Many of the vehicles had to be replaced in CG, including most of the helicopters. In a sequence requiring three Black Hawks to land, it was filmed using Huey's, chosen because they had a similar opening to Black Hawks for the actors to get in and out of. The Hueys were later replaced with CG Black Hawks into the real atmosphere, behind the dust that had been kicked up. Unable to access the blue prints of Black Hawks, Dneg tracked down rough dimensions from the internet. The Black Hawks were modeled as they are in real life along with essential 4K textures painted from scratch. In addition lot of roto was required, keying off of the atmosphere, with additional vfx and lighting work.

For all the CG trees they used Houdini L systems, which provided a very useful layout tool. The L system trees were baked out as series of rib-archives with varying levels of dynamics to simulate gentle breeze and varying atmospheric conditions so that the animators and the layout artists could pick pre-animated trees and plop them into their Maya scenes and then using simple bounding boxes for placement. And at render-time, the curves became trunks, fronds and leaves using custom in house shaders, which created displaced surfaces from the curves.

All the smoke and atmospherics in the background were done using Maya fluids and Dneg's own in-house fluid renderer, squirt.

Another sequence takes place at the poolside of the Republican Palace, inside the Green Zone. These were filmed on location in Morocco. A broad brush matte-line runs around the pool, made up of a set build wall on one side, a set dressed changing block on another and a hedge on the other two, beyond which the team extended with a CG Republican Palace and CG trees, along with some sizable CG dressing to the foreground as well. The Moroccan pool had its deep end at the opposite end to the real one in Baghdad, which had a knock-on effect of having the diving board at the wrong end. However, when the sequence was cut together, no one was actually using it, so it was decided to move it to the other end by painting it out of the few shots it was in and adding a CG version at the other end. A CG fountain was added to the pool that was impractical to have for real and various bits of CG shrubbery to match as closely as possible to the real thing.

The set extension work was pretty atypical, according to Noble. "You expect to have everything in the foreground for the photography and your extensions starting from the [mid-level] background. But here they placed the photography in the middle of a 3D environment, which was very challenging."

Bill Desowitz is senior editor of AWN & VFXWorld.