The firepower behind mortal engines
Discover the tools and techniques Weta Digital used to shift giant cities for the film’s post-apocalyptic world
Ian Failes talks to Weta Digital about their work on the incredible futuristic world of Mortal Engines, featuring giant cities on wheels
“We relied on proceduralism at every stage of the build” Kevin Smith, visual effects supervisor, Weta Digital
In Mortal Engines, director Christian Rivers had to move a massive city. And not just any city. In the future world of the film, the entire remnants of London have been placed on enormous tracks and been motorised. Not only that, London attacks other smaller and more vulnerable cities on wheels.
To pull off that stunning imagery, and also flesh out a steampunk world that has been ravaged by a war and extreme geological movements of the Earth, Rivers looked to the artists at Weta Digital who, among many other challenges, had to find ways to animate these enormous machines in photoreal landscapes.
3D World found out from key members of Weta Digital’s team – visual effects supervisors Ken Mcgaugh, Luke Millar and Kevin Smith, and animation supervisor Dennis Yoo – how new tools had to be built, and how an army of effects artisans were deployed to help deliver the film.
Not your usual city-building
Entire cities on wheels required a new approach from Weta Digital to be able to not only animate the mass of moving buildings, but also put it inside an environment that was constantly moving. London, for instance, was 860 metres tall, 1.5 kilometres wide and 2.5 kilometres long. It needed to be filled with 200,000 citizens and appear to move across land in a believable, yet devastating way.
“The biggest challenge for us in that regard,” says Ken Mcgaugh, “was that all of our techniques for doing large-scale environments and particularly urban environments rely upon the assumption that the environment doesn’t move. And so, once we put it on wheels and started trying to make it move, it broke everything. Nothing worked, so we had to engineer a new pipeline that allowed us to kind of combine the way you work with vehicles and environments into one system.”
“I’ve animated vehicles in the past,” adds Dennis Yoo, “and they’re always a pain. If you don’t actually have a good setup or a rig, it can end up being quite a chore to get those contacts working, and the weight, and the wheels going – if you’re just keyframing that, it’s a bit of a nightmare.”
To deal with these issues, Weta Digital devised a system of using what they called ‘layout puppets’,
in which the animation rig for the cities that animators worked with behaved largely like a vehicle. The difference, explains Mcgaugh, was that “hanging off of that rig further down the pipeline were sub-layouts, which were treated as environment layouts. It allowed us to modularise how we worked and also allowed for us to put really complex city environments onto something moving.”
Other tools that helped included Weta Digital’s ‘Gumby’, a procedural animation system that aided in collision detection and also helped keep the wheels on the ground, and a dynamic caching system that provided secondary motion on the buildings that are part of the moving cities.
Adding to the animation challenges was simply being able to render that enormous amount of detail, which was required for wide views of the city and for flythroughs. There were 933 unique assets on London which, once instanced and duplicated, could expand to 18,000 (“not including all the blades of grass and leaves and all the trees and parklands,” adds Mcgaugh). Weta Digital utilised its proprietary path tracer Manuka to render the CG elements.
“We also resurrected some technology that we originally developed for The Hobbit called Cake,” notes Mcgaugh. “Cake is a procedural level of detail baking system. It’s a much more intelligent way of baking down both the appearance and the geometric complexity of assets into something that can streamed off of disc, just at the level of detail it’s required for, for a particular frame that’s being rendered. And it underwent quite a bit of new development to get it working for our moving cities. Without it we couldn’t have rendered London at all.”
Journey to shan guo
The moving cities make their way over vast tracks of land. Weta Digital employed their environment construction system called Scenic Designer and tree growth tool Totara, among other tools, to build out a world that required these huge scales.
The studio’s procedural building approach helped to ensure that the landscapes did not look too similar, including for a huge stretch of land that London covers on its way to Shan Guo and the shield wall. “It’s 5,000 square kilometres,” notes Kevin Smith. “It’s this enormous thing and we couldn’t build that by hand, so we relied a great deal on proceduralism at every stage of the build – in modelling, in layout, in textures and in shading variances.”
“We built things so that we could slide stuff around on the valley floor as painlessly as possible,” adds Smith. “So we had little vignettes of ruined cities, and that little vignette had a lot of procedural modelling and procedural texturing. The model team took all the matte painting geo and all the valley floor geo and they ran all that through Sidefx’s Houdini, so we had a lot of procedural weathering and water flow and erosion.”
Matte painting also helped deliver this giant valley and outlands environment, using, among other tools, Isotropix’s Clarisse toolset and compositing via Foundry’s Nuke. “The compositors came up with something we called ‘six packs,’” explains Smith. “At different points along the matte painting of the action we would render six cameras up, down, left, right, front and back. We would use those high-res renders to project the matte painting back onto the geo. Close-ups of the ground went through our Manuka renderer.”
The film’s final act sees London attack the shield wall with a new Medusa weapon, causing catastrophic damage. Houdini and Weta Digital’s in-house fluid engine Synapse were used for the weapon effect and the resulting explosions. “The core of the idea for Medusa was that this energy bolt hits the wall and you get this effect that was like a black hole,” says Smith. “It destroys the wall, everything sucks in, you get this crazy purple energy and it destroys all the ships. Then everything sucks back into the wall and you just get this cool moment of silence where you think, ‘Is that it?’ And then, bang! You get a huge explosion.”
Not Just grounded cities
An airborne city, Airhaven, is also part of the world of Mortal Engines. It is depicted as a collection of buildings sitting amongst the clouds held aloft with clothcovered balloons, netting, cables and wires.
To deal with so much cloth and ensure that it always appeared to be in motion, Weta Digital ran lots of soft simulations and extracted these back out into vector displacement maps, then set it back into all of the palettes. Then, describes Luke Millar, “we had a palette offset so we could essentially apply cloth motion at a shader level. Because the palette would apply to the various different cloths within it, we up-versioned the palette, hit the render button, and suddenly all of Airhaven came to life. I remember the first time we saw it, it was incredible.”
Unfortunately, the town of Airhaven meets an untimely fiery end, which meant Weta Digital had to simulate both the cloth-like destruction and many explosion and fire elements. “We took sections, which we had run the cloth simulations on, and
we handed them to our effects department, who set fire to them,” says Millar. “We gave them the brief of, ‘Okay, set fire to it with one ignition point and let the flame burn through.’ And then, for another one, we’d say, ‘We want to set fire to it in the middle and have it burn in both directions.’ And another one might be, ‘We want it to burn with three ignition points.’”
Airships are the vehicles of choice to get to Airhaven before it is destroyed, with the hero airship, the Jenny Haniver, belonging to Anti-tractionist aviatrix Anna Fang. A practical cockpit of the Jenny was built on set, while Weta Digital also made a CG version along with the aerial environment in which it travels. This included cloud formations which were built completely digitally. “We built a Terragen sky matte painting for the wider extent of the environment,” says Millar, adding that the clouds themselves came about via close attention to shading and realistic scattering.
Amongst the chaos of the moving cities – on and above the ground – fugitive assassin Hester Shaw is pursued by a re-animated
“a big challenge Was Walking that line between mechanical and organic” Ken Mcgaugh, visual effects supervisor, Weta Digital
soldier called Shrike (played by Stephen Lang). The character was completely computer-generated by Weta Digital. Initially, Shrike was designed to be almost all metal, but as production continued it was decided to provide him with more flesh on his face.
This enabled the character to deliver more emotion. “Our animation supervisor, Dennis Yoo, worked very closely with models and the concept artists to make sure that the flesh was distributed across the face where it was needed to sell the emotional beats of the story,” outlines Mcgaugh. “And then one of the biggest challenges animation faced was walking that line between mechanical and organic, and finding out where that line needed to be depending on the emotional content of the scene.”
Lang was on set during filming and, although he wore a capture suit, Shrike’s final performance was not achieved via mocap. Instead, Weta Digital’s team took Lang’s performance as inspiration and key-frame animated the character to suit a mechanical being.
“We actually tried putting motion capture on him from the start,” says Yoo, “but it actually highlighted something really quick which was that it started looking like a guy in a metal suit. Christian wanted us to animate something much more deliberate in terms of performance.”
“So in the beginning where you start seeing Shrike, he’s very deliberate,” continues Yoo. “He’s stalker-like, we jokingly said ‘ninjalike,’ so we were trying to hit that kind of note where he was just this efficient kind of killer. Then later on he starts getting damaged, and that was a whole new performance we had to find, where he was this damaged machine, yet he still has these organic parts to him. He’s falling apart, yet at the same time we couldn’t make him too weak.”
THE scale of THE World, and THE scale of THE task
Reflecting on the achievements made by Weta Digital in Mortal
Engines, Ken Mcgaugh notes that managing the scale of the cities remained their biggest challenge, as was just selling the almost seemingly ridiculous idea of entire cities on wheels chasing and devouring each other.
“We did walk a fine line between boring and cartoony,” he says, “and I wish there was some magic bullet to solving that, but it ended up being a very holistic approach in every single shot. Wherever possible, we tried to maintain a sense of how large the cities were, but we quickly discovered too, that it is equally as important to not tell that as well. The overriding goal was to maintain the suspension of disbelief.”
Previous page: concept art for the enormous ‘traction city’ of london, complete with the remnants of st Paul’s cathedral on its peak above: the film opens with the traction city of london chasing down a smaller one
top: With london approaching, Hester shaw and other inhabitants of a smaller city run for cover
above: a major geological event has re-shaped the globe in Mortal Engines
Main: scenes aboard the Jenny Haniver airship required Weta digital to deliver fully cg cloudscapes
inset (top): Mortal Engines director christian rivers with Hera Hilmar, who plays fugitive assassin Hester shaw
inset (bottom): situating the camera low to the ground was one way to sell the sheer size of london
above: based on the wheels in Weta digital’s cg london city, VFX artists adjusted what had been filmed on set to be the right size of impacted earth
right: the film’s final act sees london attack the area of shan guo, but not before a resistance group known as the anti-traction league attempt to stop the city
shrike, a humanoid re-animated soldier, was a completely cg character crafted by Weta digital, based on a performance by stephen lang