Prometheus 6

I can think os so many ways to frustrate the very attempt...like walking with someone close enough that your shadows overlap. Or on a crowded street. Or on irregular ground.

Shadow analysis could spot terrorists by their walk
04 September 2008
From New Scientist Print Edition.

Nearly seven years after Osama Bin Laden disappeared, US intelligence agencies are still chasing his shadow. And shadows are precisely what they should be looking for, says NASA's Jet Propulsion Laboratory in Pasadena, California.

By analysing the movements of human shadows in aerial and satellite footage, JPL engineer Adrian Stoica says it should be possible to identify people from the way they walk - a technique called gait analysis, whose power lies in the fact that a person's walking style is very hard to disguise.

Video taken from above shows only people's heads and shoulders, which makes measuring the characteristic length and rhythm of a person's stride impossible. That's not true of shadows, though, Stoica told a security conference in Edinburgh, UK, last month. Shadows, he says, provide enough gait data to deduce a positive ID. To prove it, he has written software that recognises human movement in aerial and satellite video footage. It isolates moving shadows and uses data on the time of day and the camera angle to correct shadows if they are elongated or foreshortened. Regular gait analysis is then applied to identify people. In tests on footage shot from the sixth floor of a building, Stoica says his software was indeed able to extract useful gait data.