Tools Testing
The production sought a very dynamic performance in constant motion. Because the Fairies’ elusive shape is not static but defined partly by the moves in the animation and partly by the behaviour of the shards, a regular creature pipeline wasn’t suitable, at least not for the final outcome in the shots. John Moffatt, Double Negative’s VFX supervisor, explained their combined animation/effects development and the workflow the team established.

The facility started R&D on this project about three months before the shoot took place for the Dark Fairies in December 2011. A small crew began the design work from September and by the time VFX supervisor John Moffatt and the team at Double Negative received plates in January, they had a set of tools they were confident would lead to steady consistent, shot production.

The whole project had a turnaround of just 12 to 16 weeks from turnover of shots to final delivery but they were aware of the need for a robust, tested toolset to be able to meet this deadline. Alexander Seaman was the main Dark Fairies developer and has submitted a paper to SIGGRAPH 2012 on his work.

“We tried to create a form of previs for the sequence but it was difficult to know precisely enough how the action would be shot, in part because Rupert wanted freedom to work with the performance on set,” John said. “Because we knew the type and style of movement required, though not the actual blocked moves, we created a small sequence of three shots we showed production VFX supervisor Cedric Nicolas-Troyan and director Rupert Sanders as proof of concept. Cedric was able to design much of the shoot himself, after which it fell to us to adapt the characters’ performance to the action.”

Animation-Driven Effects
Because the sequence involved intense camera movement, no stand-ins were used on set to replace the Dark Fairies, which would have required too much clean-up in post. The stunt actors playing the soldiers performed to eyelines only. They wore heavy harnesses and were thrown and knocked about as if in violent combat. One was hoisted up to the ceiling by one of the Dark Fairies. The harnesses were large and needed extensive removal work, handled by Double Negative’s Singapore facility.

The plates were all matchmoved as soon as they arrived from set and each fairy character’s position was blocked out in all shots across the sequence. John said, “All of the Dark Fairy animation was keyframed. Cedric had experimented briefly with motion capture but it wasn’t successful enough to have saved them any time. Once the animation was in place, it would be passed to the VFX team and used as the basis of the material driving the effects. Even the floor-bound elements would be driven by this animation as far as possible, to limit the use of pure effects.”

For example, the first shot showing the Dark Fairies forming employs a flocking effect for the swirling shards of glass they are composed of. However, such an effect was always goaled toward the animation. To do this they needed a concept of a fully formed Fairy to have an object for both the animation and any required effects to goal toward. A proxy version of that model was built and rigged, giving the animators something tangible to work with while completing the finished shots. Viewers never saw this model itself, which was revealed instead only by the constantly moving, regrouping shards of glass.

Explosion in Reverse
The animators worked in Maya. The effects were done in Houdini and Maya, using an in-house tool called Bang, Double Negative’s version of the Bullet physics solver within Houdini’s SOPs, or surface operators. Bang has a point-based approach that was a good match for this point-based type of character, working somewhat like an explosion in reverse. “You could start with hundreds of thousands of individual shards and be able to coerce them back together to form a creature, as the animation required. At any moment, the shards were in constant motion,” John said.

“Caching out the animation was an important part of creating the Dark Fairies. When a piece of animation was passed to the effects team, they would use it to base the movement of the groups of shards upon. Then, once John had approved it, a group of shards together with their motion would form a cache file that could be passed to the lighting and rendering artists. Visually, the cache file is a grey shaded version of the moves and effects, and would typically take up to a couple of days to render, followed by compositing in Nuke.”

Following such a workflow meant that any changes to animation required about a week of work before the result could be seen. The technique involved tight coordinated between the animation and effects departments, but by setting the animation as the focus and priority they established a working language and were turning shots around quickly by the end.

Computational Heavyweight
“Heavy animation work alongside heavy visual effects is always a challenge,” he said. “I believe that this kind of work probably couldn’t have been done a few years ago due to the computational load. For example, each shot was completely ray-traced and each object was reflecting the entire environment. We had to build out the whole throne room digitally although we never needed to use it as a CG asset. But it had to be complete with textures to be able to control the reflections in the glass shards.”

Because the glass was black, these reflections were especially important to the overall effect as a means of defining their shape, form and qualities, making them readable as characters for the audience. Moulding and sculpting the CG shards by using reflected light required a controllable CG environment to reflect in them. The lighting could be handled in Maya with RenderMan using physically correct ray-tracing. “Once you have your environment and have photography of real glass in controlled lighting, you can recreate that lighting in the digital environment. Again, once we had established our routine it all happened pretty rapidly. Getting there was the hard part,” John said.

True Photographic Lights
“We were trying to light it in a true, photographic way, which caused a massive increase in CPU requirements. Recreating true photographic set-ups digitally appears to be required more often now, and makes a great start point for the rest of the work. Black shiny material only really becomes visible when you see the reflections and highlights, so sometimes we cheated the lights and bounce a bit to give the shards depth and volume while they were constantly moving.”

At any point, there could be three or four of the Fairies in one shot. As mentioned, the tracking of the plates first was an essential step and a massive tracking team tackled this immediately when shots arrived. In fact, proxy geometry was put into the scenes even before tracking because temps had to be completed in one week. John explained, “The cameras could be updated as we proceeded but they had to be right before we could create any effects. The way the sequence was shot - that is, without either the cameraman or the VFX team knowing exactly where the Dark Fairies were – meant that witness camera wouldn’t have been very useful, and would have produced another complete level of editorial.

“Their final positions in the end depended mainly on the VFX team using their eyes to judge what looked right within the space. Technical correctness was not always visually satisfying. The Fairies sometimes just looked too big or too small.”

Forestof Unknowns
The Dark Forest sequence of the story presented the team with quite different challenges, mainly owing to the fact that the shoot had gone ahead without much forward planning for the spooky, frightening effects Double Negative were assigned to add to it. Snow White runs through a very large set built on Black Park next to Pinewood Studios. The set was built with mechanical trees and other essential props for the actress to run around, and the resulting footage was cut into a sequence before it was passed to the team, who produced extensive concept work showing various ideas for the creatures and dangers Snow White might encounter.

From these initial materials, the sequence evolved. To create an intense feeling that the Dark Forest was closing in on Snow, they blocked out the animation they would need in the trees to build up this feeling, creating and adding branches procedurally to the physical, in-camera trees and hand animated them to reach out and try to grab her. John found their tree branch animations felt very similar to the trees in Disney’s forest in the original animated ‘Snow White’.

The production had in their minds only a few specific elements that they particularly wanted to include in the Forest. A cluster of mussels oozing thick, black oily liquid was one of these, for which footage of real mussels was supplied as reference to base an evil looking clutch of CG mussels on. Many of the other creepy, crawling, scary effects were the team’s responsibility. The camera was always on the move during the shoot and the lighting was fairly dark.

Aligning Looks
The footage sometimes contained practical elements that indicated what Rupert and Cedric wanted and where, but most were replace digitally for control and movement. From concept through to lighting and rendering, they modelled animated and applied different effects to get the final result. “The coherence to the scene came from aligning looks, such as oozing effects, the tree effects and the creepy-crawly feel, not from previs,” John said.

“Not only were none of these shots previs’d, but the whole sequence also changed significantly from turnover through to the end of the project, just as the Dark Fairies sequence did. Every week or so the production would review our progress, and either approve or send shots back to try something else. It was often a matter of creating something the production enjoyed. Progress on everything we designed and built was a little unusual, and developed in unexpected ways. We continued to work on and refine the oozing mussels and dripping oil, for example, throughout the project.

Bugs and Snakes
“The beetles were straightforward in their design but were driven procedurally because there had to be so many of them. One or two were hand animated for detail but a simulation was the only way to manage thousands of them. The principle challenge was to prevent them bumping into each other. Once we hit on a simulation the production liked, we needed to spend days sorting out the intersections. The tangle of snakes initially needed attention to get their arrangement, layout and overall sense of movement correct but came together well after this.”

When Snow White falls into the patch of mushrooms that emit a cloud of noxious pollen into her face, the artists had to digitally replace the mushrooms that had been captured in the footage in order to enhance their looks and improve the readability of the pollen animation. The effects concept artist re-designed the mushrooms as groups of small puffballs, each with an opening that could be animated to suddenly release the cloud of pollen, created using Double Negative’s in-house fluid solver dnSquirt. Nevertheless getting the pollen to be clearly drawn into her mouth was an edgy manoeuvre achieved parameter by parameter with some hand animation on top.

Simulation Wrangling
“While procedural techniques made all of these varied effects possible – fluids and smoky looks are procedural by definition – within the one sequence, it’s actually hard to give a simulation a very specific custom direction. While training the tools we had to do something precise, occasionally a render would pop out that simply hit the mark, and those were the ones we carefully hung onto; the pollen cloud was a tricky example of this. Our Dark Forest storytelling challenge was setting these simulations off with outcomes in our favour.”

The startling Bat Fairy was on the agenda from the start, basically as a request for a human sized bat. Unlike most of the other elements, it was created first very quickly in just two or three weeks for the trailer as a fully digital shot, as it was for most of its appearance in the film, complete with subsurface light and skin effects, fur and translucent wings. The initial idea for its zap-like animation came from the production’s sizzle reel.

The team worked through this sequence using essentially the same tools as the Dark Fairies required – Maya for modelling and animation rendered with RenderMan, with Houdini for effects, compositing in Nuke. Surprisingly, John and the team don’t mind so much that the camera doesn’t ever linger or return to any of these exciting effects, packed into about 50 shots. Their fleeting nightmarish quality told the Dark Forest story fast, furiously and effectively and that was all they had created them to do.www.dneg.com