Who let the animals out of the zoo? They did it themselves in PDI/DreamWorks’ wacky feature animation Madagascar. Four escaped Central Park Zoosters become shipwrecked on a tropical island where a tribe of dancing lemurs … well, that’s not the point of this story. This story is about how a tribe of techno-wizards, animators and artists at PDI/DreamWorks escaped the confines of 3D modeling and animation and created a ‘toon with CGI tools.

All the characters in Madagascar are 3D. When Marty the zebra turns his head, the top part moves first, then his muzzle snaps into place, and then his lips catch up a split second later. Alex the Lion leaps from one rock to another and as he lands, his fingers stretch to five times their normal length. You probably won’t be able to see this until you can step through frames on a DVD. In the theater, the poses happen too fast. But it’s this combination of speed poses with extreme range of motion that helped the CGI characters behave like cartoon characters rather than 3D puppets. Their flexibility is made possible by a technique riggers Milana Huang and Rob Vogt, character TD co-supervisors, call "regional posing." With regional posing, body parts can be moved without affecting other parts: Hippo Gloria’s belly bounces but her hips don’t.

"Animators could over-rotate, scale, and quickly move back to a default," says Huang. "They could create snappiness in posing because they had regional posing and could break the structure. The rig allowed an extreme range of motion between poses, but the surface was still smooth and the character had a nice silhouette without strange wrinkles or deformations."

The ideas grew from the character design. "This was the first time we had a character designer involved in the rigging process," says Vogt. "It really helped to wrap our brain around the design before we started, because then we could develop every stage- joints, controls, deformations- in a way that supported the style."

For the body, the team began by constructing a "skeleton" and choosing articulation points, or joints. "It doesn’t look like a skeleton," Vogt says. "We don’t draw a femur; we draw icons with lines connecting them." The icons represent a local coordinate system- a joint’s rotation and translation. Each icon has a sense of what it’s parented to.

"If you click on a knee joint, an icon appears that represents its coordinate system," he continues. "Then, if you click on an ankle, another icon appears and draws a line from the knee to the ankle, the ankle’s parent."

Because the design called for simple silhouettes, Vogt and Huang used as few joints as possible. However, the graphic style sometimes forced extremes. Melman’s giraffe neck, for example has 27 joints. And the Alex’s lion tail has 48.

"The animators wanted to shape the tail and neck into hard corner poses," says Vogt. "Alex’s tail almost looked like a stair step." High-level animation controls created by the riggers made it easier for animators to manage all 48 joints.

"This is conceptually very similar to what you’d do in Maya," Huang says. "Place all the joints, come up with animation controls that drive rotation, translation and scaling, and then use various techniques for having those joints drive surfaces and deformation that affect the skin."

But the character rigs they created were not similar to the rigs used for most 3D characters – the animals in Madagascar were not puppets or photorealistic creatures.

"We worked really tightly with Rex Grignon and the animation team to determine what animation controls they needed," Huang says. "They wanted pose-to-pose animation, so we had controls that let the hips, shoulders, hands and feet work independently. And we built controls for exaggerated poses."

For example: The animators wanted to position Melman’s head in a frame and then shape his neck and adjust the body to fit. While doing that, they didn’t want the head to move.

"An animator could move Melman’s head independently even though it’s connected to 27 neck bones and his shoulder because, under the hood, we’re figuring out where the hierarchy should be backwards," Vogt says. "All the joints between are placed by mathematical routines any time one joint moves. The model is rebuilt for every pose from the skeleton out." In other words, the animators sculpt the model with each pose.

Huang continues. "We procedurally build his neck between his shoulders and procedurally place his legs between his shoulders and his feet. They’re all attached to one another and of course the animator expects this to be smooth."

To create the smooth skin even with extreme poses, the riggers relied on an elastic mesh — a rubber suit, if you will — that was built by the character TDs like a polygonal model. "It’s kind of like a set of rubber bands that are attached to each other," says Huang. "It allows the skin to relax from pose to pose."

So when animators moved the joint, the joints moved the skeleton’s "bones," the bones stretched (or shrank) the rubber suit, and the rubber suit drove the outer layers, which could be skin or clothing. And Alex’s fingers could grow to five times their length as he reached for a rock.

With this in mind, when the team needed a cast of thousands populating the streets of Manhattan, rather than modeling a variety of men, women, and children, Vogt and Huang created variations by using the animation controls.

"The system was designed so animators could squash and stretch the characters, but we used it to make stubby characters and tall characters just by moving the shoulders up and down," says Vogt.

For faces, PDI/DreamWorks’ facial animation system uses a substructure that’s anatomically based- the model is deformed from the inside out.

"With other systems, you might use 500 shapes for facial animation," Vogt says. "We animate faces using a single modeled shape and a bunch of programmed muscle tensions." When an animator chooses a pucker in a shape interpolation system, she’s picking a combination of modeled shapes. In PDI/DreamWorks’ system the animator still chooses a pucker, but it’s shaped by muscle tensions. "When the animator changes a pucker into a smile, our system is interpolating between muscle tensions, not blending interpolation points between shapes," says Vogt. "Creating the in-between shapes is where our system really excels."

As with the body, the riggers created animation controls that allowed the same kind of regional poses. "Animators could manipulate any part of the face," says Vogt. They could, for example, scale the eyes, stretch the upper face, rotate and translate muzzles, even bring the lower jaw down Tex Avery -style.

And, again as they did with the body, Vogt and Huang used these controls to sculpt variations for characters in crowds- widen a nose, push the ears back, make the eyes larger. "We played with the same mix of animation controls that the animators had," says Huang.

The rigging systems Huang and Vogt built live in a separate layer of technology that makes them portable, from one character to another and even to other models. The tail system helped move snakes and vines, for example. A foot-pivot system that kept characters’ feet planted to the ground in Madagascar has now spread to some Shrek characters.

And the concepts have taken root in other productions: "The crew on Over the Hedge looked at the work we did," says Vogt, referring to an animated feature in the PDI/DreamWorks pipeline scheduled for 2006, "and they’re pushing much farther than we did."

Once an idea like this escapes, you can never tell what wild applications might follow.

Although PDI/DreamWorks uses its Academy Award-winning proprietary software for animation, Huang and Vogt believe the rigging and animation control system they developed for Madagascar could be implemented in commercial software. "The value of the system is really the ideas," says Vogt. "The DreamWorks system is very open, so it’s easy to prototype an idea, but I think people could repurpose these ideas in other toolsets whether Maya, or XSI, or something else. These development platforms are different from the DreamWorks platform, but they’re still toolsets in which you can develop ideas."