During the past decade's animation boom, feature-length cartoons have given us lions and dinosaurs and dragons, and, on the digital front, insects. ButIce Age, the first feature from Twentieth Century Fox Animation unit Blue Sky Studios, is also the first to weigh in to a great extent with prehistoric mammals: The main characters in this computer-animated frolic are lovable woolly mammoth Manfred (voiced by Ray Romano), Sid the sloth (John Leguizamo), and Diego, a sinister saber-toothed tiger (Denis Leary). There are also a few early humans on hand, but unlike their furrier counterparts, they don't talk.

Fur presents one of the more difficult technical challenges in digital animation. “The easiest things for computers to render are things that look synthetic, molded, or geometric,” says Ice Age director and Blue Sky co-founder Chris Wedge, who slyly offers toys and insects as examples. “Probably the most difficult thing for computers to do are exteriors, because, as simple as it sounds, the light comes from the sun and hits things and casts shadows. Getting the diffuse nature of the light that you see outside, and the atmospherics that occur as you gain distance from the camera, is much more complicated than analyzing the interaction of light and color that occurs between objects.”

So, you've got seriously hairy creatures against bright, icy, sweeping backgrounds: What do you do? “Our first approach to those complexity issues was to stylize the design of the film,” says Wedge, who won an Oscar for directing the 1998 Blue Sky short Bunny. “There's always a compulsion to go in and make things photorealistic, but we knew that it was going to be (a) logistically impossible for us to do that, and (b) not very interesting. In my opinion — and I got to use my opinion a lot, because I directed the movie — one of the best reasons for making animation is that you're going to exaggerate something or abstract it somehow. So the trick was to simplify the environments and characters as much as we could, and let the lighting add the kind of visual complexity that we associate with the real world.”

This is made possible by CGI Studio, the proprietary software of Blue Sky Studios, which is based in White Plains, NY. Primarily developed by research and development director Carl Ludwig, CGI Studio is based on raytracing, which simulates the way light rays are emitted from sources, strike surfaces, and interact — through reflection, absorption, and bounce — with the world. The software contains controls for camera placement, focal distance, aspect ratio, and depth of field, as well as position, size, intensity, beam spread, color, and modeling of light sources. It exists as a scripting language for three-dimensional visualization, and builds images using advanced geometric models, while competitors like Pixar and PDI/DreamWorks use rendering systems built around simpler polygonal shapes, and rely on lighting designers to add shading and dimension. “Our technical directors act more like photographers,” says Wedge. “We let the computer do all the complicated stuff.”

Raytracing has often been considered prohibitively time-intensive; on Ice Age, the average render time per frame was between seven and 15 hours. “Probably some of our competitors would be aghast if their frames took more than four hours,” Wedge says. “But we've found that we spend much less time in the design cycle, and the technical directors get to a solution much faster because they're working with something that simulates light more closely.” In addition, the director says, Blue Sky uses “sophisticated sampling algorithms” to determine the “smartest place and the smartest time to fire a ray.”

Now, back to Manfred and company. Says Wedge, “The fur technique we've come up with simplifies the amount of fur. Our woolly mammoth has a shaggy, layered haircut, but you see serrated edges, which are put in there by design to remind the audience that they're not looking at something real.” Lighting is still extremely complex: Each one of Manfred's chocolate-brown hairs creates highlights and can cast a shadow on the hair behind it. But light rays in a digital environment can be tricked, says the director. “You can tiptoe around the set like a ghost and shine lights on some characters and not on others, and you can tell the computer this guy's casting a shadow, just don't cast it here.”