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STEALTH – THE SFX WORKSHOP

VERTICAL CAR CHASE AND LIGHTNING STRIKES
Sydney based digital production company Animal Logic produced major visual effects for two key sequences in Stealth, the action film directed by Rob Cohen which opened last week (8/9/2005) around Australia. Animal Logic takes us behind the scenes in this fascinating SFX Workshop, to expose the making of two key sequences.

The story: A squadron of US Naval Air Force elite pilots has been trained to fly the Stealth combat plane. But the three ace fliers, Lt Ben Gannon (Josh Lucas), Kara Wade (Jessica Biel) and Henry Purcell (Jamie Foxx), under the command of Capt. George Cummings (Sam Shepard), are jerked out of training routine with the latest unmanned combat plane with AI, designed by eccentric genius Keith Orbit (Richard Roxburgh), when its system is damaged and it defies human orders, threatening the safety of civilian populations and invading foreign airspace in pursuit of attack objectives. The pilots have to neutralise the defiant machine before it precipitates a nuclear war. And they also have to deal with a rogue military commander …

Most of Stealth was filmed in and around Sydney. Animal Logic is based at Sydney’s Fox Studios and was already producing on-set visual screen displays for Stealth before being briefed on creating two key sequences of the film - ‘Kara Falls’ and ‘EDI Lightning Strike’.

KARA FALLS


When Rob Cohen spoke with Animal Logic Visual Effects Supervisor Chris Godfrey about the ‘Kara Falls’ sequence, Cohen had already approved sequence storyboards and was going to shoot live action footage in the US using two skydiving stunt people. Godfrey describes Cohen’s ambitious vision for the sequence as “one that had never been seen before”. On consultation with Godfrey, Cohen decided to expand the use of green
screen for the sequence so that they would have more control and versatility in the shoot and could get close and personal with the terror involved in falling from such a height.

In the scene, fighter pilot Kara Wade (Jessica Biel) is flying at 32,000 feet above North Korean territory and is forced to eject from her damaged Talon jet plane while upside down. The Talon spirals out of control, careens up, then rotates and starts to descend back down towards her. As Kara freefalls at 210 feet per second in her ejection seat, the Talon explodes blowing debris in all directions including around her and so begins the ‘Kara Falls’ sequence where she plummets towards earth in a shower of burning debris. As a visual effects sequence that enhances suspense, the challenge was to ensure that Kara
falls with the debris before deploying her parachute. If she deploys too early her parachute will be hit by burning debris. If she deploys too late, she will hit the ground at fatal speed.

“As Rob and I discussed this sequence, we both agreed that as a storytelling point, ‘Kara Falls’ can be likened to a car chase,” said Godfrey. “You can look back to any of the classic car chases and there is great similarity about how they’re shot. In the same way ‘Kara Falls’ is the same as a vertical car chase as the story telling points are similar. We used this analogy to plan and execute the action for this sequence,” said Godfrey.

A ‘Vertical Car Chase’

A key challenge for this sequence was creating drama and a sense of movement for Kara’s freefall. “We looked at the live action freefall footage that was shot in the US but it had little drama. It was stunt people freefalling in the middle of space. So Rob and I talked about creating the entire sequence in CG, shooting Kara in her ejection seat
before a green screen and adding everything else digitally,” said Godfrey. “Particularly important was the need for Kara to fall past something so we came up with the idea of a thunderhead storm cloud to create drama and give a sense of movement. Of course there were issues with that approach. We needed to build top clouds, the thunderhead
cloud, burning debris, trails, and a CG parachute which was scripted to be hit by debris and burn.”

Animal Logic’s Nathan Stone worked with Godfrey on set to work ‘blocking’ a 3D version of what the sequence would look like. Once Cohen had re-edited it, this pre-vis became the basis of executing a CG ‘Kara Falls’ sequence for the film.

Scenes depicting the jungles of Korea where Kara falls to were filmed in the Blue Mountains ranges, west of Sydney. Godfrey attended the shoot and shot Vista vision background plates from a helicopter and also took hundreds of 35mm stills that would later be used by Animal Logic artists to for the environments.

Then, on a Fox Studios sound stage, actor Jessica Biel was locked into an ejection seat and set on a specially designed gimble that had a two-axis rotation. Filmed in front of a green screen, the actor was violently thrown around to resemble the motion of freefalling at 210 feet per second.

Clouds, smoke and falling debris

The "Kara Falls" sequence had demanding cloud and smoke requirements as Kara falls through cloud banks and plummets past the face of a huge thunderhead storm cloud while being chased by burning debris from the exploded plane. The debris itself is billowing out trails of black smoke as it hurtles towards the ground.

A volumetric rendering system called ‘Steam’ which had only just been prototyped by Animal Logic’s R&D team became fully implemented to meet the requirements for Stealth.

VFX-R&D Supervisor, Chris Cooper, led a team of artists to create the CG clouds and atmosphere keeping in mind the brief from Godfrey that the CG elements were other ‘characters’ in the sequence. “Steam uses particle systems within the 3D software Maya to define where clouds and smoke are located and allows controlling parameters to be animated over time. These elements are then rendered volumetrically through Renderman using a custom ray matching shader. A technique for allowing fast motion blurring of volumetric renders within Renderman was an important component of the system, and helped us cope with the wild camera moves that were requested by the director and visual effects supervisor,” said Cooper.

The locations of cloud elements had been roughly blocked out in the pre-visualisation and early animation stages. Clouds were built on a per-shot basis by shot technical directors to permit flexibility in cloud placement, rather than trying to design a perfect cloud that would work in all shots. Continuity was achieved through sharing Steam
parameters between shots.

Many shots required smoke from the debris integrated with clouds so a technique was developed by Cooper and his team that allowed them to render smoke, cloud elements and intersection mattes separately. They could be later graded individually by the 2D compositing team and then re-combined.

Fine wispy atmospheric elements that we fall through or next to were added to most shots after the base cloud and smoke elements were in place. These elements helped enhance the sense of speed of the fall.

Cooper also wrote a procedural animation tool to simulate the effects of air resistance on the falling debris so they would cut across camera, slice or rotate randomly and add a level of variation and realism to the action. The Kara Falls sequence features 600 pieces of debris, of which 100 are burning.

A CG parachute and its demise…

As Kara falls to within 600 feet of the ground, she deploys her parachute. In order to realize the shot, Animal Logic needed to create a realistic CG parachute which was to appear in several shots being hit by burning metallic debris. This included one extreme close-up where the camera flies through a flaming, burning hole in the parachute.

For reference, the Stealth production team provided a real parachute to Animal Logic’s 3D team assigned to the task of creating the CG parachute. It was measured and photographed and a virtual garment was then made according to those measurements.

3D Technical Director Clinton Downs: “Initially we used Maya cloth but quickly found the dense high resolution model we were working with, around 27,000 vertices, revealed limitations within the cloth solver. The solution to this was a Maya plug in called Syflex. Syflex is a cloth simulator which proved great as it gave us very fast, accurate and stable simulations even when working with the dense parachute model.”

“As testing continued there were difficulties maintaining a sense of 'pressure' within the parachute, especially when the chute was subjected to large external forces. There was a tendency for the chute to 'deflate'. To combat this, Chris Cooper wrote a pressure force plug in within the Syflex programming interface which simulated the high pressure buildup of air within the parachute volume,” said Downs.

To help with the R&D for the burning of the chute a film shoot was arranged where a real parachute was set alight after being doused with accelerant. Using this footage as reference Cooper’s team proceeded to examine computer graphic techniques that would give them a similar look. “Some initial testing was done with the fluid dynamics flame system that is built into Maya. This was a good start but required even more control
and simulation speed. Early prototyping of a procedural system within Maya that used splines to define flames was done by Jeremy Pronk. I continued this development to allow the flames to propagate along the surface of the parachute, with controls for the shape, size and motion of the flames. The final images were rendered through Renderman as an implicit surface with a custom shader to allow full control over the look,” said Cooper.

For one shot where the camera was required to fly straight through the burning chute, a script was written within Maya by Downs which allowed an animated texture map to control the breaking up of the underlying cloth surface. This script took pre-torn strips of cloth, constrained them to the undamaged parachute simulation and then released the
pre-torn sections of cloth as the texture map spread across the chute surface. The end result is a flapping mess of rope and fabric which the CG fire has left in its wake.

Smoke which is emitted from the burning chute was created using the same Steam volumetric rendering system that was used for cloud rendering.

EDI LIGHTNING STRIKE


Animal Logic was briefed on creating another key sequence in Stealth - the ‘Lightning Strike’ through EDI's interior. Cohen wanted the audience to understand the Lightning Strike had caused a change inside EDI's functions.

Visual Effects Sequence Supervisor Deborah McNamara worked with Cohen to create a sequence that was coherent and purposeful in its narrative yet visceral in its action. "It was important to work with Rob and his editor to cut a sequence that held the rhythm and pace of the whole ‘lightning’ scenario," said McNamara. From the external shot of the EDI plane being hit, lightning sears through the cockpit down into Edi's interior central functions, showing "cause and efx" before exiting back to the exterior view of the plane.

A pre-visualisation sequence was created of the internal journey, with camera chasing lightning through various chamber environments of EDI and helped establish a final edit to work with.

The physical model of the EDI ball and its inner synapse core was digitally photographed as reference for 3D modeling, texturing and lighting design. 3D artists built full CGI chambers for the lightning to pass through as if it was entering a nuclear core reactor. On reaching the synapse core structure the lightning explodes into a macro view of EDI's sub-cellular DNA.

''Even though EDI is a computer, the change in him is bio-chemical. A.I. systems run on a script of codes like DNA. An injury or infection cause a mutant code to grow and regenerate. The change in EDI is synthetically biological, similar to the genetics of schizophrenia and brain injuries,” continued McNamara. "The DNA animation was to show the action of this ‘Frankenstein’ moment of mutation."

The DNA was modeled and animated in 3D by Daniel Murum. The mutation effects were created in Inferno, by Morgane Furio. Compositing the DNA into an utero-style environment with currents of electrical forces was a complex action completed by McNamara in Inferno.

One of the challenges of the whole sequence was creating a lightning force. "This was to be one camera rock 'n’ roll ride until we hit the DNA where the audience needs to digest what is happening," said McNamara. "The lightning lit the journey for the eye to follow. I wanted a more fluid force to the lightning for EDI's interior." To help achieve this, 3D Technical Director Larry Townshend developed in Maya a particle flow based on plasma currents rather than electrical sparks.

Even though the ‘EDI Lightning ‘sequence is short it needed a lot of elements and action to happen along the way, making it quite a complex CGI build. The exterior of the EDI ball and the interior synapse core needed to be reproduced accurately as CGI models as they also appear as a physical models in the film. Shaders and materials were built by Daniel Murum and Stefan Litterini.

"Rob Cohen is a dilettante for detail and exploring the background scientific phenomena that is always stranger than fiction. He has an incredible hi-octane approach to everything which really drives the adrenalin!" said McNamara.

We do not need to be physicists but we can understand how the lightning has affected EDI. It is an intelligent simple action that the audience can interpret straight away what has happened.

Published September 9, 2005

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