Sony Pictures Imageworks plugs Houdini and Maya together to create controllable fire
By Barbara Robertson
In Sony Pictures’ film Ghost Rider, the star, Ghost Rider himself, is an alter ego for stunt motorcyclist Johnny Blaze (Nicolas Cage), who sold his soul to Mephisto (Peter Fonda). Because of the devilish deal, when the sun goes down, Blaze transforms into a hell-blazing skeleton riding a fiery "hell cycle." Like his daytime earth being, the supernatural Ghost Rider is a leather-jacketed biker. But, flames shoot out from the demon’s skull and hands, and his bike’s wheels and motor are on fire.
Sony Pictures Imageworks handled a majority of the 600 visual effects shots, including all the shots of Ghost Rider and the fire—the two biggest challenges for the movie, according to VFX supervisor Kevin Mack. "The skull doesn’t have much expression because it’s rigid," says Mack. "We considered exchanging different skulls with different expressions, but we wound up using one all the time." Instead, they manipulated the fire emanating from the character and the motorcycle to create emotional performances.
Sometimes, Ghost Rider is the actor or his stunt double with head and hand replacements. Sometimes, he’s all-digital. "There are lots and lots of shots that have a fully CG rider and bike," says Mack. "But we also had some with a digital head and hands. Ghost Rider’s fire was never practical."
The demon’s fire was always a computational fluid simulation that used the Autodesk Maya solver for the core calculation controlled by a Maya plug-in that read Side Effects Software Houdini data at the front end. The Houdini data helped the effects artists direct the simulation.
"It took a lot of finagling to get the fire to look realistic and the way we wanted it," says Mack. For example: "Ghost Rider gets knocked around and he moves fast, so we submitted simulations to a lot of situations where a real fire would go out. You’d expect that if fire is moving fast, you’d get little bits tearing off, but it was hard to get a simulator to do that. So, Patrick Witting, our fire supervisor, and [others]developed the tools we needed."
Witting started with the presets in Maya’s fluid solver, supplementing those presets with several Maya plug-ins and MEL scripts. The plug-ins and scripts helped the effects artists place the fuel that drove the simulation and control its movement and shape. "The Maya fluids gave us the ability to emit fire off the surface, but it didn’t give us the ability to use texture maps to control where on the surface it was," says Witting. "Even beyond that, with Maya, we didn’t have the ability to place fuel out in nowhere."
Artists using Houdini painted maps to put more fuel on the sides of the skull and less over the eyes, and create fiery eyebrows and sideburns. The maps also controlled the thickness of the fuel layer and the fire. "We wanted a nice thin sheeting of fire over his face, but not so much that we couldn’t see the eye sockets," says Witting. "We also wanted to see the detail of the skull, the bump maps, the lighting, and all the rest. We didn’t want to obscure what was there."
Houdini’s procedural side came into play here as well: 3D and 2D noise functions helped push the fire away from the skull and into interesting shapes. For example, the artists often wanted the fire to flare up and create licks of flame. "We tried to leverage from what we had for free," says Witting. "So, if a character is deforming, we had a layer [of fuel] grow out away from the surface according to the normals. We used the painted textures and noise expressions to make the pockets of fuel look interesting and organic."
Maya’s open architecture allowed access to the fluid variables used for the simulation at every point in the sim. Imageworks’ Maya plug-in modified those fluid variables—the velocity, temperature, density, and fuel—based on settings in the Houdini data file.
"We’d intervene at every simulation step and emit little amounts of fuel in voxels," Witting explains. "The [Maya] plug-in would loop over all the Houdini points we gave it and figure out which voxels to emit fuel in." Then, it moved to the next frame; every frame of the Maya simulation read information from Houdini.
Thus, the plug-ins, in effect, provided fire control on a voxel-by-voxel basis. "It’s pretty much the only way of interacting with [the solver] under the hood," says Witting. "And [the project] was all about control and directability. We did whatever was required, whether it was rooted in physics or not." For example, when Ghost Rider sees Roxanne (Eva Mendes), Johnny Blaze’s love interest, his fire gently flickers. But when he’s angry, the flames become agitated and violent.
To manage the simulation calculations, Witting set up the task using several small, separate parts. Take the Hell Cycle, for example: "The fire on the back tire might be one part," he says. "We’d have another part on the front tire. We wanted to control the amount of fire coming out from the fender, the sides of the tires, the area where the spokes would be, and the frame. They all had lots of consistent aspects, but they behaved differently."
The same was true of the fire simulations for Ghost Rider and for another fiery character, who the crew calls the "Old West Ghost Rider," and his horse. The horse’s hooves and its body were separate simulations, as were the rider’s hat, his skull, and his eye sockets. The practical result was that the crew could specify how many voxels they needed to simulate certain areas, and they could get approvals on the individual parts.
"Fire has to be simulated within a volume, and the bigger the volume, the more computation that is required," says Mack. "We ran separate sims for every shot and for every element—the skull, each eye, each hand, each wheel on the bike, and the bike’s motor. For the rider and the bike, we might have 10 simulations. We’d re-use settings, but each shot was unique."
During one shot, though, Imageworks used practical fire: "When Johnny Blaze gets summoned to be the Ghost Rider character, we set Nick Cage’s feet on fire," says Ken Hahn, digital effects supervisor. "He was a real trouper. He walked a few steps with his feet on fire."
At night, the comic book hero Johnny Blaze (top),pictured here as stuntman Eddie Yansick, transforms into the skeletal GhostRider (second to top), a fiery demon (second to bottom) who uses his flames to express emotion (at bottom)...and incineratesinners. Sony Pictures Imageworks created the CG demon and fire.
All photos by Sony Pictures Imageworks. © 2007 Columbia Pictures Industries, Inc.
Rather than having Blaze be consumed by fire from the outside, Imageworks transformed Cage into Ghost Rider as if he were burning internally. "Outside on the skin, you see the flame reaching the surface and start to burn from inside," Hahn says. "We used the same CG fire for that."
The crew treated the skin as if it were dry paper rather than lurid, boiling flesh. "You see his eyes glow red, and black spots on his skin that open up like burning paper," says Mack. "Little bits of flesh blow off, sending embers in the air, and reveal the clean skull underneath."
To render the fire, the crew developed custom techniques based on the Pixar RenderMan RI point solution. "The fire was composed of many RI points," says Witting. "Steve Marshall came up with a shader based on physical fire that was implemented as a Houdini back end. So, Maya was sandwiched between two Houdini processes." The shader allowed the effects artists to heighten Ghost Rider’s emotions by changing the color temperature of the fire from a melancholy blue to an angry red.
The Imageworks fire simulation team spent between six and nine months developing the tool set for the film and another nine to 12 months in shot production. (The preproduction and shot production schedules overlapped by about three months.)
Mack believes the hot, new techniques and technology the crew devised to create Ghost Rider’s fire, all of which take advantage of off-the-shelf software, push the state of the art.
"Fire simulations have been around for a while," Mack says. "But I don’t think anyone has pushed them this far, especially in terms of dialing in the fire simulation to look right."
Barbara Robertson is an award-winning writer and a contributing editor for Computer Graphics World. She can be reached at BarbaraRR@comcast.net.