skeleton pauses at the top of the staircase, and then fearlessly
dives headfirst, crumpling in a 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
Although his animation program is just a prototype, Faloutsos
believes it one day will 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, 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.
"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 Corp. video game that makes limited use of the
As the technology matures, real stunt artists have mixed feelings
about the impact they believe it will have 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, where they appear real
as they move within environments, such as sinking ships or burning
buildings, that could put real actors at risk.
Animation systems such as the one created by Faloutsos and his
former colleagues Michiel van de Panne, Demetri Terzopoulos and
Victor Ng-Thow-Hing, attempt to trump both.
The key is using mathematical formulas that only loosely
choreograph the movements an animator wants a character to
Command 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 guides
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
In video games, they crop up in programming that simulates such
action as racing or flying competitions.
With animated characters, attaining realism is far more
difficult, however. Emotion can influence movement as much as
"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?"
asked Darren Hendler, technical director at Digital Domain Inc., a
Los Angeles special-effects studio.
In the forthcoming movie 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
"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