ILM’s work on Aquaman spanned several sequences, many of which take place in elaborate underwater worlds. It’s here where key characters played by live actors converse, fight, or otherwise interact. These actors were filmed ‘dry-for-wet’, meaning they were captured on set in dry environments that would be made to look underwater in post-production. If the characters were riding sea creatures, sometimes they were also filmed on ‘tuning fork rigs’ that allowed for the right kind of faux underwater float and movement.

That movement was also achieved by replacing actor hair and clothing with digitally simulated versions, with the aim of convincing audiences that the action was taking place deep below the sea. For ILM alone, this turned out to be an enormous process amongst their 670 shots for the film. In total, the vfx studio would produce more than 400 hair simulations across those shots.

The process started with reference of hair underwater. Luckily, White was able to source some very close to home. “I happen to have eight year old twin girls, one has very long hair and one has more shoulder length hair, so I spent a lot of time shooting reference. What we realized right away was that our tools were really built largely around animal hair, where you have lots of guide curves, but you can get away with a lot of interpolation between those guide curves because they generally move together within patches.”

Instead, ILM needed to simulate may different kinds of hairstyles underwater and deal with all the ways hair – whether it be flowing, short, or even beard hair – can be influenced by vortices within the water. “You can get a lot of strands going their own direction but you also get a lot of hair that kind of stays clumped together when it moves around,” said White. “All of that was taken into account in terms of the tools that we were building out for doing underwater hair simulation.”

“On top of that,” said White, “it is never just ‘push the button’ and you run real physics and then it looks good. You can very easily get to not very flattering hair poses and looks when you just run the simulation. Even though it looks like it’s underwater, it doesn’t look good for the character. And we of course wanted to keep them heroic.”

Furthermore, director James Wan had a very clear notion of how the underwater hair should look, says White. One notion was that the hair always needed to be reacting to body movement and to be lifting up and parting or separating on top of a character’s head as they moved back and up – “lots of volume,” observes White, was something ILM added, matching several comic book images that illustrated hair in this way, especially for the characters Arthur (Jason Momoa) and Mera (Amber Heard).

To aid in underwater hair simulation, ILM utilized and upgraded its Haircraft tools, which were originally developed for the work the studio had done on the feature film version of Warcraft. “Underwater hair definitely required some new tools for us in terms of getting much finer control over the way that the hair clumps together, what sticks together and what flows free, and then being able to run hair sims in multiple passes,” explained White. “We’d lay down a base simulation to get the right overall shape, but then we’d go in and pick out strands of hair to have it separate from the main mass.”

Before simulations actually took place, ILM artists meticulously matchmoved the live-action actors. Sometimes tracking markers fixed to the real hair were used on set, but other facial details or even crowns worn on set also worked accurately enough as tracking geometry. “Making sure that the hairline stayed locked was super-important,” said White, “because the integration of the hairline was a really tricky component of making the hair work.”

Separate cg water simulations were then coupled with the hair sims. This involved running digital replicas of the characters through a water volume and allowing the vortices of the water simulation to affect the hair simulation. Doing this gave good results, but still needed fine-tuning to allow for tufting dynamically through the shot down the length of the hair. “A lot of times, we’d get it looking pretty good up on top of their head and as you’d get towards the end, it wouldn’t be splayed quite right,” said White.

Occasionally it wasn’t just head hair that needed to be simulated – Momoa’s Arthur also sports a prominent beard, and that too was occasionally fully cg, or moved via warping and combining separate layers in compositing.

Interestingly, a major challenge with the underwater hair simulations came about in terms of vfx reviews. Usually a director will ‘buy-off’ on an animation review, and the shot then moves down the line. But in the case of shots requiring cg hair, Wan often reviewed hair sims directly.

“Your read on a character is so influenced by how their hair looks – whether they look cool or, in some cases, they can actually look very dorky if their hair is sticking straight out the sides or has a big puffy hill on top of their head,” noted White. “One of the key ways of [getting approvals] was to get it 90 per cent of the way there and then we’d have post-sim sculpting tools that we could use to finish out the shapes or take care of stray hairs that weren’t looking too good.”

“Hair actually plays a pretty big role in the film,” added White. “There are some very dramatic moments in the third act battle, where James described to us this crescendo of music and the characters’ hair just flowing up and all around them. That was really quite a challenge for the artists to get, but it looks fantastic.”