The computer science behind the first down line

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.

According to Allen St.

John’s 2009 book The Billion Dollar Game: Behind the Scenes of the Greatest Day in American Sport – Super Bowl Sunday, 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 acquired all of PVI’s intellectual property in December 2010.)

BUT HOW DOES IT WORK?

It takes lots of computers, sensors, and smart technicians to make this little yellow line happen. Long before the game begins, technicians make a digital 3D 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 3D 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 3D 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.)

The Computer Science Behind the First Down Line

It’s Thanksgiving this week here in the U.S., which means it’s time for food, family, and football. So let’s tackle (see what I did there?) a question that I’ve always had as a casual football viewer: what is the science behind that moving first down line that appears superimposed on the field but underneath the players?

In football, almost every play is aimed at passing that first down line, a marker that indicates a team has successfully moved the ball forward for a total of at least ten yards.

For every possession, a team has four chances (or plays) to cross that line.

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Ideally, a play ends with the ball all the way in the end zone for a touchdown, but falling short of that, passing the first down line at least means a fresh start—resetting their number of chances (or downs) up to four.

Since 1998, fans watching from home have been provided with a bright yellow or orange line that conveniently marks exactly where this first down line lies during each play. That way, fans can easily tell how far their team has made it (or how far they still have to go) on their march forward toward their competitor’s end zone.

So the challenge becomes how do you mark a line on the field that needs to move repeatedly throughout the entire game? How do you make sure that line appears in the correct spot no matter the viewing angle of the current television broadcast camera? Even as a single camera pans across the field, tracking a running player or a thrown ball, how do you make sure that line doesn’t move? Finally, how can you set up the line so that the players appear to run over it, rather than the line crossing over the top and blocking the view of the action?

Pre-game 3-D Computer Modeling Is Key

To get the first down line on the field for viewers at home, a lot of work is done before the game even starts.

A detailed three dimensional model of the field is made, which includes the gentle slope most football fields have in order to encourage water to drain away from the center.

This model also notes where every single possible first down line could be placed in relation to the field and its close surroundings.

Then, special mounts are made for the main television broadcast cameras that track each and every movement the camera makes so that the camera positions can be modeled as well.

When the camera makes any kind of movement—from manual movement by the camera person or panning across the field to even zooming or focusing—the extent of that movement is carefully recorded and the resulting perspective on the field is modeled in detail.

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The Yellow First-Down Line: An Oral History of a Game Changer

ESPN's Sunday Night Football broadcast on Sept. 27, 1998 — 15 years ago this week — featured an unremarkable matchup between the Baltimore Ravens and Cincinnati Bengals.

But the game bears a historical footnote most NFL fans probably don't know—it also marked the debut on TV screens across the country of the now-ubiquitous yellow line that's digitally imposed on football fields to show where a team needs to reach for a new set of downs.

Earlier that same year, Bill Squadron and Stan Honey left their jobs at News Corp to build a startup. Its name: Sportvision. Its goal: Leveraging technology's rapid growth to improve the sports-viewing experience for fans at home.

See also: 6 Questions With 'Sport Science' Creator John Brenkus

The company has since developed a host of game-enhancing products for Major League Baseball, NASCAR, the NBA, the NHL and more.

But it was a meeting with ESPN executives in New York City 15 years ago that would springboard Sportvision to instant success.

The meeting birthed the yellow first-downline—high-definition and instant replay aside, arguably the most significant technological innovation for sports fans on the couch.

That yellow stripe has since become a staple of the NFL-viewing experience and a symbol of how technology has revolutionized the ease of being a fan. But it didn't just come to be on its own, and how it was born contains factoids and tidbits that will fascinate any NFL fan or media nerd.

Fifteen years after it debuted, we spoke with key players from Sportvision and ESPN, as well as some fans, for the inside story on how that yellow line got to your TV screen and how it has changed since. Read on for the interesting tale behind an oddly endearing bit of sports history.

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An Oral History of the Yellow First-Down Line

Bill Squadron (Sportvision co-founder and former CEO, now head of Bloomberg Sports): People had been talking about the idea of an electronic first-down line on telecasts for some time. I think I mentioned in a piece I wrote for SI.com that there had been a patent filed for a different way of doing it many years before. So it was on our list of things we wanted to do when we started Sportvision.

We demoed it for all the NFL rights holders. The reaction of most was positive but cautious. There were concerns about the cost. We got a lot of positive reinforcement but people were cautious.

Jed Drake (ESPN's vice president of production): When we heard Sportvision was keen to work with us, we said, 'Why not, let's give it a go.' So we ended up meeting in ABC's offices in New York.

One of the things we've always seen as important at ESPN is seeing technology as a way to absolutely enhance our presentation when it's done right, a way to tighten the experience for viewers and give us some separation between ourselves and our competitors.

Squadron: We were optimistic going into the meeting, because we felt this idea had real potential, but we were far from thinking it was a guaranteed thing. We knew the industry tends to move slowly with new things, and there was concern that the technology could be delivered in a live broadcast. The huge step forward was being able to do this in a live broadcast.

What surprised and pleased us about ESPN's reaction was that they immediately realized how popular and groundbreaking it could be. They weren't worried about the cost. The NFL deserves a lot of credit too, because they also immediately saw the value this would provide for fans.

But before Sportvision and ESPN could team up, they had to run the first-of-its-kind idea by the NFL for approval.

Drake: I remember showing it to [NFL commissioner at the time] Paul Tagliabue, knowing there were three possible outcomes. He'd either say it needs more work, no way, or this is good.

He's a reserved guy, no jumping up and down. We showed him some demos and he said, 'This is good, this makes sense.

' We all nodded our heads very seriously, then he left and closed the door and we all started jumping up and down.

Squadron: We actually initially had the line as a little more of an orange color. But the actual decision about the yellow hue was one Jed Drake made. We showed him the palette of colors and he decided.

With an exclusive agreement in place, the final step was to fine tune the technology to make it broadcast-ready — something that took longer than initially anticipated and delayed the new product's public debut.

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Squadron: The last step before we were able to go to air was that the line was flickering enough that we had to hold off for a couple weeks to get that jitter off the line. Again to ESPN's credit, they wanted to wait to make sure it was really worth it.

Hank Adams (current Sportvision CEO): That line is actually a number of different systems working together to produce the ultimate effect. We laser survey the field so we actually have a three-dimensional map of it.

We look through all the broadcast cameras, then stretch the field into place and have camera sensors that measure pan, tilt and zoom. As the camera pans and follows runners, we literally map the three-dimensional world onto your two-dimensional screen.

Every pixel on your screen corresponds with a location on the field that we know.

How the First-Down Line Works

­Football is a major pastime in the United States. Kids play in the Pop Warner football league, some progress to high school football, some of those play college football, and a very select few play professional footb­all in either the NFL or CFL.

This funnel toward greatness continues until the first Sunday in Feb­ruary, when the elite of football's elite play in a game that people all over the world gather to watch: the Super Bowl, the championship game of American professional football.

One of the main objectives in American football — and a helpful one if you want to score points — is to gain a first down.

In order to get a first down, the offense must gain 10 yards within a series of four plays, or downs.

If the offense gains the necessary yards (or more) in four downs or less, the team reverts to first down and the process begins anew until the offense fails to gain ten yards, scores, or turns the ball over to their opponents.

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One problem that football players and officials have always had to deal with is exactly how to measure the 10 yards needed to gain a first down. First downs often decide games, but collegiate and professional football officials often measure them using a decidedly antiquated length of metal chain attached between two poles.

Television viewers have had trouble figuring out ­where the first-down line is in relation to the offense. A small arrow located below the end pole isn't usually visible on your television screen.

If you've watched any football games since 1998, however, you've probably noticed that fluorescent yellow or orange line that seems painted on the field from one sideline to the other.

In fact, the line is computer generated, representing the exact spot that the offense must reach for a first down.

Sportvision, a company based in New York City, debuted its “1st and Ten” system on during a game between the Bengals and the Ravens, broadcast on ESPN on September 27, 1998. Football fans everywhere rejoiced.

Since that first game, Sportvision has continued to provide ESPN, ABC and FoxSports with the ability to enhance their football telecasts with this technology (you can view images from actual games that used the first-down line on their Web site). Other networks use similar technology.

In this article, we'll look at how the 1st and Ten system works.

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