When one sets out to fool people about Mother Nature, they usually fail miserablly. It is very difficult to replace what we are used to. Especially when it comes to nature's wrath. The recent movie Twister, however, shows that it is possible, with a lot of work and the help of a computer.

In the world of special effects tornadoes, "the standard is The Wizard of Oz" (Cinefex Magazine #66, p.74). For its time, the tornado in that movie was a special-effects masterpiece. However, today's discerning audiences would easily see through such a simple effect. So Twister director Jan De Bont turned to the masters at movie trailers for Twister, where a large black tornado on the left side of the screen is spewing debris, and a tractor tire hits the camera. After 10 weeks of hard work, ILM presented the test shot to De Bont. Having proven with that shot that tornados could be done on the computer, Twister was given the green light to begin filming in the winter of 1995.

Unfortunately for the film crew, the spring they were filming in Oklahoma was an unusually pleasant one, so original plans to film with the backdrop of approaching storms were scrapped in favor of completely computer generated storms. To compensate, ILM added 2D and 3D clouds to many of the scenes, transforming a seemingly bright, sunny day into a cloudy, stormy one.

[Image courtesy of the VFXHQ]

Another weather-related problem was the fact that filming took place over the course of a spring, so that by the end of filming the crew had to move from Oklahoma, where the wheat fields were ripening and changing color from green to yellow, to Iowa, with its corn fields and later spring. Also, some corn ripened between shooting days, so that husks were visible where none appeared before. These were edited out by the ILM crew, as was some of the ripened wheat.

Much of the debris was made by a computer as well. For the debris that wasn't, the crew got two Boeing 707 jet engines and tossed biodegradable foam debris in the path of the exhaust, sending it flying across the scene. A tricky part for ILM was syncing up this real debris with their own animated debris. This was solved by modifying the rendering system software to be able to change the behavior of a single particle in the system, perhaps because the director wanted the car fender to bounce OVER the sign, not fly past.

As for the tornados themselves, ILM developed rendering software capable of rendering and lighting soft-shaded particles, making it possible to light individual dust particles in the tornado from different angles. This allowed them to use shading in the tornadoes that was far more realistic than anything up to this date. The ILM crew used two different techniques for animating the tornadoes, and a third for animating the water spouts early in the movie.

To judge a tornado a F-scale is used. The smallest torando is a F1, growing to the largest and most destructive, a F5. The F3 and smaller tornados were created as conical solids masked by a few layers of shaders and a few individually rendered particles. It was discovered that for smaller tornados, this technique was most effective. However, the larger F5 tornados required a number of layers of individual particles rendered to the extent that the column was no longer transparent at any point.

The ILM crew also discovered that when creating tornados, slower equaled bigger. When the resulting tornados were spun too fast, the shot ended up looking like a minature. After looking at home movies taken of a large number of real tornados, they figured out that debris makes one revolution every five to ten seconds. They incorporated this knowledge into their animations, spinning the larger F5 tornados far slower than their smaller brethren.

For the water spouts observed when the main characters drive across a bridge, only to get stuck there, the animators studied some footage of water spouts in real life. They created very turbulent water surfaces using Dynamation and RenderMan. Multiple layers of particles were animated and spun near the base of the spouts, while higher up they were smoother, more stable cones. They also animated the spouts to move around far faster than the land tornadoes, giving them the appearance of more irregular, more chaotic storms.

This is also the scene where the cow flies past the truck and one of the characters remarks "We have cows!" This, too, was an interesting scene to do, as the cow is viewed through a rain-soaked windshield. "From an animation standpoint," said animator Peter Daulton, "an animal that is flying through the air-as opposed to walking or running and having contact with the ground-gives us a tremendous amount of freedom. I think we pushed the envelope on believeability a little bit, but it was hard not to make it humorous. The cow looked as if it was not happy being up there. When we showed it to (director) De Bont, he laughed for about a minute." (Cinefex #66, p81)

After trying to animate the cow both behind a wet windshield and behind a dry one with CG rain, the crew decided on the former, because trying to make believeable rain is very difficult, and not necessary given the option of real rain. They had to isolate the rain on the windshield too, but they were able to cut a corner here and only go for the bigger drops, since the windshield wipers were moving very fast and wiped away any inconsistancy that might have arisen.

Still another scene involved a tanker truck being hurled out of the F5 twister and landing in front of the heroes' truck, exploding on impact. It was first done with practical special effects, rigging an aluminum full-size mock-up of a tanker truck with pyrotechnics and dropping it in front of a moving stunt car. This was all done on the set. Later, ILM animated a tanker truck revolving around the tornado and flying out towards the camera, removed the aluminum tanker, and animated the CG tanker twisting in the wind as it gets closer, and then cut to the shot of the original tanker landing and exploding. This is a great example of practical and computer effects complementing each other, and is discussed more in depth in the Computers -vs- Models section.

[Image courtesy of the VFXHQ]

Another problem for the special effects teams was the frequent use of computer displays in the movie. All the false computer screens, as well as any of the fake weather reports and storm warnings, the laptop computers, and the National Severe Storm Lab, were done by Banned from the Ranch Entertainment, a computer graphics company that specializes in on-screen computer animation for movies. Banned from the Ranch, of Santa Monica, California, borrowed $3 million worth of computer equipment from Silicon Graphics and custom built them to create SGI laptop computers, which don't exist in real life. They were powered via a cable to an offscreen SGI Indy, and the graphics for them were produced by a Banned from the Ranch employee sitting at the Indy watching the scene. For the "Storm Warnings", Banned from the Ranch got local Oklahoma TV Weathermen to do weather reports in front of storm data transplanted from locations where storms were occuring that spring. They also created a realistic-looking NSSL room by getting data from various sources: doppler radar, sattelite, old archives, etc. They then edited the location information and used existing software to plot and display the results.

Altogether, Twister was a tremendous accomplishment in the field of computer special effects. From the convincing, large-scale tornados to the smaller, finer details such as editing to maintain continuity, Twister was a major achievement for everyone involved. ILM had set the standard much higher than ever before. Anything to come later would be judged against the effects in Twister.

Silicon Graphics
Twister
Visual Effects HQ: Twister
Twister Media Page
Computers -vs- Models Page
Pixar's Renderman

Last Updated August 22, 1996