Football fans just call it "the yellow line." It appears out of nowhere on the TV screen, marking how far up the field the offensive team must advance to gain a first down. The stripe looks as if it is painted on the turf: when the camera angle changes, it stays in proper skew; as the camera zooms in and out, it widens and narrows; and when players run by, it disappears behind them. But once the play ends, the streak vanishes, only to rematerialize when the team lines up for the next play.

Sportvision in Chicago produces the "1st and 10" line for the ESPN, ABC, Fox and Turner networks, and Princeton Video Image in Lawrenceville, N.J., does it for CBS. An operator arrives at the host stadium with the network's video crew one or two days before game time and proceeds to digitally map the gridiron, calibrate the three main game cameras at the 25, 50 and 25 yard lines, and customize the video-graphics software so that the golden apparition will appear on every play from scrimmage.

This article was originally published with the title Phantom Gain.

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