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“Disputes over line calls used to be one of the main joys of tennis—this, after all, is John McEnroe's game. But fans rarely see players explode in rage anymore. In high-profile matches (i.e., those broadcast on TV), human umpires have largely been replaced by a machine called Hawk-Eye. The system is a kind of computerized ump that stitches together video footage from several high-speed cameras to produce a 3-D simulation of the ball as it approaches and bounces off the ground. Hawk-Eye's decisions are final: When a player challenges an umpire's call, the system displays its view of what just happened, then displays a judgment on the screen—in or out—that the human umpires are compelled to accept.” source...