If you attend a Super Bowl party on Sunday, you’ll probably hear at least one casual football viewer ask, “How do they get that yellow first-down line on the field?” While “magic” is a fine answer in its own right, the real explanation is a bit more technologically intense. Let’s have a look at the background and mechanics behind every football fan’s shining beacon, the yellow first-down line.

Like the first-down line, football fans? You owe a tip of your cap to an unlikely source: hockey. According to Allen St. John’s 2009 book The Billion Dollar Game, the first-down line actually emerged from the ashes of one of sports broadcasting’s bigger debacles: the FoxTrax system for hockey, which was designed by a company called Sportvision. FoxTrax – which hockey fans no doubt remember as the much-maligned “technopuck” that debuted in 1996 – employed a system of cameras and sensors around a hockey rink to place a little blue halo around the puck.

FoxTrax wasn’t a great fit for NHL broadcasts. Hockey purists hated the intrusion into their game, and casual fans didn’t flock to hockey just because the puck was suddenly easier to follow. However, the system inspired producers to think of new ways to insert computerized images into live sports broadcasts. The idea of using a line to mark the first down in football was a natural extension, and Sportvision debuted its 1st and Ten system during ESPN’s broadcast of a Bengals-Ravens tilt on September 27, 1998. A couple of months later, rival company Princeton Video Image unveiled its Yellow Down Line system during a Steelers-Lions broadcast on CBS. (Sportvision is still kicking, and ESPN just acquired all of PVI’s intellectual property in December 2010.)

But How Does It Work?

It takes lots of computers, sensors, and smart technicians. Long before the game starts, technicians make a digital 3-D model of the field, including all of the yard lines. While a football field may look flat to the naked eye, it’s actually subtly curved with a crown in the middle to help rainwater flow away. Each field has its own unique contours, so before the season begins, broadcasters need to get a 3-D model of each stadium’s field.

These models of the field help sidestep the rest of the technological challenges inherent to putting a line on the field. On game day, each camera used in the broadcast contains sensors that record its location, tilt, pan, and zoom and transmit this data to the network’s graphics truck in the stadium’s parking lot. These readings allow the computers in the truck to process exactly where each camera is within the 3-D model and the perspective of each camera. (According to How Stuff Works, the computers recalculate the perspective 30 times per second as the camera moves.)

After they get their hands on all of this information, the folks in the graphics truck know where to put the first-down line, but that’s only part of the task. When you watch games you’ll notice that the first-down line appears to actually be painted on the field; if a player or official crosses the line, he doesn’t turn yellow. Instead, it looks like the player’s cleat is positioned on top of an actual painted line. This effect is fairly straightforward, but it’s difficult to achieve.

To integrate the line onto the field of play, the technicians and their computers put together two separate color palettes before each game. One palette contains the colors – usually greens and browns – that naturally occur on the field’s turf. These colors will automatically be converted into yellow when the line is drawn onto the field. All of the other colors that could show up on the field – things like the players and officials’ uniforms, shoes, and flesh, the ball itself, challenge and penalty flags – go into a separate palette. Colors that appear on this second palette are never converted into yellow when the first-down line is drawn. Thus, if a player’s foot is situated “on” the line, everything around his cleat will turn yellow, but the cleat itself will remain black. According to How Stuff Works, this drawing/colorizing process refreshes sixty times per second.

All this technology isn’t cheap, of course, and it takes manpower to make it run. Sportvision’s version of the yellow line requires a four-man crew, including a spotter in the press box who’s relaying information about the first-down marker. According to St. John’s book, it costs Fox $25,000 per game to project the line onto the field. Seems like a bargain to keep millions of football fans’ heads in the game, though.

[Image credit: Stampede Blue]

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