The problem with the traditional “photofit” process is that the human brain recognizes faces holistically, not as a collection of isolated features.
In the Dark Ages of policing, an eye-witness to a crime would be asked to reconstruct the perpetrator’s likeness, jigsaw-like, from a daunting selection of isolated eyebrows, noses, and haircuts. This low-tech process tended to produce bizarre and inaccurate results.
This mismatch between human psychology and crimefighting methods may have been resolved by two research groups in the U.K. who have independently stumbled upon similar solutions as to how to produce a better “facial composite” of a suspect in a more psychologically natural manner. Peter Hancock and Charlie Frowd at the University of Stirling, Scotland, have produced a system called EvoFIT,1,2 while Chris Solomon at the University of Kent, England, has developed a similar system called EigenFIT.3 Now the race is on to commercialize both these systems and win the approval of the police and security services.
THE MEN FROM THE NORTH
So what is the problem with the traditional photofit process? Charlie Frowd from the Scottish team explains: “Peter knew that the normal method used to make composites does not work very well,” he says, “we are not good at describing and selecting individual facial features, but are very good at selecting whole faces which look like someone we’ve seen.”
Utilizing the fact that humans are better at whole-face recognition than cherry-picking of features, both systems use a computational search technique known as a genetic algorithm to “breed” together several faces that are judged to be similar to that of the suspect. Over several “generations,” the composite should approach optimal similarity to the suspect.
A genetic algorithm adopts concepts borrowed from evolutionary biology to “evolve” an optimal solution to a problem. The technique works by starting with an initial population, the search space, which can be randomly generated or narrowly specified. The Darwinian “fitness” of the individuals in the population is evaluated according to some criteria and the highest-ranked individuals are selected for reproduction. A new generation is produced from these “parents” through combination of their features and/or addition of random features (mutation). The fitness of the resulting “offspring” is then evaluated. The whole initial population, or just the worst-ranked individuals, are replaced with the offspring, and from that new population new parents are chosen to breed. This process continues until some terminating condition is reached.