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THE next time you see an action movie, look closely at the actors performing
the death-defying stunts鈥攖hey 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 鈥渟ensors鈥�.

Computer-based stunt artists should eventually replace nearly all real-life
ones, says the system鈥檚 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. 鈥淎 benefit of this is that you don鈥檛
necessarily know what鈥檚 going to happen to a character until they鈥檝e been hit,鈥�
says Faloutsos. The skeleton can be dressed up to resemble a real member of a
film鈥檚 cast鈥攐r in any other way.

The motion of 3D graphics figures is governed by a set of programs called
controllers. Each type of behaviour鈥攔unning or leaping over a wall,
say鈥攊s 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鈥檚 centre of gravity, its joint movement and any points of contact
between itself and the environment.

鈥淥ur controllers are aware of what is happening in their environment,鈥� 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. 鈥淓very controller is asked if
it can handle the situation,鈥� explains Faloutsos. For instance, when the
character has lost its balance, dive and fall behaviours take over from the
running controller.

For each type of behaviour, the controller looks at the effects of the
virtual environment on each of the character鈥檚 joints and limbs, and the effects
they have on each other to determine what would happen next. Each of the joints
in Faloutsos鈥檚 stunt artist is designed to work like those of an average
human鈥攂ased on data from a biomechanical database. It even performs
instinctive reactions, like extending the hands to protect itself while
falling.

鈥淚nevitably we will be replaced some day,鈥� 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.

鈥淚t comes down to money,鈥� he says. 鈥淚f computers can do what I do but for
cheaper then the studios will go with them.鈥� He adds that virtual stunt people
will be able to do things that real stunt people can鈥檛 do, such as falling from
a building and actually hitting the floor.