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Step by Step:
Stealth

Jul 1, 2005 12:00 PM, By Ellen Wolff

A Triumph of Terrain

Aerial acrobatics are essential in Columbia Pictures' Stealth, and director Rob Cohen pulled out all the stops. The man behind XXX wanted the freedom to follow the film's fighter pilots throughout daredevil moves, and he wanted the land below to be visible and convincing. As Digital Domain's Oscar-winning visual effects supervisor Joel Hynek notes, “Rob's commandment was clarity. He'd seen a lot of flying movies where they had footage of a guy in a cockpit on a runway with just the sky outside, and then they'd cut to telephoto shots of airplanes flying around. Rob wanted to free up the camera to give you the feeling of flying.”

Achieving this effect required generating huge tracts of photorealistic landscapes that would be visible from the planes. For that DD used Terragen, a procedural terrain-generating program developed in-house. That terrain also had to be reflected on CG planes and integrated with cloudscapes created with DD's volumetric modeling and rendering program, Storm. One 600-frame “proscenium arch” shot took this strategy to the limit.

It set the scene for a bombing mission of a fort in Tajikistan. Hynek, a pilot himself, explains, “Missiles are on the way down as the planes come in overhead and swoop down to the fort. We watch them shoot at the fort and then as one plane does a 360 we zoom in on it. As it's coming around we go right into the cockpit in a nice smooth hookup. Then we go out of the cockpit as the pilot completes the 360 and aligns himself to shoot again at the fort. We're just about a thousand feet above the ground, so this shot is a triumph of terrain.”

DD began the process with a previz created in Alias' Maya. That previz was then brought onto the greenscreen stage to drive the camera during photography of the pilot in a gimbal-mounted cockpit. Visual effects Oscar-winner John Frazier built a gigantic gimbal that had the whole front of a plane mounted on it. A Spydercam was hooked up to work in coordination with the gimbal, and a Kuper motion control rig drove the Spydercam and mimicked the camera movement from the previz.

Hynek's team also worked out a lighting scheme with DP Dean Semler. “We tried to stick to that as much as possible when shooting the greenscreen. That became our lighting direction. Dean took stills with a digital camera, and we used them later on to match his lighting.”

Back at DD, the previz also drove, to some degree, the animation of the plane. While some modifications were required to link up the greenscreen footage with the CG plane, Hynek says, “We would project footage onto our animated model. That was useful to making our greenscreen cockpits fit our CG plane. We could either use the whole nose of the plane as photographed, or we could have only the interior cockpit be photographic. All the canopies were computer-generated, which is an absolute must if you're trying to make a plane reflect an environment. We generated HDRIs [high dynamic range images] for reflections on these surfaces. Sometimes they needed to be very hi-res and very accurate.”

Designing the terrain involved a combination of Maya models and digital elevation maps taken from satellites of actual mountains. According to Hynek, Terragen can take input data from a wide variety of places. DD's Terragen team, headed by Brad Herman, set up what Hynek calls “a zillion terrain rules, like where there should be vegetation and water and snow.” He continues, “The program itself has 1,100 nodes and some of them are quite complex.”

In what could be described as a geological simulation, Terragen generates a picture that, Hynek says, “looks remarkably like a place on earth. We had to tweak it somewhat, mainly in the scale of some of the textures. In this shot we're wide at first and then very tight, so everything has to shift resolution a lot. Fortunately, Terragen has an adaptive level of detail, so it's always figuring out how much detail it has to present based on your final resolution. This particular one was based on a 20km by 20km digital elevation map. You can work with different resolutions of terrain, but in a 2k image there are about 11 million triangles in there. Terragen figures out how many of those triangles are significant, and what resolution a certain part of the image needs to be.”

Terragen also automatically mapped in rivers and roads wherever the artists wanted them, explains Hynek. “You simply draw a line between point A and point B, and it figures out how the road should go.” Trees, however, were created separately in SideFX's Houdini and added later.

While changes could be made and the terrain re-rendered (using Pixar's Renderman), Hynek admits, “Terragen renders aren't cheap. But they're not prohibitively expensive. When we started the project it took 10 hours a frame, but by the time we were done, it was down to an hour a frame for the complex stuff.”

Because Terragen also generates z-depth maps, Hynek's team was able to readily drop in clouds generated with DD's Storm. They used the studio's proprietary Nuke compositor to bring all the elements together. The compositors then dialed in the degree of motion blur that Cohen wanted to get the desired amount of sharpness. “We gave Nuke just the vectors for motion blur and then applied that to the already rendered Terragen terrain.”

The results were so realistic-looking, recalls Hynek, “I'd think, ‘Is that CG or is it aerial footage?’ I asked our Terragen lead Brad Herman if he felt like God, and he said, ‘Well, God made the world in six days, but we don't have his render farm!’”

Credit Roll

Director -	Rob Cohen
DP -	Dean Semler
Special Effects -	John Frazier
For Digital Domain:
Visual Effects Supervisor -	Joel Hynek
Digital Effects Supervisor -	Kelly Port
CG Supervisors -	Markus Kurtz, Vernon R. Wilbert Jr.
Compositing Supervisor -	Bryan Grill
Effects Animation Lead -	Garman Herigstad
Animation Lead -	Erik Gamache
Terrain Lead -	Brad Herman
Modeling Lead -	Andrew Wilkoff
Lighting Lead -	Esdras Varagnolo
Technical Integration Lead -	Swen Gillberg
Compositing Lead -	Lou Pecora
Senior Compositor -	Michael Maloney

© 2008 Penton Media, Inc.