Science: Marilyn lives!

Computers can now bring dead stars back to life. So prepare to meet the new breed of synthespians.

Last month, in a small lecture theatre in London's West End, Marilyn Monroe made her first appearance in more than 30 years. Marilyn hasn't yet been returned to the land of the living, but thanks to a team of British computer scientists, special effects experts and a Rada actor, she briefly graced the screen once again.

For many people the creation of computer-generated actors - synthetic thespians, or "synthespians" - is a typically Hollywood-style gimmick. While it is probably true that the technology required to create such effects will remain the preserve of the big film studios, the team behind the virtual Marilyn has other ideas. The technology that the team is developing will cost far less than current systems, making it available to more experimental film-makers. "We want to create a kind of digital Dogma," said Peter Martin, of Createc, the commercial arm of the National Film and Television School, which is one of the companies involved in making Marilyn live again.

Synthespians are unlike the crude characters that populate computer games. The ultimate goal is photo-realism: virtual characters that are indistinguishable from real people. This is already possible, but expensive. There were probably more synthespians than real actors in Titanic, and a virtual Tom Cruise performed stunts in Mission Impossible which the real Tom Cruise was understandably reluctant to attempt. The problem is that the cost of such creations is huge. Only the biggest studios can afford fees of more than pounds 600,000 for a single minute of footage. Createc's advantage is definitely on price.

The concept behind the creation of synthespians is simple. Every one is "driven" by a real actor. Creating a synthespian involves mapping the movement and expressions of a real actor on to a synthetic alter ego. All the computer does is to transfer the actor's movements and expression on to the synthespian. In essence, synthespians are highly sophisticated puppets - with a real actor using a computer to pull the strings.

The process starts with the actor acting in front of a normal camera. A computer analyses the acting itself and categorises it in terms of the actor's identity, expression and pose. Next, the synthespian itself has to be created. In the case of Marilyn, the team were able to use a normal photograph to create an amazingly realistic 3-D image. In principle any image can be used - the audience was also treated to a 3-D Mona Lisa. Once the 3-D model has been generated it can be made to "act" using the data from the computer analysis of the real actor.

If synthespians are ever to look and feel realistic, they will have to improve on the flat feel of existing virtual characters. Increased computing power is the key. The faces and bodies of synthespians need huge flexibility to re-create all the subtleties of human motion. Similarly, the analysis of the real actor's motion has to be sensitive enough to capture these subtleties.

But this technology is now becoming affordable. "Digitation is beginning to enter areas that you wouldn't have expected to become digital," says Andrew Berend, of Createc.

Many film-lovers may find the idea of dead actors gracing the silver screen a bit of a nightmare, but Createc and its partners have even more in store for us.

Dominic Wright, a young director working for Createc, has already produced a short film using the technology. Crow Jane - the title is taken from a Nick Cave lyric - follows a young woman who lives in a forest outside a small town. The film is entirely computer-generated and was produced cheaply using Createc software and a camera developed by Glasgow University's Turing Institute.

But what of Hollywood's plans for synthespians? Its use, especially in crowd scenes, is likely to become more common. Real extras get wet when it rains, have to be fed, and must be paid to come back if shooting doesn't go to plan; synthespians are cheaper and less demanding.

Bad news for extras, then, but the use of virtual actors will probably be limited to stunts and crowd scenes. The flaws in synthespians' appearance and movement are obvious in close-ups, and whatever the improvements in the technology, it is likely that some aspects of an actor's performance will never be digitised. Ellis Jones, vice-principal of Rada, agrees: "Hugh Grant will play the lead role in romantic comedies for some time to come."

However, Jones and the Createc team highlight the way in which synthespians can give human actors more control over their characters. The acting skills of real actors should be able to shine through into the synthespians' performance. Whatever the looks, age or sex of the human actor, only his or her skills as a thespian will limit the performance of the synthespian they control. Actors could suddenly become free to play very different roles - characters of different age, sex or even species. Digital acting could create a new era in artistic expression.

The writer is education officer of the Science Museum in London