Out of some 600 effects shots for Ghost Rider, Sony Imageworks and vfx supervisor Kevin Mack oversaw 342 shots. All images © 2007 Columbia Pictures Industries Inc. and GH One LLC. All rights reserved.

If you have the QuickTime plug-in, you can view two clips showing Sony Imageworks’ vfx work by simply clicking the image.

When Ghost Rider opens Feb. 16, Sony Pictures certainly hopes that the Marvel Comics superhero will set the box office on fire. After all, it tells the tale of stunt motorcyclist Johnny Blaze (Nicolas Cage) who sells his soul to the Devil and becomes a hellblazing vigilante. With his flaming skull and fiery motorcycle, the Ghost Rider is one of the most unique superheroes ever conceived. He also was one of the most difficult to visualize on the big screen. Contrary to most superheroes, the character couldn’t be portrayed by a man in a suit -- in any shot. It could only be created via computer-generated animation.

The challenge to generate the ultimate CG fire was assigned to Sony Pictures Imageworks and visual effects supervisor Kevin Mack, with Josh R. Jaggars acting as vfx producer. “Ghost Rider features over 600 effects shots, of which Imageworks did 342 shots,” Mack says. “We focused on the more creative effects, such as characters and key environments, and we farmed out the more straightforward effects, such as rig removals and sky replacements, to other vendors.”

For Mack and his team, the priority was to develop a totally realistic and controllable CG fire. The concept behind the Ghost Rider character was that, under certain circumstances, his bones start superheating. The heat first dries out the flesh and skin, before burning them from the inside out, leaving nothing else but blazing bones. Director Mark Steven Johnson (Daredevil) wanted a rich inextinguishable layer of flames flowing over the character’s skull and hands. This supernatural Hell fire, which was clean burning and cool to the touch, would become hot when it left his body. “Our main challenge was to create a realistic fire that had a distinct quality,” Mack recalls. “We started by shooting tests with a mannequin skull on fire. It showed us that if you expose for the scene, the fire will burn white and the colors will be overexposed. On the other hand, if you expose for the fire, you do get rich colors and details, but everything else in the scene goes dark. So, I suggested that we always render the fire with the best possible exposure, regardless of the light conditions of the surrounding environment. Since this was a fire from Hell, we had some latitude to make it look beautiful. It gave us a rich, detailed and colorful fire with a surreal quality.”

On each shot, the first step consisted of tracking the plate and then matchmoving a 3D scan of Cage to the actor’s performance. The 3D skull and hands fit inside the 3D body, allowing the fire animation to start literally within the body.

The tests revealed another problem: at 24-frames-per-second, real fire tends to create a stroboscope effect on screen. Flames actually move too fast to be properly captured by film cameras. Mack addressed this issue by slowing down the Hellfire flames. This helped the CG fire read better on screen, and it also emphasized its supernatural quality.

Combining 3D Packages
Digital effects supervisor Ken Hahn did the initial testing, and effects animation supervisor Patrick Witting took over to develop the fire system. “It was all about control and directability of the fire,” Witting explains. “Maya’s fluid engine was the core fluid solver, but it was limited in its emitter and colliders. So, we used Houdini to drive them, and we wrote a custom Maya plug-in to read and interpret that Houdini data on every frame. This hybrid system gave us controls for a variety of simulation situations, both physically motivated as well as crazy stuff. Each distinct piece of fire was typically one layer, coming from one simulation version. That being the case, characters were typically broken up into many distinct simulations. For instance, Ghost Rider had separate simulations for his skull, his neck, each eye and each hand. The Hell Cycle had a separate simulation for each wheel and so on.”

One of the major challenges for the CG team, lead by JD Cowles and Brian Steiner, was to generate a fire that would react to full speed motorcycle rides, helicopter rotors, and even burn underwater. When the fire lacked the required dynamics, counter-forces were built in to simulate motions that would make the fire react as dramatically as possible, while maintaining a level of believability. “Many of the simulations were parented to the character -- to allow the thin layers around them to travel with them – rather than being left behind – as you might expect of a motorcycle ripping down the road for instance,” Witting explains. “To make parented simulations look realistic, we calculated counter-forces on every frame -- one for velocity and another one for acceleration. Key-framable multipliers on these motion channels drove Maya forces to give realistic and/or artistic responses to a character’s motion.”