The next time you see an action movie, look
closely at the actors performing the death-defying stunts –
they may not be real. Virtual stunt artists are being
developed that could ultimately leave the genuine article
looking for a new career.
Unlike previous computer-generated characters, which have
to be laboriously generated frame by frame, these virtual
actors respond to the physics of the real world thanks to the
use of a novel array of virtual sensors.
Computer-based stunt artists should eventually replace
nearly all real-life ones, says the system's developer, Petros Faloutsos,
now based at the University of California Los Angeles. They
can perform a vast array of acrobatic stunts that allow
directors to create complex yet realistic feats, without
anyone risking his life.
The basic virtual stunt artist takes the form of a properly
jointed skeleton figure that responds to forces produced by
gravity, friction and impact with other objects in its virtual
environment.
"A benefit of this is that you don't necessarily know
what's going to happen to a character until they've been hit,"
Faloutsos
says.
The skeleton can be dressed up to resemble a real member of
a film's cast – or in any other way.
The motion of 3-D graphics figures is governed by a set of
programs called controllers. Each type of behavior – running
or leaping over a wall, say – is governed by a different
controller. When these behaviors are combined, things become
even more complicated, which reduces the realism of
computer-generated figures.
To overcome this problem, Faloutsos
and colleagues Michiel van de Panne and Demetri Terzopoulos at
the University of Toronto developed a program to supervise the
individual controllers and make them work in concert. Each
controller has virtual sensors that keep track of variables
such as the character's center of gravity, its joint movement
and any points of contact between itself and the environment.
"Our controllers are aware of what is happening in their
environment," says Faloutsos,
who developed the idea in collaboration with Vancouver-based
computer animation company Motion Playground when he was at
the University of Toronto. This enables them to sense when
they fail, such as when the balance controller is unable to
recover after the character is knocked over. When this
happens, the supervisor program looks to the other
controllers. "Every controller is asked if it can handle the
situation," explains Faloutsos.
For instance, when the character has lost its balance, dive
and fall behaviors take over from the running controller.
For each type of behavior, the controller looks at the
effects of the virtual environment on each of the character's
joints and limbs, and the effects they have on each other to
determine what would happen next. Each of the joints in
Faloutsos's stunt artist is designed to work like those of an
average human – based on data from a biomechanical database.
It even performs instinctive reactions, like extending the
hands to protect itself while falling.
"Inevitably we will be replaced some day," says a resigned
Andreas Petrides, a stuntman based at Pinewood Studios near
London who was responsible for coordinating the stunt fighting
in "The Phantom Menace."
"It comes down to money," he says. "If computers can do
what I do but for cheaper then the studios will go with them."
He adds that virtual stunt people will be able to do things
that real stunt people can't do, such as falling from a
building and actually hitting the floor.
