LOS ANGELES -- 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, too," Faloutsos said.
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 video game that makes limited use of the
technique.
As the technology matures, real stunt artists have
mixed feelings about the impact they believe it will increasingly
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, like sinking ships or
burning buildings, that could put real actors at risk.
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.
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."
