"The Wall of Fire required a lot of highly-complex fluid simulations, all created within Maya. We opted not to use practical fire elements, as we felt the scale of the event was so large that no real element could match it. The fluid simulation was built in multiple layers, but it was all driven by the main simulation, and then rendered together. In order to generate correct interactions with the buildings, we had to rebuild sections of Manhattan in the computer. So, I went to New York and shot aerial footage of the city. Then, we modeled the geometry of the buildings and projected the footage onto them."

Around Christmas time, a mere month before deadline, production requested a new series of shots for this sequence and others, which stretched Animal Logic's capacities to the limits. "Initially, the intention was that we would do the whole movie," Jackson observes. "But towards the end of the project, the scope of work grew considerably and got close to 400 shots. We felt more comfortable assigning those new shots to other vendors. In the end, we produced 247 shots for the movie."

Joining production to help guide the additional visual effects effort was Eric Durst, who also served as overall visual effects supervisor. "As in many films with extremely tight deadlines and a very large cinematic vision, the work often requires additional people at the end," Durst suggests. "In this case, enlarging the crew, including expanding the teams at Animal Logic, really helped preserve the high degree of detail and care that this project required. Andrew had been on since the beginning and was fully engaged in scenes such as the subway collision and the plane crash, among many others. My role was to help by supervising additional shots and scenes that required 100% attention."

Additional vendors included Postmodern Sydney and Haiku Post, which mainly focused on straight greenscreen composites and fix-up shots, and Buf Compagnie, which contributed five dramatic shots to the Wall of Fire sequence.

Fire and smoke wasn't just limited to the Wall of Fire sequence. Fluid work was also needed for other elements of the film as well. Images courtesy of Animal Logic. 

The Paris-based team was lead by VFX Supervisor Antoine Deschamps, VFX Producer Pierre Escande and R&D Supervisor Xavier Bec. "We had to create the wall of fire itself, as well as all the destruction in its path, and the fiery sky," Deschamps explains. "The fire had to look like a wave that would flow in the streets while buildings would explode and collapse. We opted for a full fluid simulation for both the fire and the smoke. Our R&D team also developed a new shader to handle explosions and smoke. In order to get a more organic feel, we added a lot of practical fire elements to the very front part of the wave."

The sequence necessitated the creation of up to 600 buildings for the really wide shots, which led the team to procedurally animate as many elements as possible. Low-resolution buildings were rigged to collapse via rigid-body dynamics, while high-resolution buildings, and their complete inner structure, were destroyed via matching animation. All the work was carried out using proprietary tools. The simulation was divided into various modules, some representing one hero building, others a whole city block. Yet, the shots were so complex that rendering still took up to 10 hours per frame.

"For closer shots on Time Square and Grand Central Station, we recreated the location using photogrammetry techniques," Deschamps says. "We then painted out all the pedestrians, the vehicles, and changed the billboards. For the aerial shots, we used helicopter footage and modeled the corresponding buildings. Every time a building would collapse, we had to reconstruct the building that was revealed behind it. Also, we completely relit the city by adding fiery reflections on the windows and on facades. It made the scene look much more realistic. For the space shot, we used texture elements that had been created at Animal Logic, and we added our own simulation."

In the end, Durst believes that one of the very interesting aspects of the project was doing the digital intermediate at Park Road Post in Wellington, New Zealand. "I was very impressed with the work of David Hollingsworth and his team, and the way he color timed the RED camera footage, building extraordinary cinematic and filmic feel to the movie. I felt it played an important role in the final result."

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, both print and online, and occasionally to Cinefex. In 2004, he organized a major special effects exhibition at the Musée International de la Miniature in Lyon, France.