A POLICE car rolls up to a house where the doors and windows are smashed in, rooms are ransacked and numerous high-value items are missing. Calming the home-owner, an officer begins to investigate: "Did you see the person who did it?" The shaken victim replies: "Yes, he had massive purple dreadlocks, green lips and was dressed like Michael Jackson."

Such an unusual perpetrator would be easy to identify in the physical realm, but this break-in took place in a virtual world, where odd-looking avatars are the norm. It may sound like an odd crime, but Japanese police have previously arrested virtual muggers, and the FBI has investigated casinos based in the virtual world of Second Life.

Virtual crimes will become more common as we venture more and more into these worlds, says computer scientist Roman Yampolskiy. To prevent this, multinational defence firm Raytheon, based in Waltham, Massachusetts, has a patent pending on fusing a person's real biometrics with their 3D avatar, so you know for sure who you are speaking to in a digital world.

Yampolskiy and colleagues at the Cyber-Security Lab at the University of Louisville in Kentucky are going one step further: they are developing the field of artificial biometrics, or "artimetrics". Similar to human biometrics, artimetrics would serve to authenticate and identify non-biological agents such as avatars, physical robots or even chatbots.

In Second Life, avatars are easily identified by their username, meaning police can just ask San Francisco-based Linden Labs, which runs the virtual world, to look up a particular user. But what happens when virtual worlds start running on peer-to-peer networks, leaving no central authority to appeal to? Then there would be no way of linking an avatar username to a human user.

Yampolskiy and colleagues have developed facial recognition techniques specifically tailored to avatars, since current algorithms only work on humans. "Not all avatars are human looking, and even with those that are humanoid there is a huge diversity of colour," Yampolskiy says, so his software uses those colours to improve avatar recognition.

The team also investigated matching a human face to an avatar generated from that face - previous studies show that avatars often resemble their owners. Combining their colour-based technique with existing facial recognition software produced the best results, suggesting it might be possible to track someone between the physical and virtual worlds. They will present the work at the Defense, Security, and Sensing conference in Baltimore, Maryland, this month...