The Computer Science Behind the First Down Line
What is the science behind that moving first down line that appears superimposed on the field but underneath the players?
Sabrina Stierwalt, PhD
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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. 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.
The computer system that generates the line (usually the 1st and Ten graphics system but also possibly the Live Video Insertion System) requires one final set of inputs: the detailed color palettes that describe what colors should be covered by the line and which ones should not. Each field will have grass or artificial turf with its own specific range of green hues and the system is programmed to cover pixels with those colors with the yellow line. The colors found in the players’ uniforms, as well as the referees’ clothing, is also programmed into the system ahead of time so that the line is not generated over pixels with those colors.
Game Day Action
When first implemented, the system took at least four operators to carry out. Now the system is more streamlined and often only one person is needed to enter the new position of each first down line once a play begins and to spot check that everything is working properly. Occasionally new color palettes have to be added on the spot—for example, if it starts to snow on the field.
To keep up with the movement of the game in real time, the line is drawn as often as 60 times per second! The game programming video is also delayed by several frames in order for the system’s response with the drawing of the line to keep up. In 2011, SportVision, the producers of 1st and Ten determined their line had a margin of error of 1.38 inches compared to the actual location of the first down marker.
Originally, the programming cost between $20,000 to $25,000 per game, and in 2001, Fox Sports even threatened to get rid of it to cut costs. Luckily, the line is such a fan favorite that it is here to stay and has even won Emmy Awards.
Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.
Image courtesy of Shutterstock.
