Facial-recognition technology is no longer a gimmick in dystopian science fiction movies or CSI-style cop shows: It’s increasingly used in more pedestrian ways. Your face can unlock your iPhone X, for example.

Or, if you’re flying with Jetblue from Boston to Aruba or the Dominican Republic, you have the option of using your visage as your boarding pass, a system that involves an offsite U.S. Customs and Border Protection algorithm making the matches. And now, the tech—featuring a camera attached to sunglasses— is being used by police officers in crowds in China, The Wall Street Journal reported on Wednesday.

In addition to the glasses, the Chinese system involves a connected mobile device that the police officers carry that contains offline face data, allowing the system to work quickly. According to the Journal, at one city’s railway station, they’ve nabbed seven people associated with crimes using this method, as well as others traveling under false identities.

Here’s how artificial-intelligence-powered technology like this works in general—and what one potential pitfall of it is. (Besides, you know, the whole surveillance-state thing.) Software that powers facial recognition generally uses a two-step process, says David Alexander Forsyth, the chair of the computer science department at the University of Illinois at Urbana-Champaign and an artificial intelligence expert.

Step one is to figure out where the faces are in the image in question; the system is looking for a window-like section of the image that also has someone’s countenance in it, and not the other stuff of modern life, like stop signs and cars. Step two: it needs to see if it can match the face to any in its database. Read more from popsci.com…

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