By Karen Moltenbrey

Over the years, Weta Digital has made a name for itself, creating vast imaginative worlds for highly acclaimed feature film franchises such as The Lord of the Rings and The Hobbit. However, for the recently released Mortal Engines, not only did the studio have to construct wide swaths of land the size of countries, but the crew also had to build supercities that move at head-spinning speed.

Mortal Engines, produced by Universal Pictures and MRC, takes place centuries after a cataclysmic event known as the Sixty Minute War destroys civilization as we know it, leaving behind few resources. Eventually, survivors learn to adapt, and a deadly, mobile society emerges whereby gigantic moving cities roam the earth, preying on smaller towns they hunt down across a landscape called the Great Hunting Ground, basically the size of Europe. It is now a period of pre-revival, as the earth begins to renew itself, and the survivors become nomads on wheels.

Eventually, London, a traction city, emerges at the top of this vicious food chain, consuming resources from other cities and towns it devours, including fuel, food and human labor. It’s a dog-eat-dog world. But there are those who want to end this vicious cycle; they are members of the Anti-Traction League, who advocate for static, self-sustaining homelands.

Based on a book by Philip Reeve, the film is directed by Oscar-winning visual effects artist Christian Rivers (King Kong). Simon Raby (Elysium, District 9) served as cinematographer, while Weta created the visual effects, led by Ken McGaugh, Kevin Andrew Smith and Luke Millar, with Dennis Yoo as animation supervisor.

Ken McGaugh

A New World Order
In all, Weta delivered 1,682 VFX shots for the feature film, most of which pertained to the environments.

How did this work compare to some of Weta’s other world builds? “I can’t speak as to The Hobbit because I didn’t work on that. But on The Lord of the Rings, New Zealand’s landscape was used for Middle-earth, so there was a lot of location work, and most of the world building was all in camera,” says McGaugh. “On Mortal Engines, because earth has been destroyed and manipulated by these giant cities moving over it, there’s nothing left that resembles the earth that we know. So, there was no location for us to shoot; we had to build it from scratch.”

How does one go about building such a world — and then setting it in motion? “In a book there is a lot of metaphor, but film has to be fully literal,” says Rivers. Fortunately, he and the crew had the vast experience as well as the technological genius to get it done.

Such a goal, however, required new rules and workflows, even for a veteran studio like Weta, which has a history of breaking new ground, especially when it comes to animated characters and amazing landscapes. Here, those diverse elements would converge like never before.

“We have quite a bit of experience doing computer-generated vehicles as well as digital environments, but most of our workflows assume that an environment is not a vehicle, that it doesn’t move. So trying to bridge that gap was a challenge. We had nothing that would allow us to do that until we first started,” says McGaugh. “So, we had to invent some new workflows and technology internally to allow us to bridge that gap so the animators could animate the city as if it were a vehicle, but we could build the city and dress it as if it were an environment.”

The Land
The environments in Mortal Engines are CG — built and animated using Autodesk’s Maya and composited in Foundry’s Nuke — with practical set pieces used for filming embedded into them.

In addition to the unique cities, there are some large tracts of land, including the Great Hunting Ground, scarred with massive tread marks left by traction cities over the centuries. Here, the once-organic environment had been reshaped and now appears man-made, but life is establishing a foothold in this once-barren landscape.

“It has all these layered plateaus with hard edges and embedded track shapes that we placed everywhere,” says McGaugh. “Our rule of thumb was that the higher the level of the plain, the more foliage there was, since it had been a long time since it had been driven over. However, on the lower level, at the bottom of the trenches, it was also green, but more marshy and full of reeds, since that is where water accumulates.”

Some survivors of the war pushed into the mountains and founded settlements there, rather than living a nomadic existence. One such settlement is Shan Guo in the East on the Asian Steppes, protected from the mobile cities by mountain ranges. In addition, there is a massive two-kilometer shield wall (6,561 feet high) situated between two of the mountains that protects Shan Guo and the static cities in the Himalayas. This environment alone was daunting to create, as it covers 5,000 square kilometers (over 3,000 square miles).

On one side of the shield wall, the environment is very lush, fertile and green, and the buildings influenced by Bhutan monasteries. On the other side of the wall, there is a lack of foliage, with the landscape strewn with decayed ruins of traction cities that have unsuccessfully attacked the wall. And while the shield wall is massive, it had to appear smaller in comparison to the mountains surrounding it.

While constructing these mountains, the Weta artists used available geographical data, increasing the resolution through erosion simulations that would shape the mountains more naturally. “That gave us extra detail that we could use to make it look more organic,” McGaugh says. The simulation also was used to embed the ruined traction cities into the crater environment as well as situate the crater environment into the surrounding landscape.

Cities on the Move
The moving cities can cover a great deal of ground in very little time, gouging and scarring the earth in their wake with deep crevices; above, airships dot the skies. The relentless ploughing of the traction cities over the landscape has driven layers and layers of mud, debris and waste into the ground. Weta re-created this effect by starting with a precisely coupled fluid simulation with multiple viscosities; this could accurately simulate the combination of solid and liquid layers of the mud. They then began laying tracks and eroding them, then laying more tracks and eroding, repeating the process until the desired result was achieved.

In the film, there are numerous homelands, including Airhaven, a fantastical city in the clouds with a jellyfish look that is home to the Anti-Tractionists.

“Airhaven didn’t have a lot of movement, so we didn’t have to use our new layout puppet technology. But when it crashes, that had to be animation-driven, so we built a lightweight puppet with a large section of the city on each piece of the puppet, so animators could choreograph the crash,” explains McGaugh. “Then they handed that off to our effects department, and they would simulate all the individual pieces breaking apart and exploding, and then add the explosions, fire and all the dynamics on the balloons and the cloth.”

So many of the traction cities have multiple moving parts that it was impractical to animate them by hand. Alternatively, Weta developed a tool called Gumby, which is a vehicle-ground interaction toolset that allows animators to move a city from point A to point B over uneven terrain along a curve. The Gumby system then made sure that all the wheels stuck to the ground, thereby driving the suspension system that causes the infrastructure to move appropriately.

A dynamic caching system allowed for secondary bounce and wobble to occur on various pieces of a city in response to the motion from the Gumby system. “It wasn’t perfect, but it allowed for very complex animation in the blocking stage and made the motion more believable and closer to what the final version would look like,” explains McGaugh. Once blocking was approved, then the animators would refine the motion as needed.

London Lives!
According to McGaugh, the single biggest challenge was constructing London and executing it in a way that maintains its enormous size while keeping it in the realm of believability. “Concept artist Nick Keller came up with a design for London that looked like it could be self-supporting and was scalable, so we could make it as big as it needed to be in order to house 200,000 people, and when it moved, we could sell that as believable, too,” he explains.

London is the largest of the traction cities. It incorporated approximately 17 live-action sets and is a mile wide and a mile and a half long, and over a half-mile high. It is divided into seven tiers, with life aboard London progressively more desirable farther up each tier.

“This is a place where the glass is gone but stone statues have survived,” Rivers says. “We decided to make anything we see in our world today archaeological and then skew and twist things from there.” As a result, some iconic landmarks are recognizable but have an altered appearance.

“The design had to lend itself to believability for being so large and moving, but it also had to evoke a sense of contemporary London through recognizable features, such as the Trafalgar Square lions acting as sentinels on top of the outriggers, so they’re visible from a distance,” McGaugh points out. London was then crowned with a reconstructed St. Paul’s Cathedral.

A contemporary feel was evoked through the architectural style. As McGaugh notes, London is known for its diverse and contrasting architectural styles juxtaposed against each other. So, the designers followed that style when laying out the buildings atop the digital London. “That was also carried out through the front façade of London and at a much larger scale, so that from a distance, you could still feel that diversity where it’s kind of rusty and brutalist at the bottom with a layer of architecture that is reminiscent of the houses of Parliament, and then is topped with chrome and steel construction shaped like a coat of arms,” he adds.

Because of this diversity of architectural styles, the group was able to source from its library of existing buildings — whether Victorian, Georgian, contemporary office buildings, tower blocks, row houses, Buckingham Palace — and mix them together without having to maintain uniformity from building to building.

But with so much detail, it became prohibitively difficult to render, and that’s where Weta’s Cake technology came into play — which used an intelligent way of breaking down geometric and material detail into a format that could be streamed into the renderer, using just the level of detail required. “Before that, it wasn’t viable to render London,” says McGaugh. “But Cake allowed us to process all the data into a format that enabled us to render it, and render it quite efficiently.” Rendering was done within Weta’s proprietary Manuka renderer.

Lighting was also tricky, as the team was following the lighting direction from Raby, who used backlighting — which is not easy to do in CGI when using hard edges, especially when there is shiny glass and metal involved. As a result, the CG lighters, using the studio’s Foundry Katana-based pipeline, had to do tests on almost every shot to find the appropriate angle that sold the backlighting and kept the visuals interesting and not too flat, while maintaining continuity with the camera shots.

London on the Move
A city constantly on the move, London can travel at approximately 300 kilometers (186 miles) per hour, bolstered by massive engines. While that speed sounds ridiculously fast according to real-world physics, it was necessary to hold audiences’ attention, as physics and cinema were often at odds on the film. “There was a lot of testing, and we tried 100 kilometers per hour when London is chasing down [the mining traction city of] Salthook across a vast landscape, but it looked like a couple of snails racing. It was too boring,” says McGaugh. “Indeed, 300 kilometers sounds ludicrous, and if you think about it, it is. But that is what allowed us to keep the chase exciting while constantly selling that there is movement.”

Indeed, London had to move faster than physics would allow, yet just how fast depended on the camera shot. Nevertheless, this wreaked havoc on the effects that simulated natural phenomenon, such as dust. The key, however, was to use visual cues to make sure the cities felt massive and other cues to make sure audiences were not distracted by the fact that the cities are moving so fast.

When constructing the massive city of London, Weta devised the concept of so-called “lily pads,” representing 113 sections of London. Each was rigged and animated independently and contained millions of components that had to be tracked and moved. Each lily pad was constructed modularly, enabling artists to add clusters of buildings, parks, shops and so forth on each platform. More and more detail was then added to areas as needed.

These lily pads were supported by complex suspension systems for individual movement; at times there was some inter-movement among them, as well. “[The movement] was pretty subliminal at times, but if it wasn’t there, you’d have noticed it and everything would have felt static and locked,” McGaugh says.

Shrike
While Weta’s work on the film was heavily focused on environments, Mortal Engines does contain one digital character, Shrike, who had raised the movie’s heroine, Hester Shaw, after her mother’s murder. Half-man/half-machine, Shrike was a dead soldier resurrected by technology. He stands at seven feet tall and weighs close to 1,000 pounds.

Shrike’s anatomy is not human — he has extra appendages and extra mechanical bits that had to be rigged to move differently from that of a typical human. “It was determined early on that we could not use motion capture because we needed him to be inhuman, so we had to invest quite a bit of effort into finding his motion through keyframe techniques,” McGaugh notes.

Shrike’s face comprises metal parts and human skin. To achieve a realistic tug and stretch of the skin against the metal, Weta developed a custom facial-muscle rig so animators could use the visible muscles and skin to allow him to emote in some particularly dramatic moments in the movie, inspired by the performance from actor Stephen Lang.

A New Day
While the scale of the world building for Mortal Engines was not at the level of The Hobbit, it was not without big challenges for the VFX veterans at Weta. Initially, the concept of massive cities on the move was difficult to wrap one’s head around. But, as always, Weta’s artists and animators were able to bring that unique visual to life in a realistic way.

Now with Mortal Engines in theaters, the studio remains on the move with a number of other mega projects in the works, including the Avatar sequels and more on the big screen as well as the final season of Game of Thrones for the small screen. All resulting in more expansive, unique worlds brought to cinematic life.

Karen Moltenbrey is a veteran VFX and post writer.