Once the ship was lit and textured, it was populated by about 100 digital passengers. Individual animations were derived from a motion capture session done at House of Moves. In the challenging opening shot, the trickiest aspect was the integration of Lucas in the scene, both as a digital double in wide angles, and as a live-action element in close-ups. The shot was first previsualized in 3D, and the camera move exported into a massive 400-foot square Cablecam system that was used to shoot Lucas.

ILM used a new fluid dynamics engine developed in cooperation with Stanford University to create the ocean, including wave, turbulence and bubbles.

Multi-processor Fluid Simulations
As for the ocean, ILM used a new fluid dynamics engine that had been developed in cooperation with Stanford University. The Physbam simulation system had first been used for the chrome T-X in Terminator 3, and then for the ship sequence in Harry Potter and the Goblet of Fire. On Poseidon, ILM was able to employ a new generation of Physbam. Cooperating on the project were Stanford’s associate professor Ron Fedkiw and ILM’s senior R&D engineer Nick Rasmussen. The ocean was entirely created with this fluid dynamics engine, including wave, turbulence and bubbles. “We used a hybrid approach to simulate fluids,” says associate visual effects supervisor Mohen Leo, who oversaw the water simulation and rendering on the movie. “The volume of the fluid is simulated on a regular three-dimensional grid. In addition, particles surrounding the surface of the water are advected with the fluid flow and help improve mass conservation. In highly dynamic areas where the resolution of the grid can’t resolve the surface anymore, these particles are removed from the fluid simulation. When a simulation is run at a sufficient resolution, the removed particles are ejected in areas that visually match areas where, in reality, the water’s surface tension breaks, so they can be used to represent spray and bubbles.”

The main difference with the Goblet of Fire version of the software was that it was now possible to run multi-processor simulations on eight or 16 processors. This dramatically improved turn-around and allowed the team to work at much higher resolutions. “At low resolutions, the fluid motion appeared very viscous,” Leo notes. “Only at fairly high resolutions did the fluid begin to resemble water. Unfortunately, high resolutions meant longer simulation time and higher memory demands. Due to the change in viscosity based on resolution, low-res simulations were only of limited use for faster tests. Until Stanford implemented multi-processor simulations, we struggled with turn-around (simulations could take four or five days to finish) as well as memory limitations. With the new distributed calculation, each processor only deals with a section of the full grid, which means that memory requirements decrease as well.”

Full integration with Zeno’s particle solver made it possible to set up a semi-automated system — designed largely by CG supervisor Willi Geiger — in which the artist could create water surface, spray, foam, bubbles and floating debris, all based on one core fluid simulation.

Digital artists had to solve how to visualize the difference between a 50-foot and 200-foot wave. For shots where the whole wave is visible, ILM took a “traditional” approach by sculpting geometry to represent the main body of the wave.

Taming a 200-foot CG Wave
One of the key aspects of the capsize sequence was how to make the difference between a 50-foot wave and a 200-foot wave on screen. Initially, ILM’s team tried to run full simulations for the 200-foot wave approaching the ship, but this soon proved to be impractical. Shermis prevised the sequence of shots leading up to the impact of the wave, with a strong focus on dramatic effect, not realism. Thus, many of the shots required the wave to have a shape and motion that defied physics… Since the fluid solver was designed to create physically accurate behavior, forcing it to match a somehow “unrealistic” previs turned out to be too difficult. “For these shots where the whole wave is visible, we employed a more ‘traditional’ approach by sculpting geometry to represent the main body of the wave,” Leo remarks. “Fluid simulations were only run for the areas where this main body had to interact with the ship. On the other hand, for the final sinking of the ship, the water was not required to exactly match predesigned motion, but only to react to the ship motion in a realistic manner. So, many of these shots were done using full simulations for all the water surrounding the ship.”

The capsize shots required dozens of render passes, all quite complex: full ray-tracing and global illumination on the ship, multiple scattering on tens of millions of particles, etc. In order to maximize flexibility in the composite, the renders were broken up into groups of lights (moonlight, different groups of ship lights, ambient light, etc.), as well as different aspects of lighting (diffuse, specular, reflection, global illumination, etc.). This increased the number of render passes, but ultimately allowed compositors to do the final balancing of the lighting.

CG supervisors Henry Preston, Lindy De Quattro, Joakim Arnesson and Kevin Sprout oversaw other aspects of the sequence. Finally, the shots were put together in Shake with compositing supervisors Patrick Brennan and Mark Hopkins overseeing the effort.