The fire­power be­hind mortal en­gines

Dis­cover the tools and tech­niques Weta Dig­i­tal used to shift gi­ant cities for the film’s post-apoc­a­lyp­tic world

3D World - - CONTENTS -

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.

MEET shrike

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

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