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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".
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The virtual stunt actor
falls and dives realistically |
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 in Los Angeles. They can
perform a vast array of acrobatic stunts that allow directors to
create complex yet realistic feats, without anyone risking their
lives.
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," says
Faloutsos. The skeleton can be dressed up to resemble a real member
of a film's cast - or in any other way.
Combined controllers
The motion of 3D graphics figures is governed by a set of
programs called controllers. Each type of behaviour - running or
leaping over a wall, say - is governed by a different controller.
When these behaviours 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 centre 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 behaviours take over from the
running controller.
What happens next?
For each type of behaviour, 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. | 
