The allure of LAIKA’s The Boxtrolls perhaps lies in its mix of both analogue sculpting and stop motion and digital techniques in bringing the film to life. One technology the studio has embraced is rapid prototyping, an innovation pioneered by the studio for enhanced facial shape modeling and manufacturing to help carry out replacement animation - to the point that the Scientific and Technical Awards Committee of the Academy of Motion Picture Arts and Sciences is currently considering it for awards consideration. To find out more about how LAIKA used these techniques on The Boxtrolls, we spoke to director of rapid prototype Brian McLean.

LAIKA's director of rapid prototype Brian McLean.
LAIKA's director of rapid prototype Brian McLean.

If you’ve seen LAIKA’s previous films - Coraline and ParaNorman - then you’ve already seen replacement animation in action. Here, various parts of the puppet's face are replaced during stop motion animation to provide the illusion of movement. Those various face pieces - 52,000 in total in the case of The Boxtrolls - are 3D printed as part of the rapid prototyping process.

So, how does it work? First up, hand-sculpted models of the puppets are made, which are then scanned. They are then retopologized in TopoGun and brought into Maya and ZBrush for detailing. “We actually exaggerate a lot of the details in there,” explains McLean, “since the scan data and 3D printer softens things, so the model needs to overdrive a lot of the detail.”

A stop motion animator, who will later work for hours on stage with the puppet, sits with CG animator to pick facial poses. These will be presented to the directors in a playbast. The chosen faces are then pulled from a library, printed and delivered on set. “We struggled with at the beginning,” admits McLean. “It’s counter-intuitive for an animator - they’re having to separate out facial animation from the body animation. They have to make decisions on what the face performance is going to be days if not weeks before they’re out there shooting.”

The rapid prototype machine - a 3D printer - in action.
The rapid prototype machine - a 3D printer - in action.

A preliminary 3D print of the head is made for review, rather than reviews being done on the computer model since the detail can be different. Once the modelers have artistic buy-off on the outside of the head and its various pieces such as the eyes, mustaches and so on, they begin work on the inner components, acting like engineers and industrial designers to work out rigs, armature strengths and other inside pieces. Rigging is carried out in Maya, with texture painting done initially in Photoshop but now mostly in MARI.

The 3D printing process is the equivalent of rendering the CG model. The studio relied on five color 3D printers during production. The printers spray a liquid down onto a gypsum powder in layers on multiple puppet pieces. During the process, the printed pieces are tested and QA’d against other pieces for consistency before they are handed off for stage animation. “The testing process is very much an analogue one,” says McLean, “with a head on a stick and locked off camera to capture each frame and toggle back and forth to see if colors match. The team also uses dremels and X-acto blades and pieces of plastic for fixes, grinding and shimmying these faces - it’s brute force getting faces to work for registration. They even have a whole bunch of pastels and mix them in powder form, frame by frame brushing over the faces to make them more consistent.”

A tray of faces covered in 3D printing powder.
A tray of faces covered in 3D printing powder.

Despite the work required, the benefit of rapid protoyping is the detail that can be implemented into the facial features as well as the range of emotions that can be expressed - ultimately more than a million possible expressions for the character Eggs, for example. “We are taking advantage of computer animation in that we can choose unbelievable subtlety,” says McLean. “We can choose to just move a tiny amount of the lips, and when you print that out that can show up. One thing that replacement animation has been used for is broad expressions, but the subtlety was always near difficult to capture because these things were hand-sculpted. But because we’re harnessing the power of the computer, we have the benefit of getting the subtle expressions combined with the broad expressions.”

Some of that detail comes also from incorporating the underlying musculature into the 3D facial models. “We were trying to find ways to get these characters to move in a naturalistic way and we wanted to make the cheeks open, for instance,” says McLean. “The more detail you add into the faces, the more geometry you’re trying to carry through - there’s this very fine balance between knowing that we are working in a polygonal workflow. We have to have the geometry there in the rig - in the animation - not just something we’re adding in and a displacement map rendering later.”

A face after it has be cleaned of all the powder.
A face after it has be cleaned of all the powder.

Interestingly, LAIKA had to, for this film, scale down the faces - and add even more detail - compared to the work on ParaNorman. “Eggs' head in this movie is about the size of a ping pong ball,” states McLean. “Norman's head, on the other hand, was the size of probably a small apple or an orange. Just scaling Eggs' head down and making the head more proportional to the body meant that we suddenly had ping pong ball heads where we’re still trying to maintain detail. We were also asking these faces to have a broader range of expression than we’ve ever had and had to add subtlety to it, but the scale was so tiny - that ironically if you wanted to have a subtle mouth change, all you had to do was grab a face FV (representing phonemes F and V), and another face with the exact same pose that was printed at a different time, and just the differences in the print job and those little differences would give you the subtlety that you needed.”

Indeed, maintaining consistent detail among thousands of parts had its challenges. The 3D printers LAIKA used on the film had to be “whipped into shape,” says McLean, noting that variations between batches of prints did require revisions and strong QA processes. The reason relates to the sheer volume of printing work required. “How it works is,” relates McLean, “they buy HP black printer heads and bleed out all of the black ink, and then they use that printer head to pump through their own binder. Because they’re pumping through so much volume of binder, what happens is that those tiny little metal holes that are used to push out ink expand and start to get larger and larger. So just by design these things were never conceived as being about this kind of printing, they were about putting color into a three dimensional model.”

A tray of cleaned faces getting ready to be stored on the set.
A tray of cleaned faces getting ready to be stored on the set.

“Also,” adds McLean, “you’re printing on a powder and that’s recycled and when it’s on an area that’s not sprayed on they get sucked back up and used for the next printing tray. Suddenly you’re combining all these technologies that can use a wonderful model once certainly not a consistent color or part dimensional accuracy all the time. It’s like printing the same photograph on a hundred different pieces of paper and using that as a flipbook - you’re going to see crazy color differentiation too.”

In addition, variations in humidity can also play a role in making the 3D printing less consisent, as the powder reacts in different ways. This can obviously prove problematic in a location in the Pacific Northwest such as Portland where rain can happen frequently.

To deal with these inconsistencies, LAIKA carries out significant ‘post processing’. “Step by step the models are sanded, dipped, reinforced, crystal-cleared and baked in the oven,” says McLean. “We also have a strict pre-press process where the powder is baked before it’s printed on to get it to the right consistency.” The visual effects team, too, would aid in 2D fixes for many shots.

LAIKA's face and parts library.
LAIKA's face and parts library.

For LAIKA, the rapid protoyping and replacement animation process has continued to evolve with each new project, with McLean noting that he hopes further development takes place for even more detail and achieving other things such as realistic motion blur. “When we started this on Coraline,” he recalls, “at the time we were asked to budget how much it would take and money it would cost to do replacement animation. We said we can do it with three people and $30,000 of materials. Reality quickly eclipsed that and at the same time we realized the power of this and so did directors - there was no going back to hand sculpting faces. This afforded us to tell unbelievable stories to tell greater emotional range and more subtle character animation than we ever had in stop motion alone.”


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