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Step by Step:
Poseidon

Jun 1, 2006 12:02 PM, By Ellen Wolff

The Next Wave in Simulation

Movie fans have known the Poseidon story since the 1972 original, but director Wolfgang Petersen brought some ambitious ideas to this tale for the new Warner Bros. version. This time, tank work would be augmented by digital effects from an armada of studios, but Visual Effects Supervisor Boyd Shermis chose Industrial Light and Magic (ILM) for hero shots of the 1,200ft. ship being swamped by a wave 200ft. high.

“There was no way to shoot a miniature big enough to capture that event,” says Mohen Leo, ILM's associate visual effects supervisor. “Boyd Shermis was convinced that miniature water always betrays itself as a miniature. So we were faced with the challenge to do the vast majority of the ship and the water completely in CG.”

The first step was previsualization, handled with Autodesk Maya and ILM's proprietary software Zeno. This process of choreographing the Poseidon's demise demonstrated that advanced water simulation would definitely be required.

ILM also had to develop new approaches to handling the huge ship model, which required 180,000 pieces of geometry. “The only way we could do both the level of detail of the ship and the water was because we switched our pipeline to 64 bit,” says Leo. “We had to add more processors, more disk space, and more memory to deal with the amount of data that Poseidon required.”

Modeled primarily in Maya, the ship contained 6,500 set pieces, including lifeboats and cables that gave way when the ship rolled over. Fortunately, ILM's layout experts developed a system that enabled them to build the ship in a modular fashion. Then at render time, those “lightweight” models could be switched out with the full model.

But the biggest challenge was simulating the complex water effects. Leo credits the close collaboration between ILM and Stanford University, where Professor Rod Fedkiw's team developed a fluid dynamics solver, with achieving this task. “It's a whole integrated system that allows us to simulate, among several things, smoke and water,” explains Leo. “It's only been used on a handful of shots before. I think Poseidon is the first to use it as a centerpiece of production. It's a computationally costly approach — the simulations can take an extremely long time. Because of the nature of the system, at lower resolutions, the liquids behave in a way that's viscous. To get something that looked like water, we had to run really high-resolution simulations. But one of the big advantages over previous approaches was that this system not only got us the water surface and spray, but also underwater bubbles and foam and debris floating on the surface. Stanford actually had two people running simulations here at ILM for months.”

Poseidon represents a major step beyond ILM's water work for Petersen's 2000 release, The Perfect Storm. “3D fluid simulations were only used then in a limited fashion,” Leo, who also worked on that film, recalls. “The main body of the water was generally a displaced patch with some particles on top. That movie had a small ship that caused minor splashes that could be done with particles. But with a 1,200ft. ocean liner, we had massive displacement of churning water and big splashes against the ship. The water is really a character here — it's the major source of threat.”

Yet, even the most time-intensive simulation is often something a director wants to tweak. Leo admits, “That's been challenging. A single simulation for a shot could take four days to run at a reasonable resolution. Then production would look at it and say, ‘It's not quite right.’” So re-rendering would be required. To help expedite the process, Leo explains, “The Stanford people managed to make their fluid software run on multiple processors in parallel. That allowed us to run some high-res simulations overnight and turn things around faster.” The water simulations in Poseidon also benefited from a new particle simulation module in ILM's Zeno software. It allowed the effects team to simulate collisions of 20 million particles — in just seconds per frame.

“On top of that, there's still a lot of artistry involved,” Leo says. “There were parts of the movie where the director wanted the water to behave in certain ways for dramatic effect. So we'd employ different techniques to get the right shape and motion. With flowing things like cloth and water, you can't force them too much. If you push water really hard, it doesn't behave like water anymore.”

Some of the artistic touches involved painting textures in Adobe Photoshop, along with some flashes of underwater explosions. The art department created those using Avid's Softimage XSI, and then the effects team merged them with the other elements.

At render time, ILM employed Pixar's RenderMan for the water and particulates like spray and bubbles. Since the cruise ship is full of colorful lights, the film's supervisor wanted to see those lights scattering through the sprays of water. “It looks like the sprays are glowing inside,” Leo says.

The Poseidon itself was rendered largely in Mental Images' Mental Ray software, with full raytracing to get accurate reflections and refractions. “We ran a global illumination pass so that we got proper bounce light from one surface to another,” Leo says. “That added a lot of complexity to the lighting and helped create scale.” The lighting passes were broken out as separate renders so the compositors could balance them during compositing, which was done in Apple Shake.

Leo says, “[The final result was a look that] would have been too time-consuming and expensive to achieve just a few years ago. But trying to do things that haven't been done before is really where the fun is in visual effects.”

Credit Roll

Director	-	Wolfgang Petersen
Visual Effects Supervisor	-	Boyd Shermis
For ILM:
Visual Effects Supervisor	-	Kim Libreri
Associate Visual Effects Supervisor	-	Mohen Leo
Digital Production Supervisor	-	Pat Conran
CG Supervisor	-	Willi Geiger
Layout Supervisor/Previz	-	Colin Benoit
Color/Lighting Lead	-	Phillippe Rebours
Asset Layout Tool	-	Vince Toscano
Water Simulations	-	Lee Uren, Frank Losasso, Andy Selle
Splash Development	-	Joakim Arnesson, Dan Pearson
Water Rendering	-	Dan Pearson, Dan Piponi
Software Support	-	Nick Rasmussmen

© 2008 Penton Media, Inc.