The magic of the movies

"Keeping separate what actually happens from what you [the audience] think is happening," is key, according to John Cini, president of High Output, Inc., of Canton, Mass. He works on special effects for film and theater companies.

"I could look up explosion [in a book] and it might say, 'an explosion is what happens at 2,000 degrees F.,' " he says. "But what is going to make you think explosion might be different – it might be light and sound that makes you believe it's real. It has nothing to do with heat."

Perception – how the audience interprets what it sees – is what makes special effects work. In "Star Wars" (1977), a spaceship the size of a small city cruises effortlessly through space. In reality, this was an elaborate, small-scale model filmed in a studio by a specially designed camera. "Star Wars" also used a technique called "forced perspective" in a scene in which a storm of asteroids soared directly toward the audience. The asteroids were models, and the scene was built so that the models closest to the camera were larger than those farther away. When the camera zoomed in, it created the illusion that the asteroids were racing closer by the second.

When it comes to special effects, things are rarely as they seem. The snow that clumps on Frodo's cape as he climbs the mountaintops in "Lord of the Rings: The Return of the King" (2003) is made of cornstarch, says Weber. Actor Christopher Reeve's cape in "Superman" (1978) billows realistically when he flies because it has a radio-controlled flapping device inside it. The crystal ball in Dorothy's tower prison in "The Wizard of Oz" is really a hollow glass bowl with a small screen inside that projected the faces of the Wicked Witch and Aunt Em.

Even actors' costumes can involve a bit of trickery in the movies. Crews can now draw on clothing with large, crayonlike markers, in colors such as "grass stain," to make them look lived-in. They can use a special powder to create the illusion of snow on an actor's face. "You sprinkle it on a beard, mist it with water, and it fizzes and turns into ice and snow," explains Mr. Poleszak.

But of all the innovations in special effects, it's the computer that's changed the industry the most. "Thirty years ago, effects were largely about what could be done in front of the camera," says Mr. Cini. "Now, effects 'on camera' are a small fraction of the work that is done, with most effects happening after the actual filming."

The green screen, for instance, is a computer-based technology where live action is shot in front of a surface that's a bright lime-green color. "It's a very pure green, not usually found in nature," explains Cini. (This is so that it's not the same color as anything an actor might wear.) "We can later delete that green color and have it replaced with a previously created image," he adds.

The green screen made possible many of the effects in "Harry Potter and the Goblet of Fire" (2005). "When Harry Potter's flying around, he's likely holding onto a stick and they're blowing wind up his cape in front of a green screen," says Poleszak. The clouds and mountains Harry flies by were added separately – as was the dragon he battles (another computer creation) during the Triwizard Tournament. Harry, dragon, stadium, mountains, and clouds all are meticulously edited together to create one seamless scene.

"In theater you can cheat by painting clumps of snow on an actor's boots," says Poleszak. "But in film it has to look real. The image is 30 feet across."

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