The magic of the movies

The purpose of special effects is to create illusions that are believable, no matter how fantastic.

"How about a little fire, Scarecrow?" The Wicked Witch of the West taunts a terrified Scarecrow with her flaming straw broom in "The Wizard of Oz" (1939) – and then sets him on fire. But how could she? Wouldn't the actor get hurt? Not likely. This scene, like many in the movies, was accomplished with film tricks known as special effects.

"The actor [Ray Bolger] probably had an asbestos sleeve on one arm," explains Andrew Poleszak, a costume designer in Boston. "They padded out the arm of his costume, put combustible fluid on the cloth, and set the edges on fire." Dorothy doused the Scarecrow with water to squelch the flames, which "melted" the Wicked Witch, played by Margaret Hamilton. For this effect, Hamilton stood on a small hydraulic lift that was lowered into the floor. Dry-ice fog made it seem as though she was melting, and her long black skirt hid the trick.

The purpose of special effects – in theater, on television, or in the movies – is to create illusions that are believable, no matter how fantastic: a boy wizard flying through clouds, a tornado scooping up a farmhouse, or an intergalactic spaceship racing through infinite space.

How do they do all that?

Before modern technology, such as computers, special effects involved some unusual ideas. When director Cecil B. DeMille needed to create the illusion of Moses parting the Red Sea in "The Ten Commandments" (1923), he poured thousands of gallons of water down the sides of a U-shaped tank. DeMille filmed the action as the torrents crashed together – and then ran the film backward to get the desired effect. (The actors were added later.) To stage elaborate car chases by a wacky gang of early-20th-century movie characters called the Keystone Kops, movie crews dumped liquid soap on real streets and hired stunt drivers to skid their cars through the slippery mess.

Moviemakers also combined painted backdrops with live-action sequences to make scenes more dramatic. The tornado that rips Dorothy's house from the Kansas prairie in "The Wizard of Oz" was constructed from muslin (a strong fabric) built into a large cone shape. A machine suspended from the ceiling of the studio moved the fabric, and colored talc (called fuller's earth) was blown through the fabric with a hose to create a dust storm. Once the tornado action was filmed, the crew shot the background of a stormy sky (large panes of glass with painted cotton for clouds). This was then projected behind Judy Garland, who played Dorothy. Wind machines added realism to the scene.

"You simulate through machines what Mother Nature does," says Douglas Weber of Theatre Effects, a special-effects business in Cincinnati. Mr. Weber says that he uses all the science, math, and English he learned in school in his special-effects work. "If someone needs a lightning effect, you have to understand how lightning happens. All those things in school I never thought I'd use – there's nothing that I learned that I don't use now."

For "Pirates of the Caribbean: The Curse of the Black Pearl" (2003), Weber and his colleague Chris Wyllie cooked up an ominous fog to fill the screen when the Black Pearl pirate ship first appears. They also created the fog that surrounds Jack Sparrow and crew as they row into a cave where pirates' gold is stashed. "We created a wispy haze you can see through," says Weber, "so the actors weren't obscured." His and Mr. Wyllie's fog concoctions were made of what they call "people-friendly chemicals."

"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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