Car Talk
Issue: Volume: 29 Issue: 6 (June 2006)

Car Talk

Pixar's seventh feature animation sends a stock car racing down Route 66
by Barbara Robertson

From day one
, Pixar has put CG animation on Hollywood’s fast-track. Now, the multiple-Oscar-winning studio puts pedal to the metal with Cars, a Walt Disney Pictures/Pixar Animation Studios feature film that stars an ensemble cast of hot rods, stock cars, sports cars, and rust buckets.
The drama begins at a fast and fl ashy NASCAR night race on the East Coast, and then follows one participant, Lightning McQueen (actor Owen Wilson), afterward as he takes a trip to the big Piston Cup Championship in California. On the way, he crashes just outside Radiator Springs, a desert town along Route 66 in Carburetor County where he must spend time repairing a little damage. There, he learns that while achievements are fine the journey in life is the reward.
Cars was directed by John Lasseter, the driving force behind Toy Story and Toy Story 2 who has overseen all Pixar’s creative endeavors, and co-directed by the late Joe Ranft, who also served as story supervisor for the film and voiced several incidental characters.
The ambitions of the character McQueen center on beating his main racing competitors:The King, a 1970 Plymouth Super bird voiced by Richard Petty, and Chick Hicks, voiced by Michael Keaton. McQueen's epiphany arrives via Radiator Springs’ notable citizens. Doc (Paul Newman), a1951 Hudson Hornet, is a quiet country doctor (mechanic) with a secret past. Sally, a 2002 Porsche 911 (Bonnie Hunt), is a California refugee now operating the Cozy Cone (traffic cone) Motel. Fillmore (George Carlin), a 1960 VW Bus, is the resident hippie. Sarge (Paul Dooley) is a 1942 Army Jeep, and Mater (Larry the Cable Guy) is a good ol’ boy tow truck. These are but a few of the town’s four-wheeled residents. There are no human characters in this film. In fact, with the exception of a few bugs (flying VWs with wings), the only characters in the film are cars.
Any other studio given a “car” theme for an animated feature might have created a cartoon. Not Pixar. To the animators, the Cars stars aren’t cartoon characters.
“John [Lasseter] wanted a story in which the cars are humans,” says Scott Clark, supervising animator. “The doors don't open; you don’t look inside their heads. They become humans; a human drama unfolds in front of you.” At the same time, Lasseter had another requirement, one born of the 3D medium in which they’d be working. “John’s edict was ‘truth to material,’” Clark says. “
"So, on the one side is realism—we knew we could animate a car that looked believable. But, the car had to emote.” So, the Cars characters couldn’t be as caricatured as, say, Disney’s short animation “Susie the Little Blue Coupe,” but they had to come alive. “We do exaggerate,”Clark says. “But because the medium has such dimensionality, a car doesn’t get on its back tires and gesture with its front tires.”
To turn the cars into characters, Pixar made the front of each car the head and put eyes on the windshield. Any yet, although the eyes have an iris, sclera, and dimensionality, they still look like they’re made of plastic and glass. The metal above the windshield acted as a mix between an eyelid and an eyebrow, and provided a hint of a furrowed brow. The mouth, where a grill might be, became the most plastic facial feature. The cars pivot from their back axles and steer with the front. “So the back became the hips,” Clark says, “and the front tires became hands. By steering or tipping, the tires became gestural, and then, by pivoting the car’s front from the back axle, the whole car became a head.”

All images © 2006 Disney Enterprises, Inc. and Pixar Animation Studios.
Male cars had angular shapes; female cars had softer curves. Sally, the Porsche, for example, has a small, cute mouth.“Humans are the hardest characters to animate because there are so many layers of believability,” says Clark. “We had to strip away the noise and find the simple stand most elegant way to get across that this is a car, but it’s alive, and this is a male or female car. It was almost a Zen like way of animating.”
Body Shop
A team of between 35 and 40 animators worked on the ensemble cast, each person animating all the characters in a scene. All the cars used the same basic rig. “We didn't want to spend forever creating variations,”says Eben Ostby, supervising technical director. “We needed to produce large numbers of different models from one kind of rig. We could take the facial rig for one car, move it to another car, and it would still work. We’d just stretch it to fit.”
Built into the rig were two systems that helped the cars act like real cars. By using a path-based driving system installed in every car, animators could create a line through space, and the character would follow it. While the car was moving down the road, ground maps showed the road’s elevation. So, whether the roads were curvy, hilly, or bumpy, the system locked the wheels to the ground and handled the physics. Tires squished and the cars bounced as they drove over bumps; they swayed around corners, and behaved like real cars when they screeched to a stop. “It would have been impossible for animators to look at that in every frame,” says Clark. “But we had the choice to use it or not. We could even run the simulation and adjust the animation later.”
As with a real car, the suspension system could be changed by the animators to suit the character. Thus, Sarge lumbers and rolls, while racy McQueen stays low to the ground and grips the road.
A spatial weighting system for various car parts provided the performance controls. “By and large, many of the controls have analogs in creatures,” says Ostby. For example, the crew rigged the cars’ facial animation system using controls similar to those they might devise for animating a creature’s face. “The cars have a jaw and cheeks,” he says. “But there are also differences. For example, the eyelids are on a sliding panel on the windshield.”

Pixar used custom crowd-simulation software to cue specific actions
within the thousands of cars in the stadium, and
Softimage XSI’s Behavior software to control cars queuing up in line.
To animate the stadium crowds for the races at the beginning and end, Pixar used Softimage XSI’s Behavior software and a proprietary system. Behavior handled cars with such particular actions as queuing up in line. The proprietary system managed the massive crowds and allowed the animators to give individual cars specific actions that they would perform on cue. “Making a crowd look alive and not planned is an art in and of itself,” says Clark. “The crowd has a personality. We didn’t just create a bunch of cycles and hit the random button.” The animators discovered, for example, that if they multiplied an eye-blink cycle and applied it to the stadium cars, thousands of cars all blinked at the same time. “The windshields are bigger than people’s eyes,” explains Clark. “We couldn’t have that. There was a lot of really carefully considered animation done by artists in the art department.”
In the Dust
The opening stock car race happens on a small track at night. “It gets the movie off to a rip-roaring start,” says Ostby. “As the cars pass through all the light sources around the track, we see tiny shadows. It looks almost like the light is strobing. It helps make the start of the film energetic.” By contrast, the middle act in quiet Radiator Springs is relaxed. And then the last act puts us back at the track—this time, in California. The California racetrack has a different feel,” adds Ostby. “Accidents happen. We have smoke and dust, and McQueen races through the smoke.”
To create the smoke and dust, Pixar developed a new rendering model for the aerosol fluids. “That wasn’t the key thing, though,” says Ostby. “The key thing was that we used a lot of it, and we art-directed it.”
Effects supervisor Steve May led the teams that created these effects and others. “The main effects were dust,” he says. “John[Lasseter] gave us reference footage from rally racing in Europe, with cars on large expanses raising huge clouds of dust that could be hundreds of meters long, but at the same time, had this complex behavior in them.” With that footage in mind, the effects team created giant dust trails for the cars racing through Cars’ deserts.

Thousands of tiny Maya particles rendered with PRMan
point primitives kicked up dust behind the 1951 Hudson Hornet,
Doc. Large particles rendered volumetrically formed distant,
billowing dust clouds.
To do this, the crew tried, at first, putting 3D car models into virtual wind tunnels and running full fluid-dynamic simulations, but the lack of control over the simulations persuaded them to use two types of particle simulations instead. To create the big clouds of dust, which would have required too many particles to produce the fine details in the reference films, the effects timegenerated between 20,000 and 40,000 large particles that they rendered volumetrically. “Some of the particles were as big as the car, but most were the size of a wheel,” says May. “We used a sophisticated shader written by Erdem Taylan first for underwater explosions in Finding Nemo, and then for explosions in The Incredibles. It can add nice details that create a rotational feel.”
For smaller scale dust, the effects crew generated as many tiny particles in Autodesk’s Maya as they could push through the system using small point primitives in Pixar’s PR Man to render them. Disk space, not rendering memory, limited the number of particles. The crew produced turbulent particle dynamics for the dust clouds and for the smoke in the car crash scenes by using Maya hair dynamics to set up the fluid fl ow. “We attached hairs to the bottom of the cars, and they gave us overlapping dynamics as a basis for generating the particle dynamics,” Ostby points out.
Road Crew
Animators handled the car crash at the beginning of the film without help from a simulation. Instead, aptly named animator John Khars masterminded the physics. “We could have hit the‘simulate’ button and gotten something interesting,” says Clark.“But we wanted to be in control. We wanted caricatured jokes.”
Procedural animation helped, though, when McQueen crashes through Radiator Springs. He’s out of control, runs through a barbed wire fence, and becomes attached, literally, to a statue of the town’s founder. When that happens, he pulls so hard that the statue topples off its pedestal and the post that held up the state lands in front of him. He’s still attached to the statue, but it's night, he’s scared, and he’s not sure what’s going on, so he keeps driving. As he heads down Main Street pulling the statue behind him, he wreaks havoc. The statue rips up the road like a knife pulling through brittle icing on a cake.

The Fiat Luigi, the stock car racer Lightning McQueen, and the Italian
fork lift Guido have the same basic suspension rig, individually adjusted,
that helped animators keep the cars’ tires on the road in the film.
To tear up the street, the effects group used Voronoi tessellation to procedurally break the road into little chunks of asphalt.“Ferdi Scheepers created a clever algorithm that uses McQueen’s path,” explains May. “The chunks are smaller on the center line, and get bigger as they get farther away. Then he placed all the tiles back in an unbroken state and seamed up all the shading so you can’t tell it’s pre-broken.” As McQueen drives on a long path that winds for blocks through the town, animated forces push the pieces away, starting with the smaller pieces near the center and then the larger pieces later. “It’s like a zipper,” says May. “And, it goes on for at least a football length or two, kicking up rubble and spray that hits the buildings.”
The effects team also created a waterfall. In a scene that May describes as particularly majestic, Sally and McQueen drive up a mountain and come upon a magnificent waterfall.“John [Lasseter] wanted this to be an awe-inspiring moment,”says May, “one of the most beautiful things McQueen has ever seen.”
For reference, Lasseter sent the crew photographs that he had taken in Yosemite of a waterfall that showed the distinctive strands of water he wanted. By emitting particles from layers of maps, technical director Jason Johnston simulated the water falling at different speeds in the huge waterfall. “One level of particles became a generator for other levels,” May says. “We didn't want anything viscous. We wanted distinct parts—independent groups that moved as a group.”
To generate the waterfall, the crew used so many particles that the simulation shut down the renderfarm. “It was the network traffic,” points out May. “We have an adequate renderfarm, but bringing the data to the renderer made the system grind to a halt.” To reduce the load, the group quarantined the shot so that only a few frames would render at a time.

Sally Carrera, a 2002 Porsche 911, drives past a waterfall that used so
many particles the simulation shut down the renderfarm. Particles emitted
in layers simulated water falling at different speeds.
In addition to these large particle simulations, the effects team also modeled individual pieces of geometry to create a fine layer of dust in Doc Hudson’s garage. They distributed the geometry using shaders, although sometimes Lasseter wanted the individual bits of dust back lit.“It’s fun to develop new technology,”says May, “but it’s also great to focus on artistic aspects. The dust clouds are beautiful. It's not just dirt; it’s like clouds rising from the cars during the golden hour of the day.”
For the first time, Pixar rendered with ray tracing throughout the film—the cars demanded it for the reflections. “We used ambient occlusions to help bring out the shapes and crevices in diffusely lit objects,”explains Ostby. “And, we used raytracing for diffuse radiance—to spill colored light from one object to another.”
The studio renders with the commercially available Render Man software, PRMan, although Ostby points out that they tend to be early adopters. “The trickiest thing for us was in using these techniques in a film with as much stuff as in this one,” he says. “We used a multipronged approach to get our renders to be reasonably cheap.”
The studio calls one of those approaches “shrink-wrapping,”which is a texture-mapping technique that reduced the complexity of rendering fully detailed cars in a scene. “We bake information onto the surface of the cube so the process becomes, essentially, the process of rendering the cube,” Ostby says. Displacement maps made the cubes render as if they were a fully detailed cars. The rendering team also helped speed rendering, particularly of raytraced reflections, by becoming early adopters of PRMan’s brick maps. “We baked out whatever we could into brick maps,”says Ostby. “We baked raytraced reflections, occlusions. You compute the reflections only once, but you can filter them when you read the map in. So you can add an extra layering of filtering.”That helped them more easily repair any aliasing and noise problems that occurred.
To reduce the amount of complexity in the desert, with it smiles and miles of sagebrush, the crew used what it calls “stochastic pruning.” “We wanted to build the sagebrush accurately, with thousands of leaves on each,” says Ostby, “but we also wanted hundreds of thousands of plants in the desert valley.”
With stochastic pruning, the sagebrush plants didn’t lose volume;they became less complex. “As the object becomes small on the screen, we drop off leaves, but we compensate,” Ostby explains. “We dropped them off stochastically across the surface so we didn’t have bald spots, then we compensated the size of the leaves, the amount of surface area, and the shader, which need a different response to light as the leaves changed size.”

Stochastic pruning reduced the amount of geometric complexity in
the sagebrush-littered desert. Note the tail fin-shaped mountains
rising from the plains behind Lightning McQueen.
For Cars, the R&D group wrote a new lighting tool dubbed Lumos that allowed the lighters to manipulate much of the lighting interactively. “We could change light directions and shadows, all those good things, but not the final touches,” says Ostby. The crew rendered out a small number of passes—usually three or four, sometimes as many as 10. Using Apple’s Shake, the compositors could dial the intensity of reflections up or down.
In addition to making the cars look real and the landscape magical, lighting also helped tell the town’s story. “We contrast the town from the way it used to be in its heyday to how run down it is now through the use of neon lighting,” says Ostby. To do that, the crew used new area light sources for particular neon tubes, along with RenderMan light shaders. “It was a lot of work making the neon lights look right and cast the right light on the set,” Ostby says.
Motoring Down Route 66
In 2001, the two directors, producer Darla Anderson, and other members of the production team drove a caravan of four white Cadillacs on a nine-day trip along Route 66 from Oklahoma to California. “When John went on the road with the story guys, they went to meet the people on Route 66,” says Clark. “They knew the story they wanted to tell about the cars. They didn't know the through-line. There’s a lot of Joe Ranft in this movie, and he felt there was something really human about how the towns were forgotten when the interstate system was built in the ’50s. Route 66 is a physical representation of American culture, and one of the stories is about the town. It’s kind of an appropriate story to tell.”
With Cars, Pixar took a risk by sticking to John Lasseter’s mandate“truth to materials.” Turning the cars into cartoon characters might have been easier, but the crew believes the result wouldn't have been as engaging; people wouldn’t have become as immersed in the world. In any animated feature, the technical sophistication and animation skill all goes to waste if the story doesn’t capture people's hearts, but Pixar always has a story to tell. That’s why the studio’s films have been box-office hits, Oscar contenders, and Oscar winners. And that’s likely to be true for Cars, as well.

Let the races begin.

Barbara Robertson is an award-winning writer and a contributing editor for Computer Graphics World. She can be reached at