What is facial recognition - and how do police use it?

This is a catch-all term for any technology that involves cataloguing and recognising human faces, typically by recording the unique ratios between an individual's facial features, such as eyes, nose and mouth. The technology can be applied to everything from emotion tracking to animation, but the most controversial involve using facial features as biometric identifiers, that is, to identify individuals based on just a photo or video of their face. After a trial of the technology, the Metropolitan police have said they will start to use it in London within a month. On Friday, the force said it would be used to find suspects on "watchlists" for serious and violent crime, as well as to help find children and vulnerable people. Scotland Yard said the public would be aware of the surveillance, with the cameras being placed in open locations and officers handing out explanatory leaflets.