To promote the new Cars 3 film, which opens on June 16, Disney/Pixar organized a Road to the Races publicity tour that is currently making its way around the United States. The feature at each stop of the tour is a group of life-sized replicas of the film's stars: veteran racer Lightning McQueen, trainer Cruz Ramirez, and up-and-comer Jackson Storm.
The other, less obvious feature is a massive truck trailer to the left of the set-up. Inside, attending guests can watch an exclusive clip of Cars 3 that is still unseen by the general public. In the clip, Lightning and Cruz find themselves trapped in a part-race, part-destruction derby. A motley crew of hardcore vehicles chases them around a figure-8 track, including a massive, frightening school bus named Miss Fritter and a patched-up "rambulance" named Dr. Damage.
The arena is filled with dirt, which quickly turns to mud. And that mud is one of Pixar's most impressive visual renderings yet. It's wet. It's gloppy. It slips and spatters against the cars' tires. There were about 160 mud shots in the film, and to prepare, the visual effects team spent six months just experimenting with the mud—determining its nature and its physics, and building the rig that would adhere to those principles.
"The first part of the process was going out into the world, leaving our desks, and playing in the mud," said Jon Reisch, effects supervisor on Cars 3. "What about this material did we need to get across? How would we simulate it and recreate its physics? How does it move? How does it break? How does it interact with the characters?"
Getting our hands in it and having that sort of physical connection is really important. You're not portraying what you think it should be. You're portraying what it really is.
Pixar always informs its animation with a real-life source, if one is available. Take, for example, Lightning McQueen's big wreck in Cars 3, which features prominently in every trailer.
Kyle Larson's infamous 2013 crash at the Daytona 500 was the primary inspiration for this sequence.
"There were a few other [sources], but this one really inspired and scared us," said Reisch. "You see the smoke, and the debris, and just how fast it all happens. It's over in just a few seconds, but the devastation to the car and the fence is just…"
Reisch trailed off. Truly, the wreck speaks for itself. No one died, although 28 spectators were injured after Larson's car hurtled towards the stands.
To research the mud, Pixar's effects team members took a more hands-on approach. They went to local parks and played with the mud near the banks of streams. They also went behind the Pixar studio with a garden hose, and they made their own mud to play in.
"Getting our hands in it and having that sort of physical connection is really important," said Reisch. "You're not portraying what you think it should be. You're portraying what it really is."
As they did for Lightning's wreck, the team also watched a lot of source footage—of tractor pulls and destruction derbies—to learn how mud reacted to vehicles running through them. Some team members even attended a monster truck rally to get as close to the action as possible.
But when they ran the first mud tests on Side FX's Houdini software, they ran into problems. And the effects team leaned on its uniquely hands-on research to solve them.
"The thing we kept finding," said Reisch, "is that [our rendered] mud didn't have variation in it. The mud looked like cake frosting or chocolate. And so we started to introduce the idea of varying the viscosity and adding layers to the mud puddles themselves."
The team started individually programming different parts of the mud to behave in different ways. The effects programmers set a water table so that when a car ran through the mud, water would rush in and pool to fill the tire tracks. One section of the mud might be a bit more solid. Another section of the mud might be mostly water, mixed with a little dirt.
"[Mud is] a really tricky problem," said Reisch. "It's one of those things that's not really a liquid and not really a solid. It's just somewhere between, all of the time."
Making the mud react like mud is only half of the problem. The other half of the problem is to make it actually look like mud. Reisch credits the lighting and the set shading departments for making the material look wet, reflective, and glossy, and for directing the audience's eyes to where they need to focus.
And ironically, after so much focus on making things look realistic, the team must sometimes step away from realism, especially if it serves the film best.. Reisch recalls a shot where a car's tires were kicking up massive waves of mud, since the wheels were spinning at a simulated speed of 200 miles per hour. The team went in and changed the effect, so the car would create 60 miles per hour mud waves instead. It read better on-screen, and that's an important factor in an action film like this one. The audience might only have a split second to visually interpret each shot.
"On the whole, we're always willing to break the physics if we need to serve the story," said Reisch. "That's really the main aim."