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I Didn't Know That...

September 21, 2000

Q: How does television put the flags in the swimming pool at the Olympics and the yellow first-down line on the field in football games?

A: The technology is virtually identical in both instances. The first-down technology for NFL games was developed by Sportvision Inc. of New York City, while the Olympic flag technology is provided by Orad Hi-Tec Systems Ltd. of Israel, which also provides first-down lines for the Canadian Football League.

A powerful central computer examines every frame of video in real time--about 30 frames per second--using a built-in model of the football field or the pool to determine which pixels in the picture need to be replaced. Each camera in the system is equipped with sensors that determine pan, tilt, focus and zoom--i.e., exactly where the camera is pointing and how much the image is magnified. This information is fed to another computer, which lets the central computer know precisely what it is seeing.

In football, the central computer is keyed to remove the green pixels of grass--ignoring the green of players' jerseys--and replace them with yellow. Player colors and other colors on the field are left unchanged, so the yellow appears only where there is grass. This is why players and officials remain in the foreground, rather than having the yellow stripe run across their bodies. The entire process requires eight computers.

In swimming, the computer is keyed to the blue on the bottom of the pool, so it ignores the water and the swimmers. That's why it looks as if the flags are under water. But instead of generating a yellow line, it puts in the flags of the competitors' countries. The system is also tied into the timing devices so that, at the turns, it inserts the flag of the leader.

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