Image: Image Metrics

Now Playing: 'Ilana' is a computer-generated character designed by IMage Metrics using standard modeling tools. To animate the character, Image Metrics of Santa Monica, Calif., used proprietary software to analyze videos of actress Ilana Rogul [inset] and create descriptions of their movements; it then attached these descriptions to the computer model in order to make the character move realistically.

Imagine Marlon Brando in Godfather IV, Sylvester Stallone boxing with a younger version of himself in Rocky VIII, or Marilyn Monroe playing opposite Johnny Depp. Such scenes may hit the big screen a lot sooner than you think.

Realistic human bodies created by computers already travel naturally across movie and game screens. Portraits rendered by computers are equally believable. To date, however, when animators have tried to make those portraits move, the illusion breaks down. Computer graphics have not been able to conquer the human countenance. Synthetic faces on the big screen have been more odd than realistic. Crossing that frontier will take not only advances by technologists but also the brilliance of artists and perhaps a lawyer or two.

On the technical front, developers have, during the past year or so, focused on motion-capture technology-that is, methods for converting the performance of a live actor into the framework for a computer-generated being-as the final step that will make a digitally rendered human believable. Two recent advances have filmmakers excited. But to understand the possibilities of the techniques, we first have to consider how one builds a computer version of an actor and why the results so far have fallen short of directors' visions.

A science-fiction dream for many years, Hollywood's quest to generate a realistic image of a human on screen began in the early 1990s, with the 1991 release of Terminator 2 and the 1993 release of Jurassic Park. These movies-and the money they made-excited the film industry's interest in the possibilities of computer graphics.

Previously, computer graphics had played only supporting roles, generating scenery or special-effects sequences. After Terminator 2 and Jurassic Park, computer graphics moved to the forefront of mainstream cinema. In The Mask (1994), the director put a digitized version of Jim Carrey's head at the center of the screen, doing impossible things in great green detail. In 2001, Gollum's computer-generated face started waxing lyrical in The Lord of the Rings trilogy.

And last year, few moviegoers who saw Pirates of the Caribbean: Dead Man's Chest noticed that the head of Davy Jones, a strange, part-human, part-cephalopod being, was a digital creation.

But the digital creatures in The Mask, The Lord of the Rings, and the second Pirates of the Caribbean movie had no counterparts in reality. We know that magical masks, creepy Gollums, and humanoid octopuses aren't real, so we don't overanalyze them; we don't compare what we see with our expectations. We just focus on the entertainment. The same principle makes Saturday morning cartoons entertaining: in a stylized medium, we ignore the faults and just have fun.

But the industry couldn't stop there. People thought that if something simple worked so well, something incredibly realistic had to be better. Ironically, however, the more realistic the computer-generated human, the more difficult it has been for an audience to accept it.