The animators would keyframe a number of Tooth Fairy guide creatures for the technical directors to follow.

Of Stones and Swarms
Among the movie's many unusual characters, the stone creature was one of the most challenging to visualize. Its very nature implied a tricky combination of hand animation and simulation. "Our goal was to achieve a complete sense of performance driven by keyframe animation enhanced with dynamics," says Andrew Whitehurst, CG lead on the sequence. "To that end, we did the key frame animation on a non-dynamic giant first to get the basic performance. In order to add the rocks breaking up in a controllable way, we developed a system that would allow us to make certain chunks of rock become dynamic at defined moments. For example, if the giant slammed his hand down, we could choose some rocks around that arm to become dynamic when that event occurred. It enabled us to direct when the dynamic moments happened in a way that emphasized the keyframe animation, rather than trying to simulate everything at once.

"We engineered the system so that we were able to fine-tune the simulation on any particular boulder. We could make all the other rocks in the simulation non-dynamic again, and just tweak the simulation for that one boulder until we got what we wanted. It was a case of using simulations to get something naturalistic, but controlling when and how the dynamics happened to make them work aesthetically and narratively."

Due to their sheer number, the tiny Tooth Fairies presented their own set of unique challenges. The hero creatures were created using hand animation to obtain a defined performance. The rest of the swarm was animated via a particle-based system that used cycles of hand-animated behaviors. "The animators would keyframe a number of guide creatures for the technical directors to follow," Butler explains. "The TDs would then implement Double Negative's proprietary swarm tool to create a particle simulation. This sim would form the basis of the speed and direction of the creatures. Animation cycles and behaviors were sampled and combined to create the creature's actual motion. Many iterations were needed at the start of production to find the behaviors that worked best."

A Mammoth Effort
During the climax, the colossal Elemental makes a spectacular arrival through a city street, ripping pavement apart and tossing cars around. The destruction effects were created using dynamics, with cloth simulations added to crumple the cars panels. Additional dirt and debris passes were rendered using Double Negative's fluid simulation and volume rendering pipeline.

The Elemental itself was particularly challenging from an animation standpoint. "It was mostly tentacles with no jointed limbs," observes Colin McEvoy, animation lead on the sequence. "It took a lot of work to animate appealing shapes and motion with its many tentacles because of the complexity of the animation controls. It was also extremely large, but needed to move dynamically."

Adds Butler, "The process of animating the Elemental was centered around the primary animation principles of line of action, silhouette and negative space. As the size of the creature meant its entire form was not in the frame for most of the sequence, we tried to build appeal in the animation of the creature by focusing on the abstract shapes created in the composition by the flailing tentacles. Once we had established the basic poses, we would build on them by animating passes to create weight and dynamism."

The vegetation on the creature was a secondary animation system that was run as part of the lighting pipeline. Other effects passes, including steam and drips, added extra layers of detail and realism. "There was a huge amount of work in compositing putting all this together, bringing in other elements of steam and atmospherics, pyrotechnics, etc., to make it all work," Martin remarks.

Digital Enhancements
Parallel to this full CG effort, the team was dealing with humanoid creatures that "only" required digital enhancements on practical make-up effects. It included creating Abe Sapien's eye blinks in 2D, a task assigned to Budapest's Cube.

The character that needed the most digital enhancements was Wink, a huge creature built on steroids. "Wink often needed to have his mace fist replaced digitally for his fight scenes with Hellboy," Butler observes. "We also provided a completely digital Wink for shots in the Troll market where the actor couldn't safely perform the stunt or action required. In one particular shot in the sewer, we are introduced to Wink as he emerges from the shadows. The director shot it practically but didn't like the lighting when he saw it on film. He wanted to keep the character in shadow and reveal him later in the movie. We ended up tracking the practical Wink and replacing him entirely with a differently lit Wink!"

Adds Martin: "The Troll Market sequence is a good one for the combination of practical and digital effects. It's predominantly practical, but there are a lot of digital additions -- from something as simple as adding digital eye-blinks, to replacing part of a main character (like the mouth and jaw, or a mechanical fist) to putting full digital characters into a shot among practical creatures (guys in suits)."