Alain Bielik discovers how Sony Pictures Imageworks mastered the ultimate in totally realistic and controllable CG fire as the centerpiece of Ghost Rider. Includes Quicktime clips!
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 couldnt 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 characters 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.
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. Mayas 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 characters motion.
For aesthetic and dramatic reasons, there was not any smoke associated with Hell fire. There was heat distortion for each fire simulation, though, Witting continues. It was a separate particle simulation, based on the approved fire simulation, which drove compositing warp nodes for the background and characters behind the fire. Similar artistic license enabled the character to keep his clothes intact although his entire body was on fire
During plate photography, the crew used special rigs to simulate the presence of Hell fire on set. I didnt want having to replace entire parts of the environment to create interactive light effects in CG, Mack observes. So, we used interactive lighting rigs that allowed us to capture, in camera, the flickering light of the fire being cast on the actors and on the environment. Nicolas Cage or his stunt double wore a mask that was covered with powerful amber-colored LEDs. We had a program that fluctuated their brightness randomly. The bike had the same LED rig mounted on the wheels and inside the engine. It gave us great interactive light effects on the Ghost Riders shoulders and on the other characters around him, as well as on the ground around the motorcycle. It made our CG fire look all the more real.
On each shot, the first step consisted of tracking the plate, and then matchmoving a 3D scan of Cage to the actors performance. The 3D skull and hands fit inside the 3D body, allowing the fire animation to start literally within the body. The first visible effect of the transformation is steam, which then turns to smoke, both being computer-generated. The team rendered the skin burning away like paper, with burn holes growing all over the body, head and hands and revealing the bones underneath. Throughout the transformation, Cages image was re-projected onto the actors animated 3D replica. This allowed the team to retain the original live-action element as long as possible, and to progressively switch to 3D animation as the skin burned away.
During the transformation, Johnny Blaze touches his motorbike, igniting a similar transformation on the vehicle. To this purpose, the team built a CG replica of the two motorcycles: the regular version and the Hell version. The Hell cycle was later used in many shots in which both the Rider and his vehicle are entirely CG. This approach was employed whenever the character had to reach full speed or in specific environments, such as in the desert scene. This was one of the most complex sequences, Mack notes. We had the Ghost Rider and the Old West Ghost Rider, a blazing horseman, racing along in the desert at night with great swooping camera moves. The sequence was so ambitious that we couldnt even shoot any plate for it. It was entirely created in the computer: horseman, biker, environment and all. It was quite a challenge because we had this completely CG sequence that had to blend in the middle of a live-action film. Photorealism was thus of paramount importance. Using Maya, we built a 3D horseman that was key-framed throughout the shot by animation supervisor Marco Marenghi and his team. We didnt use any motion capture on this project at all. The animation turned out to be tricky because the character had to appear as if he was riding at 70 mph The far background, the mountains and the sky were created as a huge 360° matte painting. The closer desert environment was then modeled in 3D with plants, rocks, bushes and dry riverbeds.
Whenever the Ghost Rider left a long fiery trail behind him, this signature effect required a specific approach. As you increase the volume of the area in which the fire is being simulated, the complexity of the simulation goes up exponentially, Mack says. We couldnt create a simulation volume that was big enough to hold the whole trail. So, we ended up creating the trail in little sections that were then combined in compositing.
The CG fire was rendered in a proprietary render engine, while the other elements were handled in RenderMan. Most of the compositing was carried out in the proprietary compositing software package Bonsai. We had a variety of render quality settings - one appropriate for evaluating motion, another for final rendering, and many in between, Witting explains. As the memory and render times went up with increased quality, our main tool for keeping things manageable was sliced rendering: slices perpendicular to the camera direction were rendered on separate processors, then glued back together.
Besides the fire simulation, Sony Pictures Imageworks also had to create three complex characters: Wallow, a demon that forms out of water and dissolves into water at will; Abigor, the ethereal wind elemental demon; and Gressil, the demon that forms out of swirling bits of dirt. Each elemental demon involved building a 3D replica of the actor in Maya, and then using this body as a template to create the effects in Houdini. The template would be used to turn the character into water, dirt or wind, Mack says. For instance, the Wind demon looks solid, but when the Ghost Rider swings his chain at him, the weapon passes right through him, as if he was made of smoke. We first matchmoved our 3D demon to the actors motions. Then, we used the body to emit particles, and the particles picked up the color of the area that they came from. We made those particles maintain their coherence for a moment after they left the surface, which made for some cool effects in which you see part of the character drift away leaving the original body behind. For some flashes, we used Inferno to add a more demonic quality to his face: his mouth would become distorted, his eyes would change shape, his teeth would get bigger, etc.
Understandably, the team is quite proud of what has been achieved in terms of fire simulation. CG fire and fluids tend to look unreal on screen, but I think we broke it with this project, Mack concludes. Also, CG fire is generally used for daily life elements, such as a curtain on fire, a candle flame or a camp fire. For Ghost Rider, we actually created a tool that allowed us to do a lot more with it in terms of moving around quickly and realistically reacting to many different forces. We had complete control over its movement and color, all the while keeping it photoreal. I consider this our greatest achievement.
Alain Bielik is the founder and editor of renowned effects magazine S.F.X, published in France since 1991. He also contributes to various French publications and occasionally to Cinefex. In 2004, he organized a major special effects exhibition at the Musée International de la Miniature in Lyon, France.