Attack dogs weren’t the only part of the animal kingdom that Cinesite tapped for reference for World War Z. The standout shot in the trailer (above) is that of thousands of zombies swarming over the walls of Jerusalem; clawing, climbing and tumbling until they pour our over the city. For this, the team studied ants and other creatures that move with a hive mentality, where each is autonomous but move in conjunction with a mass of others.

“The interesting thing about the way that they move is how they react as a crowd and their lack of a sense of personal space,” says Matt. “It would be virtually impossible to get a crowd of people to run like that because [the actors] would get completely freaked by it. By removing [the zombies'] humanity, you make them feel different – even if the audience can't put their finger on why they are different.”

Another sequences that demonstrates the film’s approach to zombies is when Brad Pitt’s character and his family get rescued by helicopter from the roof of a housing project in New Jersey. The most obvious example here is when the zombies chase Gerry onto the helicopter, which starts to fly away, and zombies continue to attempt to chase him – and end up running off the side of the building.

Footage for the rooftop escape sequence was shot on a greenscreen set, with the environments added in post.

Having no sense of self-preservation, the zombies chase the helicopter off the side of the building.

More subtle is a previous part of the sequence where the zombies are running up a metal staircase to get to the roof – as the zombie performers proved to be too careful when climbing for Matt’s liking.

“We found it very hard to get the stunt performers to run up the stairs, as they're always being mindful of where the next step is,” he says. “Whereas if you're a zombie, you don't care where the next step is. If you break your ankle, so what.”

Cinesite ended up using a mix of performers and CG zombies, with the performers largely obscured so their care couldn’t be seen. Careful choices were made in the editing and framing to ensure that any bumps taken by the zombies along the way didn’t come across as humorous.

One of the main issues Cinesite found with creating the scene that ends on a rooftop of a housing project in New Jersey is that there aren’t any projects in that state – “I know this,” says Matt, “as I went and looked for them.”

Instead, the sequence was shot on a small set surrounded by a greenscreen. Rather than build the rest entirely in CG, to get the level of authenticity Matt wanted, they built the geometry in 3D and then projection mapped elements captured from the real world at very high resolution onto that.

From these stills taken from raw footage on greenscreen and their final composites, you can see how much of the world Cinesite created.

Members of the team photographed rooftops up-close in New York from a cherry-picker, while longer shots were taken of New Jersey for a faithful backdrop to the scenes. Combined they make a coherent whole that isn’t truly any one place, but feels true even if you know the place being pictured.

“We created a world that doesn't exist, informed by the projects in Harlem – as aesthetically I thought they were cool,” says Matt. “In the mid-ground is Lower East Side and then in the background you've got Jersey.”

To apply these to the scene, Cinesite used the 3D tools of Nuke in what Matt describes as “a really cool way”. As well as projecting the photos as textures onto the 3D geometry of the scene, the team ensured that all of the technical data from the set – camera position and types, real-world lens data, aerial perspective and much more – was brought to make the final result as accurate as possible.