ILM builds a giant CG model and new methods for simulating water to help create the remake of Poseidon
When director Wolfgang Petersen wanted to film a 200-foot wave capsizing an 1100-foot cruise ship for the remake of the classic 1972 disaster movie Poseidon, his visual effects supervisor, Boyd Shermis, contacted Industrial Light & Magic. Petersen had previously worked with ILM on
The Perfect Storm, which called for a big wave as well, but for Warner Bros.’
Poseidon, he wanted something unique.
“Shermis said [Petersen] wanted to knock over the boat in a graphic way, this time using waves approaching the boat,” says Kim Libreri, visual effects supervisor at ILM. “He wanted to build the shot in a way that hasn’t been built before: He wanted a dynamic, destructive wave hitting the boat from many angles.”
The boat’s size meant that the ILM crew couldn’t use a miniature boat and real water; they had to create the shot digitally. “We like to shoot water at quarter scale,” explains Libreri. “It wasn’t practical when it’s meant to hit an 1100-foot-long object, so [Shermis] asked if it would be possible to do a digital boat and digital water.”
Images ©2006 Warner Bros.
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This isn’t the first time a visual effects studio has created digital water or a digital boat. But this time, the amount of water colliding with a large object and reacting in complex ways was especially massive. “We had a giant body of water around the boat,” says Mohen Leo, associate visual effects supervisor. “Near the end of the movie, we had to run all the ocean around the ship through the fluid simulator to move the water and foam around, and move all the debris in the water.”
That sequence, during which a 200-foot wave rolls the Poseidon, was one of three created by ILM, one of several studios working on the film. (The Moving Picture Company, for example, handled the water inside the ship.) ILM’s other two sequences centered on the boat: a long, opening shot in which the camera rises from underwater to follow actor Josh Lucas jogging around the deck, and nighttime shots of the luxury liner. In all, it took a crew of 80 visual effects artists at ILM, 12 of whom were CG supervisors, a year to create the cruise ship, the digital water, and the three sequences.
To build the digital ship, modelers worked from concept art and blueprints provided by the production unit for the required set pieces; ILM’s art director Wilson Tang refined the final ship design. The Poseidon measures 1106 feet from bow to stern, and 224 feet from hull to funnel. All told, modelers built 181,579 renderable pieces (see “Boat Builders,” pg. 34).
“The triangular face count in the basic ship is 1.3 million faces,” says Vince Toscano, CG set supervisor. “We used a reference library system that gave us a 10:1 savings. If we hadn’t, there would have been 11 million faces.”
When the camera draws close, you can see ashtrays, steam in the hot tub, cabin interiors, posters on the wall, martini glasses on tables, cameras, cabling, deck chairs, towels, light fixtures, and people walking on the deck-all computer-generated. “You can even see people watching TV inside their cabins,” says Libreri. “And at night, with over a thousand CG lights, it looks like a floating Las Vegas.”
The 1106-foot luxury liner and surrounding water are digital, created at ILM for an opening shot during which the camera follows actor Josh Lucas as he jogs around the deck.
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Modelers built the ship in Autodesk’s Maya, and set dressers assembled the luxury liner in ILM’s proprietary Zeno software system. Meanwhile, painters used Zeno and Adobe’s Photoshop to create texture-80 percent of the textures were painted, five percent were photographed, and the rest were procedural.
Toscano decided on a Lego approach to building the Poseidon, using such modular units as cabins, railings, decks, and interiors that snapped into place. To replicate a bridge interior set that included a library, a bar, and an exercise room, ILM projected photographs of that set onto geometry within the digital model. Inside the cabins, curtains appeared closed and open, set dressers varied the furniture, and CG people walked around. “I could look at the ship in profile and never see repetition, even with 200 cabins on the side of the ship,” says Toscano.
The team building the digital sets selected parts from an on-screen catalog, for example, placing a green cushion on one style of lounge chair and a red one on another, hanging paintings, and adding light fixtures. Towels folded in various ways had pre-set simulations to blow them wildly or slowly. “We had hot spots where pieces could snap onto each other,” says Toscano. Set-dressing kits varied depending on the shots. Once the set dressers finished snapping pieces onto the modules, they could use a second kit for parts, such as cabling, that extended across more than one module.
Artists could view modules on screen in low, medium, and high resolution, depending on how much of the behemoth boat they needed to work with, and could see the entire ship in proxy mode. Proxy mode approximated the model-a lounge chair, for example, would look like a bent square with arms, while a cabin window would be opaque with boxes inside that represented the interior.
Lighters working inside ILM’s proprietary Lux, a lighting module within the studio’s Zeno system, could assign materials and lights to the proxies that the real geometry would use for final renders; the artists could click on a proxy element and see it in full resolution. “Even though it’s referencing an archived piece for rendering, they can still turn it on and see what it looks like, drape something on it, or put a light on it for rendering,” says Toscano. “The model lets the artists open up an entire scene.”
In addition to level of detail, the ship builders and texture artists created variations for each piece of geometry that changed the look. To help with lighting, the basic surface changed depending on whether the shot was in daylight or at night. In addition, anticipating the water simulation, modelers optimized some pieces by capping parts that didn’t need to have water flowing inside. And, because the wave smashes the ship, the set dressers had a special kit filled with damaged pieces. “We took the original models and broke them,” says Toscano. “We had broken glass, broken chairs, bent arches, metal panels, and wooden floors that broke away and buckled. We stripped the boat, exposed the understructure, and ripped it up.”
The most intense shot of the ship appears in the beginning of the film and, at 4300 frames, it’s the biggest shot in the movie. “It starts out underwater,” Libreri says. “Sun streams through the ocean surface, and we see this massive structure move. The camera lifts out of the water and reveals the 1100-foot-long cruiser. The shot lasts for three minutes.”
Actor Josh Lucas was filmed at Sepulveda Dam near Los Angeles while running in front of a massive greenscreen. He’s the only live element in the shot, and a digital double replaces him half of the time as the camera follows his jog around the deck. During the journey, the camera zooms in close enough to see ashtrays and bubbles in the hot tub. “It took a year to get the shot together,” says Libreri. “We rendered it all with global illumination using raytracing in [Mental Images’] Mental Ray. I don’t think anyone has run global illumination to this level.”
Philippe Rebours set up the materials and the lighting method, and was CG supervisor for the daytime shot. “The boat was like a huge creature made of 400 parts,” he says. “It had to be completely realistic. There were tons of self-reflections-it’s made of painted metal. And, it’s so huge that it becomes the environment itself.”
Lighting conditions included daytime, underwater, and nighttime scenes. At night, 1000 CG lights illuminated the ship-cabin lights, deck lamps, and so forth. Rather than have the raytracer determine where the light bounced from all 1000 lights onboard the ship, Rebours’ team developed a system that, based on the intensity of the light, automatically defined the geometry illuminated by a particular light. When the boat was underwater, the lighters could override previously set parameters.
For the daytime shot, the lighters created materials, environment lights, and a key light that would work with lighting from the greenscreen shot. “We’d do a prepass to get all the indirect lighting,” says Rebours, “not just the ambient light coming from the world, but from the ship. For the nighttime shots, we needed to gather light coming from those thousand lights. They all needed indirect lighting-especially along the decks.” The lighters worked with one of 25 model sections at a time. When they achieved the look they wanted, they’d duplicate it for the next section and vary it slightly.
Rendering the daytime shot took three days using 300 twin CPU dual-core 64-bit machines-the equivalent of 1200 processors. “The whole shot takes 5tb,” says Pat Conran, digital production supervisor. “We needed 1.4
tb just to store the fluid simulation. It’s such a large boat; we had to see massive turbulence in the water.”
To create that turbulence, ILM used its Physbam simulation system. “[Shermis] asked us to take CG water and CG water simulations to the next level,” says Leo. “He wanted us to make sure our water simulations were at a level of detail and realism that hadn’t been seen before, so we spent a lot of time working with Ron Fedkiw at Stanford University and his Ph.D. students, and with Nick Rasmussen in our R&D department.”
ILM had previously used the Physbam Particle Level Set (PLS) system for computational fluid dynamics described by Fedkiw and others in many SIGGRAPH papers. The simulator had melted liquid chrome in Terminator 3: Rise of the Machines, helped a skeletal pirate drink a glass of wine in
Pirates of the Caribbean: The Curse of the Black Pearl, and poured water off a magical ship in
Harry Potter and the Goblet of Fire (see “A Draconian Test,” January 2006, pg. 26). Now, the studio needed something that would create all the key elements of dynamic water-the ocean surface, splashes, spray, foam, bubbles-from a unified, physically accurate, high-resolution fluid simulation. But, ILM had never used Physbam at a Poseidon scale. “In the past, for something of this scale, we’d typically fake it by sculpting a shape and using particle emissions on the crest,” says Libreri.
They faced one problem, however: Physbam had a big appetite for memory and processors. Simulations are difficult to control, and they’re sequential-that is, they start on frame one, go to frame two, and so forth to the end. What happens before dictates what happens next, and that adds up to oceans of production time. “When you’re simulating a volume, everything scales cubically,” says Conran. “We were scaling by a thousand; we had to find new methods of working.”
So, the Stanford and ILM teams solved the problem by splitting the simulation in a frame into multiple pieces that could run on different processors; that is, they parallelized the fluid solver code. “That gave us fast turnaround,” says Libreri. “The simulations became scalable, and we got higher detail. We were able to take two waves, collide them against each other, and as they interacted, the waves would break and particles would pour off in a simulated way. It was a real breakthrough. We started to see things we never thought we would see.”
A proprietary particle system made it possible for ILM to hand sculpt and choreograph a 200-foot wave of water that moved slowly toward the ship.
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The key was high resolution. At low resolutions, when the simulation grid was large, the movement looked like that of a viscous liquid-more like syrup than water. At high resolutions, the liquid approximated water, inside and on the surface. And, when the simulated water became the most turbulent, when it moved with more force than the high-resolution grid could handle, it ejected particles. Conveniently, the solver spit out those particles in places that matched those areas in a real ocean where the surface tension would break: the areas where water turned into droplets and air into bubbles.
“So, we added gravity and buoyancy to these particles, and used them to represent spray and bubbles,” says Leo. “By tracking where the particles hit the main water surface, we could define areas for foam particles, which were advected with the fluid.” In effect, underwater bubbles followed the churning water; when spray landed on the waves, it became foam, and the foam moved with the water surface.
“You automatically get a very good first take of cresting waves, foam on the surface interacting with the boat, and large events like a wave crashing down on the boat,” says Conran. “But, if you have a huge particle splash, there are a lot of dynamics going on that are different from surface foam. The foam is turbulent. It pulls apart and forms into cellular patterns.” Thus, Willi Geiger, who helped mastermind the particle-based fiery lava for Star Wars: Episode III-Revenge of the Sith (see “Dark and Stormy Knight,” June 2005, pg. 10), worked on perfecting the surface foam by feeding the removed particles from the fluid simulation into another simulation system in Zeno.
Most of the time, the simulation used “one-way coupling.” That is, the water moved rigid bodies like floating deck chairs, but the deck chairs didn’t affect the water’s movement. “We could have had two-way coupling, but it wasn’t necessary for the most part,” says Leo. In fact, in some shots, although the simulator moved the life rafts and debris around, to protect the director’s framing, the crew treated the boat as a hand-animated rigid object and splashed water against it.
Physbam couldn’t solve everything, though. The simulation engine was best at moving water surfaces and objects that displaced water. For white water caused by big splashes, the team used a new particle system in Zeno. “We upgraded the system to render more particles than before and to do smooth particle hydrodynamics,” says Leo.
ILM created the final composite (above) by (from image A through D): first, previsualizing the animation and camera; second, simulating the turbulent ocean around the ship; third, rendering particles for spray, foam and bubbles; and, last, rendering the sinking ship using raytracing and global illumination.
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And for the giant wave, the team started with a hand-sculpted wave and choreographed the particle system. “You couldn’t simulate a 200-foot wave that moves slowly and reaches threateningly for a ship,” explains Leo. “A real wave would break or collapse. But we used the fluid solver for the interaction around the boat.” By the end of the film, the crew was using the solver even for small elements. “It became our Plan A,” says Leo. “It was the most reliable way to create bow wakes and minor splashes.”
Indeed, the solver worked so well that the crew mimicked practical methods to control it by setting initial conditions and velocities and letting it run. They dumped “waves” onto the ship. They pulled a “plug” to make the water disappear. “In the early days, sims weren’t at the resolution we needed, so we had to cheat a lot of things,” says Libreri. “Now we can use all the same tricks that practical effects technicians use. It gave us such accurate simulations that when we dumped a million gallons of water into a tank, it wiped out the camera.”
Libreri, who was a visual effects supervisor on The Matrix trilogy, believes that simulation systems such as ILM’s Physbam and also the custom system used by Munich-based Scanline for various projects represent an important evolution in visual effects. “It’s something I’ve talked about for years,” he says. “Our industry is evolving from emulation to simulation. We’re beginning to mathematically model the real world, rather than cheat and pretend it looks right.”
“It’s not the easiest path,” Libreri adds. “It’s the hardest path. It’s stressful for the director and the producer, but they stuck by us. Wolfgang [Petersen] and Boyd [Shermis] really helped us push the state of the art forward.”
Barbara Robertson is an award-winning journalist and a contributing editor for Computer Graphics World. She can be reached at BarbaraRR@comcast.net.
681 lounge chairs
456 deck chairs
348 tables
45 umbrellas
32 lifeboats
31 life preservers
31 security cameras
44 first-aid boxes
413 signs with directions and warnings
20 newspapers and magazines
73 miscellaneous towels
37 bar glasses (most used glasses: juice glasses)
2 full bars
8 bar stools
Some Poseidon Stats:
Size: 1106 feet from bow to stern; 224 feet from hull funnel
Cabins: 382 on 6 floors, including 2 lofted penthouses
Portholes: 876
Cabin interiors: 14 unique furniture layouts in 220 lower cabins
Swimming pools: 3
Hot tubs: 2
Parts: 181,579 renderable pieces in the base ship model; 2117 archived Mental Ray files for the instantiation of the ship
Texture maps: 11
gb of mip-mapped textures
Computer Graphics World April, 2006
Author(s) :
Barbara Robertson