Video: Animated walking
CHILDREN do it with ease, but walking on two feet is challenging for robots. And while animated characters stroll along quite happily, they rarely look human when they do.
That's because the many joints of a human body can move in multiple directions, creating a bewildering array of potential poses. Marshalling them to make a humanoid walk is no simple matter. "It's like driving a car with 50 steering wheels," says Michiel van de Panne, a computer animation researcher at the University of British Columbia in Vancouver, Canada, who is one of many trying a new approach to giving robots and video game characters more realistic gaits.
Simulating physical processes helps to make virtual worlds look realistic. For instance, a technique called ray tracing mimics the way light reflects off objects. But animating characters is generally a more superficial process. Although motion-capture technology can record real-life movements for virtual characters to mimic, it is impossible to give a character every variation of movement it will need. A better approach would be to use the simulated physics approach to enable virtual characters to "just generate motion on the fly", says Martin de Lasa of the University of Toronto, Canada.
Animators first considered this approach back in the 1990s, but slow computers and a limited understanding of biomechanics deterred them. These limitations no longer apply, says de Lasa. For instance, UK firm NaturalMotion has developed virtual characters that react to whatever they encounter in a realistic way through an approach called "simulated evolution".
The process begins with a population of virtual skeletons controlled by a network of virtual nerves. Each skeleton has a slightly different network, affecting its ability to walk. Those that can walk furthest are declared "most fit" and are used to spawn the next generation, in which a subset of the nerves are slightly altered. Over several generations the skeletons automatically "evolve" into better walkers. The firm's animations appear in many computer games and movies.
However, because the NaturalMotion control system self-evolves, it's not clear how it works, which might cause engineers to pause for thought before applying it to an expensive robot. Van de Panne and de Lasa are converging on a more readily understood approach - and both presented their results last week at SIGGRAPH, the annual computer graphics conference, in Los Angeles.
Instead of animating a character by controlling each joint independently, van de Panne and de Lasa have developed overarching controllers. These create simple rules that specify how the character should behave, and the individual joints simply move to obey them.
In van de Panne's model, for example, once the path of the swinging foot is specified by the controller, the angles between various joints in the leg and hip are automatically calculated. And because the path of the swinging foot governs the movement of the limbs - not vice versa - the controller readily adapts to different body types.
De Lasa, working with Igor Mordatch and Aaron Hertzman, also at the University of Toronto, has focused instead on how the position of the feet and torso affect the centre of gravity. It might seem simplistic, but when applied to a virtual body with realistic limb proportions and biomechanics, other behaviours emerge naturally. For instance, as the feet move during walking, the centre of gravity shifts in a way that threatens to break the controller's rules - to compensate, de Lasa's character (pictured) automatically began swinging its arms, just as humans do. It will even "windmill" its arms if pushed. "You essentially get all of these things for free," de Lasa says.
Van de Panne and graduate student Stelian Coros are now working on implementing their work in humanoid robots.
De Lasa knows very well the challenges of robotic walking, being a former member of the Big Dog robotic mule project at Boston Dynamics. Big Dog exhibits eerily realistic walking behaviour, recovering its balance after slipping on ice, but it has one major limitation: its sophisticated control system works only for the robot's dimensions and design.
Chris Atkeson, a robotics researcher at Carnegie Mellon University in Pittsburgh, Pennsylvania, says that such bespoke control systems require expertise to design. "It's an art," he says. "You essentially need a Michelangelo to do it, and there aren't that many Michelangelos." He says that the new strategies could spur the development of controllers capable of adapting to different robotic forms.
For now, de Lasa is encouraged by the fact that different research groups are converging on similar methods to achieve realistic walking. "It gives us confidence that we're on the right path."
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