means breaking down the celluloid picture into an electronic signal of 1s and 0s, similar to what's on audio compact discs. Sounds simple enough, yet one film frame--1/24th of a second--takes up 20 megabytes of computer memory, or enough to write an abridged version of "War and Peace." ILMers count in gigabytes--thousands of millions of bytes.

Once in the computer, the film image can be manipulated by a graphics artist in limitless ways: shape, color, density, all the while maintaining proper angle and perspective. Moreover, that image can be combined with ones invented solely by the computer. In "Terminator 2," all shots of the liquid T-1000 cyborg are creations of a person at a keyboard. And they look utterly realistic, quite unlike the primitive images used in, say, TV news logos and commercials. Indeed, before the age of lightning-fast computers, the only way T-1000 could ever have been born was through the use of animation and miniatures, and that wouldn't have looked believable.

To complete their electronic sleight of hand, ILM technicians have to merge the film--and computer-generated images seamlessly through another trick called morphing, short for metamorphasizing. That's how T-1000 mutates between humanoid and molten forms before our very eyes, with no cutaways. When the computer work is done, the composite images must then be translated back to the celluloid form--reversing the digitizing that began the process. For a film sequence of a minute or two--the length of the T2 tile scene-- digital compositing takes dozens of ILMers several weeks, rooms and rooms of memory disks and a bundle of money. There are about 40 computer-doctored sequences in T2. According to the trade magazine Cinefex, ILM invested in $3.5 million of new equipment for T2 work.

Compositing has been used before at ILM. Computers gave life to a dead planet in "Star Trek II: The Wrath of Khan" (1982), let a sorceress morph her way through the animal world in "Willow" (1988) and aged the diabolic Donovan 400 years in "Indiana Jones and the Last Crusade" (1989). Then came Cameron's "The Abyss" (1989), with the digital invention of Pseudopod, an undulating column of sea water that could mirror the faces of people. What makes "Terminator 2" a new breakthrough - and again confirms Cameron as a pioneer of special effects - is that the T-1000 cyborg moves with the whole range of human motions.

The next step? Faster machines and better software will refine the technique. Stuntpeople will become superfluous as real actors are able to "appear" in dangerous scenes. "Creature" shops that built E.T. and Darth Vader will be phased out as computers take over. But at ILM, where the techies have worked on six of the 10 most successful films in history, the vision is a bit more grand. Ross, whose office is along the "billion-dollar corridor" where posters of ILM spectaculars are displayed, says the day is coming when movies will be entirely the pigment of a computer's imagination. "We'll be able, God forbid, to put Marilyn Monroe starring alongside Arnold," he says. "And you won't believe your eyes."

T2 is just the latest film to use new technological tricks to dazzle audiences. Among the other legends of cinematic special effects:

French magician Georges Melies pioneers the field.

Forget Fay Wray. A furry model and a skyscraper stole this show.

The first of the modern outer-space films, by Stanley Kubrick. Part sci-fi, part fantasy.

George Lucas's legacy. The Force that created ILM and a new era in FX.