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Step by Step:
Kingdom Of Heaven

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

Catching Fire

Modern visual effects techniques like CG sets, virtual extras, and digitally enhanced stunts are making it possible to mount period pieces on a scale that would have pleased Cecil B. DeMille. These techniques have enabled director Ridley Scott to finally film the 12th century Crusades saga, Kingdom of Heaven, for 20th Century Fox. “This is a pet project that Ridley has always wanted to make,” says Peter Chiang, visual effects supervisor for London-based effects studio Double Negative, which handled 145 shots for the film's visual effects supervisor Wesley Sewell.

Typical of the challenges in Kingdom of Heaven were the effects for a massive siege of a medieval fortress. In the heat of battle, the fortress' defenders pour hot oil on the attackers, some of whom catch fire and plummet to the ground far below. While actual stuntmen took the fall, it was Double Negative's job to make their immolation appear real. The fire, notes Chiang, “was a device to draw your attention to the action that Ridley wanted to highlight.”

Double Negative began with a basic Cineon file of the plate photography, explains Double Negative's 3D Head Rick Leary. “Then we took it into Baselight, which is software developed by FilmLight [London] which is basically a color grader. We then applied that converted Cineon file into the XR file format, which is an enhanced high-dynamic range format. We implemented that format from scanning all the way through final compositing.”

Next the Double Negative team established the speed of the shot in order to do a 3D matchmove and track in their 3D-CG fire simulation. For this particular shot, they had only some rough camera notes, so they used Science-D-Visions' 3D-Equalizer to track the movement of the camera as it followed the falling stuntmen. The 3D-Equalizer software, like most of the software used at Double Negative, runs on Pentium PCs with the Red Hat Linux OS.

Chiang's effects team then blocked out the main forms that the stuntmen were interacting with in the scene, including a tower and drawbridge. They then had to rotoscope the figures in 3D because the flames had to appear to come from the stuntmen's clothing and wrap around their bodies. For virtual stand-ins, Double Negative used generic 3D polygonal forms that could be re-proportioned to match each stuntman's body. Rotoscope expert Helena Masand “moved the figures into the perfect poses,” says Leary. “She did all the animations for the virtual stand-ins and all the tracking and proxy geometry on the shot.”

To make the fire a true 3D effect, says Leary, “We made a decision to do fluid simulation. We wanted to drop the figures in, do a fluid simulation pass, and from that we could then set them on fire. Instead of using a standard particle emission, which would be the old-fashioned way of doing it, with fluid simulation we actually gave it a temperature and an amount of fuel and dictated at what temperature that fuel caught fire. This is an approach that's been gaining momentum since Alias introduced Maya's fluid dynamics engine.

“We used our digital character to actually punch a black hole in the middle of the fire,” Leary continues. “As the 3D rotoscoped character falls through the simulation, the fire actually comes out as you would expect, and wraps around the character. It moves and dissipates as you would expect.

“There were stages to running this simulation,” says Leary. “The first stage was the movement — the way the fire itself emits from the skin of the object. We had about four iterations of the movement simulations. It took awhile to crunch those numbers, but once we had those, we could cache them and then concentrate on what the fire looked like. Getting the scale of flames and their correct turbulence is always difficult.”

Double Negative also used fluid simulation instead of particles to generate smoke. Likening it to a cloud tank effect, Leary says, “When you do an airborne effect like smoke, it's entirely appropriate to use fluid simulation.”

Once the movement and scale were correct, Double Negative faced the challenge of rendering. “The thicker the effect has to be, the more difficult it becomes to render,” says Leary. “The main problem with Maya fluid is actually rendering it for film. Maya has its own software renderer that's very adequate for broadcast work, but it didn't quite provide the details we needed for film.”

While Double Negative uses Pixar's RenderMan as its primary renderer, that couldn't solve this particular problem either. “RenderMan is still very much a surface renderer, and doesn't handle true volumes,” says Leary. “So we've developed our own renderer for fluid simulation called DNB.” Double Negative developed DNB to address the volume issues of smoke and steam in the upcoming feature Batman Begins. “Since then, we've built on it to address volume issues like this fire,” says Leary. “It relies on the same core technology.”

Double Negative used Apple's Shake to composite the fire and smoke simulation into the plate photography. The final shot was delivered as a digital file, ready to be dropped into the film's digital intermediate.

The final impact that Scott wanted the shot to have “was to increase the theme of danger in the plate,” says Chiang. “Ridley also used visual effects to make the continuity right for the edit that he had. All of his films make good use of technology. When you add to that the Ridley Scott eye, you've got amazing cinema.”

Credit Roll

Director	- Ridley Scott
DP	- John Mathieson
Visual Effects Supervisor	- Wesley Sewell
Visual Effects Producer For Double Negative Ltd.	- Victoria Alonso
Visual Effects Supervisor	- Peter Chiang
Visual Effects Producer	- Jennifer Silver
Visual Effects Art Director	- Mike Ellis
2D Digital Supervisor	- Adrian Banton
3D Digital Supervisor	- Rick Leary
3D Technical Director	- Helena Masand
2D Lead Compositor	- Tristan Myles

© 2008 Penton Media, Inc.