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Computers are taking the fall for stuntmen

By Duncan Graham-Rowe

January 30, 2002

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.

This article is excerpted from New Scientist, a weekly science and technology magazine based in London. Visit

Copyright 2002 Union-Tribune Publishing Co.

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