By ANDREW BRIDGES, AP Science Writer

LOS ANGELES (AP) - The gangly skeleton pauses at the top of the staircase and then, fearlessly, dives headfirst, crumpling in an apparently bone-jarring fall.

Petros Faloutsos chuckles as he replays the clip on his laptop computer. Again and again, the UCLA scientist commands the virtual character to dive.

The animation is primitive, the technology complex.

Beyond the initial command to jump, the fall is completely unscripted. Physics, not the computer animator's mouse, controls the action.

Although just a prototype, Faloutsos believes his animation program will one day allow virtual stunt artists to replace their flesh-and-blood counterparts in performing otherwise deadly feats of derring-do.

"Maybe people will be directing virtual actors, and we'll have to give them Oscars (news - web sites) too," Faloutsos mused.

The brief clip is a glimpse into the nascent field of physics-based animation. The technique, whether used for movies or video games, strives to create a virtual world consistently guided by the same physical laws that give order to the real world (news - Y! TV).

"It's the Holy Grail of character animation. Everybody wants to do it, but there's not a whole lot of it out there right now," said Damien Neff, senior artificial intelligence designer for NFL Fever 2002, a Microsoft video game that makes limited use of the technique.

As the technology matures, real stunt artists have mixed feelings about the impact on their craft.

"There's a positive side and a negative side: To talk positive, it's made it safer to do a stunt — you don't have to lay your neck out on the line as much as you used to. But it's taken some cash away also," said Ben Scott, a Hollywood stuntman who works on the HBO series "Six Feet Under."

Traditionally, animators have relied on their own talents to draw characters that appear to move naturally.

Movie studios and game developers also bank increasingly on libraries of hundreds of stunts amassed by filming the sensor-studded bodies of real performers. Those "captured motions" can then be matched to virtual characters and inserted into movies or games.

Animation systems such as that created by Faloutsos and his former colleagues Michiel van de Panne, Demetri Terzopoulos and Victor Ng-Thow-Hing attempt to trump both methods.

The key is using mathematical formulas that only loosely choreograph the movements an animator wants a character to undertake.

Command, say, a character's arm to move and the momentum will force its torso and head to shift as well.

The range of motions available to a character ultimately guide how it behaves, as does its own computer-generated sensitivity to both gravity and any forces imparted by its virtual surroundings.

Different environments, for example, will prompt the same character to move differently — and unpredictably. A fall on slick ice won't be the same as one down a steep flight of stairs.

In movies, physics-based animation techniques have been used to render inanimate things like the waves in "The Perfect Storm" or the shock of blue hair that coats James P. Sullivan in "Monsters Inc."

In video games, they crop up in programming that simulates such action as racing or flying competitions.

With animated characters, attaining of realism is far more difficult, however. Emotion can influence movement as much as gravity does.

"You can tell from how someone is walking if they're effeminate or angry. How would you account for that in a physics-based system?" said Darren Hendler, technical director at Digital Domain Inc., a Los Angeles special effects studio.

In the forthcoming film "The Time Machine," Digital Domain used a physics-based animation technique to render the collapse of thousands of skeletons of people turned to dust and bone.

Animators still shy away from using physics to model the movement of people, however. They say the human eye is just too good at spotting even the slightest hint of fakery.

But Faloutsos believes future systems will allow directors to guide characters as they do live actors. "The ultimate goal is to have a totally complete human inside the computer that you can direct," he said.

Until then, officials with the Screen Actor's Guild know there will be work for the more than 6,600 Hollywood stunt artists the union represents.

"People, quite honestly, like to see human beings on the screen," said Ilyanne Kichaven, a guild spokeswoman. "There's still something an actor can bring to the screen that a computer-generated person cannot."

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