WORLD WAR Z - Crowd Source

Part 1 of Digital Media World'stwo-part featureon the VFX of 'World War Z'

MPC's first involvement began in early 2011. MPC’s VFX supervisor Jessica Norman carried out on-set supervision mainly at the Malta shoot location, which was to stand in for scenes in the story unfolding in Jerusalem in Israel.  This footage became the basis for some of the most violent, complex crowds seen in the film, in which frantic zombies pile themselves into massive pyramid mounds, tentacles and other shapes, overturn a bus and turn the streets to chaos.

From Form into Action

Following the rapid world-wide spread of an infection that transforms people into vicious, emaciated zombies with a voracious appetite for human beings, the story follows lead character, Gerry Lane, to Jerusalem where a ‘safe’ zone called Busland has been established within a concrete barrier wall. But shortly after he arrives, the zombies outside manage to breach the wall - and havoc breaks loose.

To help the team understand the scale, size and overall impact the zombie masses were to have within the live action plates, MPC received some concept work from the Art Department, mainly as sketches showing essentially what they did achieve in the end. In particular, the team achieved a very effective way of animating the individuals to collect themselves into the giant mounds and hanging tentacles.

However, much of the challenge came from translating the formation of those shapes into an action sequence. Insect references and schooling fish helped suggest appropriate movement, while animation supervisor Gabrielle Zucchelli and crowd lead Marco Carboni worked together on the animations required to form the zombie crowds.

Early in production they captured numerous action clips as a motion capture base for specific scenes. For the piles and mounds, actors would climb up nets, for example, and for the bus sequence they scrambled up and fell over a ramp. Their moves and actions were then used to populate the crowd, starting off by defining the final shape with geometry or a shot layout, always bearing in mind the speed the animations should have as well.

Zombie Tracking

‘World War Z’ was a two-and-a-half year project for MPC’s team, who produced 450 shots for the film. The facility’s ALICE crowd software has now been developed and used on many projects over several years and this time, again, various improvements were called for. For example, the extreme density of many of the crowd scenes needed attention. Normally, an appropriate level of avoidance between agents is built into the program, but in this case they needed to let the agents crush together, while still avoiding actually intersecting each other.

They also devised a means of identifying individuals in order to track certain ones more closely. By the end, they established their system to include 3,000 different agents, none of which were the same. In fact, Jessica recalled, as they got into the character development and build, they began to call some of them by name as they followed them through the shots.

To control the falling and landing zombies realistically, the crowd team used PAPI, MPC’s physic API, which is well-integrated in the pipeline so that PAPI and ALICE are bridged together. This allows them to create behaviours and animations in the crowd engine which could then also be used to drive the physics simulations at the same time.

Rigid Body & Crowd Simulations

“The crowd team could weight in and out of ragdoll behaviours, create partial ragdolls and entirely or partially driven animations to choreograph the hordes of zombies as they plough through Busland and the other scenes. PAPI is exposed as both a scriptable and node based physics system to the TDs, which they can use to drive all manner of effects. Although originally conceived for ‘Kingdom of Heaven’ many years ago, WWZ provided by far and away the most challenging combined rigid body and crowd simulations we have ever tackled,” Jessica said.

Prompting this number was the need to get a diverse system up and running well before they knew how many how many shots would feature the zombies or exactly how close they would be to camera. These agents weren’t used as hero characters but at any moment might need to fill the frame while in motion. Thus, modelling and texture artists in the asset team created 24 different body types, men women and children, varied with textures and multiple clothing options.

All characters were built from reference photography and scans gathered on set, mainly in Malta. Initially, they were all built as humans and from there, the zombies were created in three progressive stages of infection. Level One was for those that just had been bitten showing bite marks or small wounds, and Level Three were emaciated with missing hair, dark veins and large wounds.

Crowd Choreography

To accommodate the camera work, the characters were also built at four different levels of detail, hero standard for close ups through to those at a distance requiring only geometry. Intermediate levels were given a groom with dynamics and full cloth simulations. Where needed, some specific photography was shot for foreground action.

The CG supervisor Max Wood devised a way to work out the 3,000 variants using presets. This helped quickly decide what clothing combinations worked with which characters, for example. The wardrobe was designed so that any items of clothing could go together and each item would fit any body type. Clothing was also built to match the zombie levels - dirty and torn for level three and clean for humans. For the largest crowd, the team developed a bigger wardrobe, allowing them to change items on any character to suit individual shots. For the larger crowds an in-house cloth solver, integrated with ALICE, was used, while they could go to Maya nCloth for hero cloth simulations and finer detail.

Jessica said, “When the CG zombies needed to interact and fight with live action stunt actors, as the humans, their moves were choreographed in detail by the fantastic stunt team. MPC took measurements of all the performers for our match move and roto animation team. The animators used the roto animation to get close contacts and matching movement into our CG characters. This roto animation was also used in lighting and compositing to create shadow interaction in between the characters.”

Lighting Layers

Lighting the characters as they piled up in the bright desert sunlight presented special challenges. “Typically, crowd scenes can be divided up and lit in a logical progression from backgrounds to mid-ground to foreground and then each level rendered in separate layers. But these shots needed a new rendering approach because some characters would move rapidly from the back to close up to camera, or front to back. These characters had to be identified and the moves understood,” Jessica said.

“Otherwise it became very hard to sort out different layers within, say, a pyramid of zombies, and then work out how to render and release these layers. The larger pyramids were usually made up of about 5,000 agents. To be able to render these scenes, we split the crowd into many layers, a complex process because the agents are all interacting. Sometimes, zombies would start behind other zombies, and end up in front or fall on top of other zombies.

“With traditional holdout passes we would have had to re-render most layers and holdouts if we wanted to change one or a few zombies. The matting or cookie cutting would not have been correct, as the holdout pass only is relevant to the exact position all the zombies were at when it was rendered and assumes that they all are there.

Deep Passes

“With deep passes, you have the information about each agent’s position in space that you can use to matte different agents or objects against each other without having to re-render every layer or the holdout. During shot production we would often want to change the animation of one or a group of zombies, or maybe remove a few. Using deep passes created a more flexible workflow were we could change the animation on a few zombies without the heavy rendering of all passes. At times, we ended up pre-compositing all the layers, maintaining the various AOVs within, to produce one large, combined crowd.”

Jessica pointed out that the different departments at MPC always try to align their work closely together, but it was particularly important on WWZ. The team work and work in between departments was invaluable and helped piece all the different aspects of zombies and zombie crowds together – the crowd and animation teams working on behaviours, for example, and the lighting, crowd and compositing teams planning for the render.

The compositors in particular had final responsibility for making the bizarre, unnatural pile-ups of scrambling zombies look like real, existing parts of each scene under bright sunlight, so every stage of the work coming before that had to mesh and work together. The shoot included a fair amount of hand-held style moves as well, which could make the tracking - done with 3DEqualizer in most shots, Boujou for others - harder.

Busland Barrier

MPC’s team built the 70ft high wall surrounding Busland as a digital piece, securing the neighbourhood from zombie invasion and infection. The wall asset was represented on set by smaller, practical pieces positioned where they needed to attach visibly to the set. MPC extended this out and added scaled sections to the tops of buildings, a solid featureless concrete barrier made to look like a newly built structure.

The team also surrounded one of the main shoot locations in Malta with a digital environment comprising a margin of wasteland with a backdrop representing Jerusalem, seen from a distance. This Jerusalem environment was a 2.5D matte painting projection based on photography. The scenes shot at this location included large numbers of extras for the humans, which MPC developed as more typical crowd shots by filling in with denser, combined human and zombie crowds.

Runway Chaos

MPC has worked on many realistic airport and aircraft sequences but the scene at the airport out of Israel needed to express panic and chaos, not controlled take-offs and landings. Numerous types of planes all rushed to the runway at once, abandoning most safety procedures. “The scene was shot at a small airport in Malta. We added a terminal building using 2.5D matte paintings and also slightly changed the layout of the runways to work in the movie,” said Jessica. “Our environment team added a city in the distance with smoke and fire elements in the composite.

“The aircraft animation in these circumstances took extra coordination to ensure it look believable. In some shots, the main plane would be live action, in which case we changed its logo and removed the truck towing it. In the majority of shots we had to remove the live action plane to match the new runway layout but it was it was still good to have it there as perfect lighting reference. We also built a full hero Antonov plane based on scans and reference photography. In some shots it was live action, but in others it was full CG built in Maya, with Photoshop and Mari for textures.”

Crash Landing

Once the lead character Gerry Lane manages to get away in a passenger plane, he discovers that zombies have boarded with them and begun to attack the crew. As they advance on the passengers, he is forced to throw a grenade through the side of the plane, hoping the zombies will be swept out. Blasting the hole through the set was achieved as a practical effect by the special effects team and shot with several cameras to provide enough angles for the post work.

It was tricky for MPC and the camera crew to know in advance how people should be placed in this shot. In fact, most of the individuals seen right at the moment of explosion were digital, seen getting sucked out of the aircraft combined with a few stunt actors pulled out on wires. Green screen placed behind windows and, later, the hole itself was replaced with aerial footage from Israel during take-off, and then from Wales as the plane comes in for a crash landing resulting in a broken engine, wing and nose.

“The descending plane is CG through the full shot,” said Jessica. “It was important to create a crash that viewers would believe the story’s characters could survive, but would still be quite dramatic. We started off by blocking the animation and trying out different paths for the plane, pre-impact, and also various ways for it to come towards camera once it has impacted with the trees and ground. We looked at a lot of plane crash reference and studied the way the plane body reacts at impact.

“Once the plane animation blocking was approved we started looking at the trees in the plate and worked out which parts of the live action element that we would replace with CG trees and matte painted background. We added additional trees closer to camera than what originally was in the plate to help create depth in the shot and elements for the plane to interact with.”

Zombies Up Close

Just before rushing onto the plane, Gerry cuts off the young Israeli soldier Segen’s hand to prevent the spread of infection from a zombie’s bite. In the following shots on board, the actress wears a blue sock over her hand, which was painted out and replaced with a CG bandaged placed over the end of her shortened arm.

At the WHO installation we get a view of individual zombies at close range and moving quite slowly. Here, the performances were shot in-camera and the actors wore prosthetic make-up. MPC helped enhance their make-up and retimed a few of the moves to accentuate their strangeness, but Jessica said their looks and performances were excellent and essentially caught live. The team also added video inserts to the building’s many security monitors.ww.moving-picture.com