When Pixar animators were creating this year's hit movie ''The Incredibles,'' they noticed a certain limpness in the movements of a key character, the diminutive fashion diva Edna Mode. Her skirt appeared to sag and crumple as she walked. The animators could have taken the trouble to iron out the glitches frame by frame. But they devised a more clever solution: the studio fitted Edna with a virtual petticoat. While her underwear is never actually seen onscreen, it nonetheless helps keep her clothing in place.
Welcome to the world of invisible animation. Hollywood's computer animators have had great success when it comes to depicting the human body in motion. Their portrayal of shirts, pants and jackets has proved to be equally lifelike and impressive. But when animators program computer systems to mimic the way interwoven fibers interact with skin -- that is, when virtual clothing is put on the virtual person -- the results are hard to predict and often go awry. Simulated cloth routinely gets snagged in armpits and groins or flutters and tangles spontaneously. Directors simply do not know in advance what an ordinary shirt will do once it is fitted to a moving torso.
In the face of persistent wardrobe malfunctions, animators have discovered the virtues of introducing a virtual garment that cannot be seen onscreen but nonetheless alters the computer modeling in a desirable way. For instance, when Tom Hanks's conductor's jacket in ''Polar Express'' kept flapping violently in the wind, it was easier to wrap him inside an invisible shroud than to smooth the jacket out by hand.
And while testing a scene of ''The Incredibles'' in which the once-dashing Mr. Incredible is demoted to a dead-end insurance job, animators noticed that his barrel chest kept tugging his button-down shirt out of his trousers. Rather than repeatedly halting production to tuck the shirt back in, they fell back on an old costuming trick and simply sewed his shirttails into a custom-fitted pair of virtual briefs.