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The famous yellow line which shows television viewers how far the team with the ball must go to make a first down has become part of the culture of watching football. The line is created by a patented video overlay technology that was developed by Sportvision, Inc. in 1998. It gives the illusion that a yellow first-down line is painted on the field; however, it is only visible on TV. The players cannot see it.
How is it done? First, a laser is placed in the center of the field to collect data on elevation points. A football field is not flat; it gradually elevates from each side, leaving a 12-inch crown in the middle that runs the entire length of the field. This information is used to draw a three-dimensional (3D) computer map of the contours of the field, and the map is adjusted and overlaid onto the camera's view of the actual field.
Anywhere from three to five separate broadcast cameras are outfitted with custom sensors and encoders to capture camera movement data. As cameras pan, tilt and zoom, this data enables the virtual line to follow suit, staying in perspective and getting larger and smaller as needed. Finally, after adding the precise location of the first down marker to the system, a yellow line magically appears on your TV screen.
At least seven computers and a crew of four people are typically used to generate the yellow line. Two crew members—one inside the stadium and one in front of a computer—communicate the position of the real first down line to locate it properly on the TV screen. A third crew member is a troubleshooter, and the fourth monitors the various colors that make up the palette from which the line is drawn.
Drawing the yellow line is accomplished through a sophisticated process of color keying, which allows the operators to tell the computer what colors to draw on (grass, dirt) and what not to draw on (skin, uniforms). The only pixels that should change are the ones the same color as the field, typically several shades of green.
As a result, there are two difficult situations. One is when the player's uniform color nearly matches that of the field (for example, the Green Bay Packers' jersey on a bright, sunny day). The other is when the field itself changes; when it rains or snows or if the field becomes very muddy. In these cases, the color palette may need to include brown or white shades.
In recent years, the system has been upgraded to add more features. Down and distance appear on the field inside an arrow pointing toward the offensive team's end zone. On one network, a color-coded chart showing a field goal kicker's success at various distances may appear. Some broadcasts show a second, computer-generated line, which marks the line of scrimmage.
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