'Open Season' Diary: Water, Hair & Cloth VFX

From the AWN/VFXWorld Exclusive Open Season Diaries.

Open Season contains some of the richest detail in hair/fur, cloth and water technology in CG animation. Fortunately, the directors (Roger Allers, Jill Culton and co-director Anthony Stacchi) were able to call upon the talents at Sony Imageworks to devise new tools and methods for achieving the desired looks seen in the film.

Hair

Imageworks custom hair system uses a Maya interface and some Maya tools, but the combing, visualization controls, lighting and the link to RenderMan are all Imageworks proprietary technologies. As with most hair and fur systems, this one relies on guide curves interpolated during rendering into millions of hairs, and texture maps to control the look of the hair.

For the stylized look of the furry characters in Open Season, the team needed additional tools for clumping and matting the hair to achieve specific designs.

For clumping, the combing artist could designate an area of influence around a curve that would attract all the hairs within it. Additional clumping algorithms were added to the hair system to help matte and breakup the hair. This allowed artists to sculpt specific shapes into Boog's fur. Geometry was created to represent the hair volumes; this helped to simplify the contours of the comb, and later filled with curves that then became the final hair. Combers could visualize how the fur would look before rendering using a custom gl viewer within Maya; the guide curves also provided the stylized profile.

Although the dynamics in the hair system would cause the hair to bounce, relax and settle into an initial shape as a character moved, sometimes the animators wanted the hair to snap into place. For this, the animators used target shapes. They could increase the percentage of dynamics that would be affected by the targeted shape during the simulation and the affected fur would try to adhere to that shape.

Hairstyles were needed for scenes in which the furry characters were underwater, wet or dirty. Specific wet combs were developed that had sharper clumping, while tighter specularity and more reflection in the fur created a wet look.

Fur Volumes

Fur volumes were created to help visualize the characters true size. This was very important for our animals that had longer hair. Using low-res proxy versions of the skin, the artists were able to approximate the length of the hair. This was used initially for buy off in the character modeling stage; later the surface was bound to the characters skin for animators to properly stage the performance, and this fur volume was also used as the guide for initial combing.

Another challenge was that both Boog and Elliot had to be able to go from a biped to quad pose within the same shot. The technical directors first created the two separate combs for standing and then for the character on all fours. Once they had both combs they then needed to blend them together using the same set of control curves; this was tricky because the technical directors did not want to compromise either design.

Because the fur grew from the skin, when shapers pulled the skin into exaggerated contours, the fur moved as well. For extreme motion, however, the technical directors would simulate different looks and blend between simulations.

Cloth

Because the characters costumes needed to adhere to the shape animators gave them, and to maintain a simple, graphic style, the crew selected only the clothing that was absolutely necessary to simulate. Shaw, for example, wore a thick, puffy vest that was simulated, but the shirt underneath was not. To maintain the vests contour, the technical directors used a targeting system for the cloth that was similar to the one used for hair to blend the cloth into predefined shapes. One area of cloth could be fluid; another stiff. To keep the vest looking puffy, an inner piece with a collision offset to the skin rested between two layers and a volume was bound to the inner piece. As Shaw moved, deformers compressed his vest to maintain volume and thickness.

Similarly, Park Ranger Beth wore a jacket and shorts that had to maintain a bell shape, so target shapes helped keep the character on model. Beths shorts, in particular, needed to retain a bell-like silhouette whether she was standing still around or in motion. Her sleeve cuffs had to retain the same bell-like quality, yet add some secondary dynamics to create a cloth-like feel. To avoid the wrinkle detail and folds that would normally occur in a typical cloth setup, the crew needed to define separate regions of control for the targeting system and set the constraints to stronger values throughout the costume.

The hunter (Shaw) proved to be a unique challenge to the crew. They needed to create a costume that would retain volume through a large range of motions, from normal conversations to fast moving, wildly erratic fight scenes both dry and wet. To achieve this, his jacket was comprised of simulated cloth garments driving an animation/deformation rig, which in turn will be bound to the final rendered cloth costume piece.

Character animators worked with the characters wearing a tube of cloth offset uniformly from the body. Once the animator finished, the cloth department switched dynamic cloth for the tube. If the characters had realistic motion, the dynamic cloth would move appropriately. But because the animation had unrealistic timing a character could accelerate between two poses in two frames the cloth could explode off the character. To solve that problem, the crew cooked the simulation up to the point where it exploded, processed that data and then extrapolated between the two poses.

Cloth simulation is based on a spring system that tries to limit the amount of stretch in a garment, and for realistic animation it works very well. In Open Season, however, the rigging could stretch an arm twice its length but cloth would not stretch that much. To solve that problem, the cloth was simulated with default character settings and then weighted blend tools stretched the cloth after the fact.

Because the clothing was part of the character design, special shaping tools in animation maintained those shapes throughout the movie no matter how the characters were moving.

Water

It was a challenge to keep the water on style and believable. Although the crew could simplify the look, the movement had to feel like water. In the river rapids chase sequence, for example, Boog breaks a beaver dam, which wipes out the other animals homes and flushes them onto the hunters ground. During this sequence, the animals cling to logs as a current rips down the canyon.

To simulate the water, Imageworks developed a Houdini-based 3D and 2D fluid solver that worked on a macro level for large bodies of water and a micro level for splashes and character interaction. The first problem to solve was the flowing water. Next was creating interaction with the environment, the characters and producing foam, splashes, white water, mist and waterfalls. A separate drip system, the 2D solver and the 3D solver were integrated into one package. 2D simulations were used for the rivers; 3D for water flooding dry land, and breaking waves.

Because the fluid dynamics simulator had to allow for art direction, some mathematical assumptions were made to speed up processing time, which allowed for more iterations.

For some shots, the character would be animated first and water added later. The simulator could isolate water around a character and work at a fine level to create splashes and water dripping off the characters.

For other shots, the water was simulated first and then layout would design the camera move and animators would work with the water surface. For these shots, the layout department needed a representational water surface. The water department would run a pre-sim using a simplified set of particles to generate overall shapes for shot approval. The result, a polygonal mesh, showed wave shapes, troughs and wavelets. The polygonal surface delivered to animation and layout allowed the animators to knowingly place characters within the water. Once the performances were approved and timing adjusted, the crew ran the final simulation.

The river rapids chase sequence represents a culmination of efforts from the entire crew. From the very first tree design, to the wonky factor of production design, the striking shape language, the extreme squash-and- stretch animation and, of course, the effects. The directors knew they had an opportunity to create a dynamic chase sequence, but when you look at it logically, how could they get away with a truck driving through the river? Then again as the directors often pointed out its a cartoon with talking bears!

Various artists at Sony Pictures Animation, who worked on Open Season, have contributed to the writing of this series of production diaries on the making of the film.