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Issue: Volume: 24 Issue: 6 (June 2001)

Out of Sync




A digital car crash blends real-time and slow-motion effects

By Karen Moltenbrey

It's not unusual for victims of violent car crashes to report having experienced the event in slow motion, as if they had been propelled into another dimension where seconds passed like minutes. Just such an experience unfolds vividly and dramatically in the opening crash scene in the race-car film Driven.

"Director Renny Harlin's vision for the scene was to show the audience what a driver sees and feels during a crash," says Ray McIntyre Jr., visual-effects supervisor at Pixel Magic (Toluca Lake, CA), which created the digital effects for the scene. "We did this by mixing real-time and slow-motion effects. Everything occurring around the crash-such as the other cars moving on the track-is in real time, but the entire accident itself unfolds in slow motion. When Renny explained his idea, we thought it might look odd, but once we did it, we knew it was a brilliant idea."

Pixel Magic created 50 digital-effects shots for the movie, nearly all of which are for the mixed-motion collision and a similar crash that appears later in the film. Other studios also contributed effects that are dispersed throughout the film, mostly involving back ground elements. In Pixel Magic's blended-time sequence, the character Memo Moreno brushes wheels with another racer. At 150 mph in the open-wheeled race car he is driving, it's enough to send his car skidding into a guardrail, then flying back above the racetrack as debris from the impact scatters in all directions. At the precise moment the car lands, another car strikes it, and the impact catapults it back into the air, before it cartwheels off the track and into the grassy infield.
To enhance the live-action drama in the film Driven, artists created numerous effects, from smoke and mist to 3D replicas of the actual cars, which were used in crash scenes.




"It would have been impossible to create this chaotic, yet strangely poetic, scene without using computer graphics," says McIntyre. "It's impossible to have one element in slow motion and the rest in real time. Also, you could never choreograph the accident to occur precisely as you'd like, even by rigging it [with pulleys]. And you'd never put the drivers in such a dangerous situation."

According to Brian Jennings, Driven's visual-effects supervisor, when he and director Renny Harlin came up with the concept of the crash, he knew part of the scene could be accomplished practically, but not all of it. First, they shot live footage at a test track in Montreal. For the beginning of the sequence, the crew filmed two professional drivers as they bumped wheels. For the next shot, they rigged a full-size replica of the race car to a pulley and wire system, and slammed it into the guardrail at 70 mph. "We had to digitally remove the big cranes, rigs, wires, and other equipment from the shot, leaving only the car on the track," says McIntyre. This was done using Pinnacle Systems' Commotion software. The effects artists also created computer-generated cars that were inserted into the background. "We tracked the scene using RealViz's MatchMover and added digital rain and mist, because the crash was supposed to occur during a downpour."

To simulate this scene from the driver's point of view also required digital intervention. First, the Pixel Magic team replicated the scene elements, such as the guardrail, track, and sky, using NewTek's LightWave running on Windows NT-based workstations. The artists then "projection-mapped" the filmed image of the guardrail on to the digital scene, which enabled them to create a computer-generated background more easily than modeling from scratch. "The computer-generated geometry had to match the film plates the director gave us, so we could put a camera move into each plate that simply didn't exist," says McIntyre. "This enabled us to generate the new camera moves for the sequence."
The initial impact of the mixed-motion crash was filmed in live action, using a pulley and wire system. Artists then digitally removed the equipment from the shot. (Images courtesy Pixel Magic.)




During the crash, the camera moves between an observer's viewpoint to the driver's point of view, then back to that of the observer. To find out what a crash might look like from the driver's level, the animators reviewed film footage of actual races and shots of crashes taken by trackside cameras.

"Renny really wanted one continuous camera motion for the entire sequence, which wouldn't have been possible without reshooting the sequence based on [digital] previews that we had made. And even then, it might not have been possible," says McIntyre. "Instead, compositing supervisor Todd Vaziri developed a transition that we coined "the slingshot," which takes you from one side of the track to the other. During this transition, Vaziri added a slight vibration to the image, and then the lens zooms in really quickly to the driver's point of view."

The rest of the crash-from the initial contact with the barrier until the car is resting on the ground-was completely computer-generated, as were all the scene elements, the sparks and debris from the car, and even the driver. Mike Hardison, 3D supervisor at Pixel Magic, managed a team of modelers and animators who built and animated 20 cars, including the one that Memo crashes into the guard rail. In fact, that car required several damaged versions that could be used throughout each stage of the wreckage. The digital models, like the physical cars used in the film, are replicas of actual race cars. "We use real race footage in the film, so the physical and computer-generated car models had to look just like the real cars from that footage," says McIntyre.
The crashing of the main vehicle unfolds in slow motion, while the rest of the scene's action occurs in real time.




With Adobe Systems' Photo shop, the artists textured the 3D vehicles, using photographs of the real vehicles for reference. Then they added dirt and grime so that the CG cars would blend into the scene immediately following the shots with the real vehicles.

All the digital elements, including the shadows, were rendered separately and then composited with Adobe Systems' After Effects. As a result, many shots contained more than 100 elements. Although time-consuming, rendering separate passes enabled the compositor to control the color and saturation of each car and the amount of debris.

"I'm not sure that anyone has ever done a slow-motion car crash like this before," says Jennings. "The execution of the car crash, with the ensuing debris and sparks, as well as the alteration of time, gives the scene a very poetic and ballet-like feel. It's an odd way to describe a car crash, but it's really quite beautiful."


LightWave, NewTek (www.newtek.com)

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