The best thing about animation is its capacity for invention and imagination, its ability to go places and do things off-limits to live-action film. But what happens when your subject isn’t a wacky creature, but a human being?

A whole new set of challenges arises as the moviemakers try to strike a balance between animation’s fanciful potential and the believability expected with the depiction of people.

Two new big-budget movies tackle these challenges with novel technology and winning results. One, Pixar’s humbled superhero saga “The Incredibles,” opened Friday with some nuanced new wrinkles in computer animation. The other, “The Polar Express,” arrives Wednesday bearing a painterly look achieved through mind-blowing (and not easily understood) filmmaking advances.

Together, “The Incredibles” and “The Polar Express” represent bold steps in the field of humanimation.

“Our goal was to do something patently unreal that feels very believable,” says Brad Bird, the “Incredibles” writer and director.

That’s a tough balance, but it’s essential to the movie’s story and themes. The heroes of “The Incredibles” are superheroes forced to enter a sort of suburban relocation program after all “supers” are targeted by a rash of lawsuits. As they navigate civilian life before jumping back into action, their dual nature as ordinary people capable of extraordinary feats comes into sharp focus. Their humanity is essential.

The biggest challenge was gauging how much detail to use, when to hold back and when to go all-out. Bird and the Pixar folks solved this problem by going beneath the surface. The computer models for the characters were given muscles that move with them. They also benefited from a process called subsurface scattering, which is based on what actually happens when light hits skin.

“The light doesn’t just hit the surface of the skin, it also goes beneath the surface,” says Bird. “It hits blood vessels, and that scatters the light under your skin. It’s a very subtle effect, but it’s the difference between flesh and plastic. You don’t have to do some of the surface details. We didn’t do hair follicles or pores. And it gives the character a kind of a glow.”

“The Polar Express” features a completely different approach to capturing human movement and emotion. If you’ve seen the trailers, you might have noticed that the characters, five of them played by Tom Hanks, look almost real. That’s because, oddly enough, they are real.

The movie’s technological trump card is called performance capture, and it’s a lot more elaborate than subsurface scattering. On the most basic level, the characters aren’t really animated. Actors wear body suits covered with reflective jewels, which are also affixed to their faces. Digital cameras record the movements of the bodies as an assortment of three-dimensional dots. The characters are then integrated into virtual sets, created in computers. (A similar motion-capture process was used to bring Gollum to life in the “Lord of the Rings” movies).

“Performance capture is key to trying to do something that has always been very difficult for animators to do, which is animate correctly proportioned characters,” says “Polar” director Robert Zemeckis. “All the performances in the movie are digitized and captured in real time, along with the voice, in three dimensions. That’s why I think you have that wonderful ability to have those subtle things that actors intuitively do that it would take an animator way too many hours to try to draw.”

How important are correctly proportioned animated characters? The question gets at the relationship between animation and realism, and the matter of whether animation should embellish reality or replicate it. Zemeckis sees motion capture as the perfect way to re-create the impressionist look of Chris Van Allsburg’s beloved children’s book. “The Polar Express” looks like a painting come to life.

“Chris Van Allsburg never wanted the movie to be animated, because he knew the animation would have to simplify and cut the heart out of his art,” says Zemeckis. “In this particular case, photo-realism (or live action) would take a lot of charm out of the movie as well.”

“The Polar Express” does look magical, but that’s mostly because of the settings and effects – in other words, the stuff that isn’t real. The characters certainly move convincingly, as well they should – they’re human. But the vacant look in their eyes is a bit eerie, like the neither-here-nor-there faces that made the 2001 space adventure “Final Fantasy” such a flop.

“Some of the characters feel cold,” says Dr. Clarke-Miller of the Art Institute of California. “They don’t seem to light up. There’s no expression.”

Bird, the “Incredibles” director, has spoken about the importance of caricature in animating humans – the art of capturing a character’s abstract essence through creative exaggeration. We see this in “The Incredibles” with Incredible’s oversized head, shaped a bit like a pineapple, and his daughter Violet’s floppy mane of black hair and big, nervous eyes.

“Animation is about catching the essence of something and being truthful in the essence, but being a little bit pushed in the execution,” says Bird. By contrast, “The Polar Express” is after something closer to reality, hence the use of real actors and motion capture.

But, at the end of the day, the same basic challenge faces animated and live-action films: If the story doesn’t work, neither does the movie. And if the characters don’t work, neither does the story. Great writing is just as key to Pixar’s incredible run as great animation. As Bird puts it: “Animated films have all the strengths and weaknesses of live-action films. Some of them just have a little more going for them than others.”

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