Green Zone Step by Step

When Director Paul Greengrass took on Universal Pictures' Green Zone, a major challenge was capturing the look of wartime Iraq without being able to film there. Iconic landmarks such as Baghdad's Hands of Victory arches, with their giant crossed swords, needed to be included, so CG versions had to be merged convincingly with live action. To meet this challenge, Greengrass tapped the visual effects crew at London's Double Negative (DNeg), which he'd collaborated with on The Bourne Ultimatum.

An archetypal example of DNeg's work was a sequence in which Green Zone star Matt Damon drives a Humvee past the crossed swords, as well as a variety of military hardware, rubble, and extras. "We started the shot panning from some journalists having their photo taken and panned with the Humvee as it drives past an armored vehicle in the foreground," says DNeg Visual Effects Supervisor Charlie Noble. "We ended up looking down the line with the crossed swords. But when we photographed this in Morocco, all we had was the Humvee and the journalists and a large expanse of tarmac. That was it."

But the DNeg crew wasn't going to wait until postproduction to tackle the CG architecture and military hardware models that they'd need to add to the shot. They used online visual references (such as Google Earth) to build rough-shaded models in Autodesk Maya and brought the models on location so that Greengrass could compose the shot with the CG in mind. "We took along a virtual overlay system," Noble says. "We had an HD camera positioned at a good vantage point on set, and we brought in the relevant CG models which had been built to a good previz level. After the camera was set up, we'd snap our CG models to the location so that Paul could pan our high-res camera around the set and see on a monitor what we'd be adding to the scene. That was a really useful lineup reference for the camera crew.

"We also had a crane on set, which we set so that it lined up with where our CG arches would be," Noble says. The crane, which would later be rotoscoped out, provided a placeholder to ensure that the space required for the CG arches wouldn't be cut off.

This overlay system was particularly crucial in a shot where the live-action photography needed to be placed in the mid-ground with foreground and background CG elements around it. "We planned to later put in a huge CG [Bradley armored vehicle] in the extreme foreground," Noble says. "The [vehicle] needed to have a driver with his head poking out of the turret. So when we shot the plate we brought in an appropriately sized blue box for the guy to stand on so he'd be at the correct height. It looked a little funny during shooting, but it's good to get as much in the plate as you can."

In times past, such a shot might have been done with the Humvee filmed against a greenscreen, but not this time. "Paul likes to let an action play out, with three or four cameras shooting the scene, racing around 360 degrees," Noble says. "We'd have needed 2 miles of greenscreen a half-mile high, and they'd be casting shadows. It just wouldn't have worked."

Since Noble previously had experienced Greengrass' dynamic shooting style, his team came prepared to capture as much camera information as possible on set. "After our experiences on The Bourne Ultimatum, we were fairly prepared for the energetic camerawork and the constant zooming and reframing that is Paul's style," he says.

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To make things easier for DNeg's matchmovers, who would have to track the camerawork, Noble's on-set crew had zoom encoders attached to each camera. "We'd built some encoders with little toothed wheels that basically locked into the focal length ring on a camera," he says. "There's a little wire that came from each one, which attached to laptops that we carried. We had a guy with a zoom encoder following each camera around. Whenever the camera operator reframed and the zoom wheel moved, our little wheels turned and sent that turning information to our laptops, which recorded it.

"We also had a guy with a survey tool standing off to one side of the set. He surveyed every point in the scene he thought would be useful for matchmoving, and each time a camera moved he would zap that position. That provided rough positions for the cameras in the scene and a starting point for the matchmovers."

Back in London, DNeg's matchmovers used an inhouse tool called Photo Fit to reconstruct the scene. "The matchmoves had to be rock-solid because so much CG had to go not only in the background but also close to camera," Noble says. "We got the photography and then tracked 2D points on the images. Then we snapped those points to the corresponding points on the survey data and the focal length data. It made the process much more efficient."

DNeg's rotoscoping team used an inhouse tool called Noodle to remove the stand-in crane so the CG arch of crossed swords could be placed into the scene. The scene was also filled out by the addition of the Maya-generated armored vehicle and CG and matte-painted rubble. Noble recalls shooting some practical rubble with digital still cameras, but he says, "If rubble had to be of a type that never existed as set dressing, then we'd build CG rubble."

To add atmospherics to the scene, DNeg simulated groves of swaying palm trees using the L-systems capabilities in Side Effects Software Houdini. These preanimated trees were then dropped into the scene. DNeg also animated dust and smoke using Maya fluids and DNeg's own inhouse tool Squirt. The smoke was rendered with DNeg's volumetric renderer, DNB, while other CG elements in the scene were rendered with Pixar RenderMan.

To finish the scene by giving it a sense of life in the background, DNeg's compositors added some 2D images of soldiers that had been photographed against bluescreen. Distant buildings were also created as matte paintings in Adobe Photoshop, and all these elements were composited into the scene with DNeg's custom version of Shake.

When it all came together, Noble was pleased that the visuals passed muster with soldiers who'd actually been in Iraq—some of whom appeared in the film. It was a relief, given that there wasn't a practical alternative to creating Baghdad in CG. "It was great that it rang true for them," he says. "Building the Green Zone would have even been beyond the budget of Cecil B. DeMille!"

Director: Paul Greengrass

DP: Barry Ackroyd

For Double Negative:

Visual Effects Supervisors: Peter Chiang, Charlie Noble

Compositing and Matte painting: Graham Page

Lighting and CG Layout: Shane Aherne, Antoine Moulineau

Rotoscoping: Adam Hammond

Matchmoving: Daniel Baldwin, Eugene Lipkin, Tim Catchpole

CG Animation and Lighting: Will Correia

Armored Vehicle Modeling and Texturing: Tom Griffiths

Armored Vehicle Lookdev: Bruno Baron

Archway Modeling and Texturing: Julian Foddy

Archway Lookdev: Julian Foddy, Romain Arnoux