"Astrid did about 80 different facial expressions and we have a new proprietary system at ILM where we can take a group of expressions and decompose them into all of the individual shapes that create our facial rig," Harrington explains. "We started by creating a 1:1 match of Astrid and it was one of the most accurate digital doubles ever done at ILM. On top of that, we wanted to either be able to do facial MoCap or to animate by hand the way we did Davy. There were some transformation shots where we were just going to copy the performance from the plate, so we needed to have both approaches and a system that could handle both [as a hybrid]. We had a new Imocap set up for her because she was going to be in water and basically nude. We applied markers using a tattoo stencil on her arm instead of the traditional bands of Velcro to capture her upper body. It would then go to animation and we would attach our mermaid to that and animate the tail [designed by Aaron McBride] using her legs as a basis. We came up with a big fin that was long and elegant that could be simulated."

New tools for water and interaction with the mermaids were also created. "The idea of taking 30 creatures with all this streaming stuff (flowing hair and tendrils), and then running them through a full-on cloth sim was never going to happen," Snow suggests. "We worked out a way to bake all of these simulations into the cycle, and even some simplified simulation tools that the technical directors could run just so we didn't have to use our creature team to do first passes on all these things. And, likewise, we developed a bunch of little library splashes and then a means of plugging those in so that as the mermaids are attacking, they could automatically generate different types of splashes. It was really layers of tools on top of the simulation engines to make them run faster and easier to do.

ILM not only used its PhysBAM fluid sim engine, but it also applied the Plume GPU-accelerated technique developed with NVIDIA for smoke and fire on The Last Airbender. This was used for mist and for overall fast turnaround. ILM also used Houdini and Maya for additional sim help. "It was handy to have automated tools to help the artists and Rob Marshall to read the animation," Snow adds. "If the mermaid was moving in space, looking almost flat shaded, you could more easily buy the hardware render. But if we didn't put the splashes in quickly, it didn't look fast or powerful enough. The water interaction was key to selling the performance, and we learned a lot about getting characters to move through water convincingly. You had to have accurate-looking simulations right out of the box to show that the animation was performing well."

ILM collaborated with Scanline VFX for the triggering of the Fountain of Youth, beginning with a droplet of water running along a leaf, and then when water flows along the walls of the cave to make a portal. However, for a spectacular liquefying death resulting in a nasty skeleton, ILM scrambled to get some extra water sim by adding a whirling effect. There were some rendered surfaces, meta-surfaces for more of a glossy look, different layers of particle sim created with PhysBAM and the simulation tools and some practical water.