About this Article
Written by: Jeff Braun
Written on: April 6th, 2005
Tags: sports & recreation
Thumbnail by: Leonard G./Wikimedia Commons
About the Author
Jeff Braun was a transfer student in the fourth year of a 3/2 double degree program set up with Biola University. He studied Physical Science at Biola and was working toward his Bachelor’s Degree in Mechanical Engineering at USC in the spring of 2005.
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Volume VII Issue III > American Football: That Magic Yellow Line
Virtual technology has brought a whole new perspective to watching football from your couch. The implementation of first-down lines in television broadcasts of America's homegrown sport has provided fans with a viewing experience that integrates cutting edge technology into the fast-paced world of athletics. This single yellow line may appear simple enough; however, its appearance is the result of meticulous work via cameras, computers, and a team of dedicated engineers. Due to this combined effort, first-down lines have become a powerful tool for viewers at home, whose perspective is often more accurate than even those of the referees.


For many Americans, football is an essential element to the perfect weekend. Whether it is congregating at local high schools on Friday nights or watching the pros play on Sunday (or quite possibly both), Americans love football. Such love has manifested itself in a profitable product for many television networks. In the 2006 Super Bowl, "a 30-second slot [was] being sold by Fox Networks for an average of $2.4 [million]" [1]. The networks are not the only beneficiaries from all these revenues: viewers like us get to experience a higher quality production thanks to the increased profits. Take, for example, the virtual first down lines. These lines have been around for a while, but most football fanatics do not realize how much engineering goes into implanting this essential feature of the game.

Basic Football: The First Down

Traditionally, on the field, the first down line is delineated by a team of officials holding highly visible black and orange physical markers on the sidelines. While this method is fairly effective for coaches, players, and spectators near the field, it leaves much to be desired for those watching miles away in their homes. Often, the camera focuses solely on the middle of the field, blocking the view of the sideline and first-down markers entirely, removing the guides of an already skewed perspective. In the past, commentators would be the primary source of information about ball position on the field; however, technology has enabled networks to create a digital first-down line. In essence, a yellow line is superimposed over the normal image of the field from sideline to sideline, allowing viewers to know precisely how far the team must go to achieve a first down. This has been a huge bonus for football fans across the nation, creating a more enjoyable experience for all.

No Easy "Feet"

Leonard G./Wikimedia Commons
Figure 1: A camera at work during a football game of University of California, Berkeley vs. University of Tennessee.
At first glance, the virtual first down line is, while helpful, not very impressive. Superimposed images, including the standard bar depicting the time and score at any point in the game, appear on television everywhere. When viewers first see the virtual first-down line, their initial response is "Wow! That's a great idea," instead of "Wow! How did they do that?"
One immediate difference is that the virtual first-down line looks stationary with respect to the players and the playing surface. It quite literally appears to be painted onto the field along with the hash marks running up and down the sidelines. Even if the camera pans to the left or to the right, the line remains in the same position on the field, unlike normal superimposed images, which base their position relative to the television screen, not the playing surface. The virtual first-down line also differs because it appears in the background. That is, when a player passes over the line, the player is actually imposed over the yellow line, strengthening the illusion that the line is actually painted on the field. Given these two challenges, creating the virtual first down line requires special equipment, custom computers, a small team of operators, and tremendous funding.

A Matter of Perspective

Making a superimposed image appear to be painted on the field no matter how the camera moves or zooms requires several things. First, a 3-D representation of the field must be created and the exact location of the main cameras must be known. This is necessary so that the line is oriented correctly on the desired yard line. For example, if the camera and the first down line were lined up at the 25-yard line, the line would appear perfectly vertical. If the camera remained at the 25-yard line but the first down line was moved 25 yards ahead, the line would have been angled on the screen. Also, this 3-D model has to let the operating computers know where the sidelines are so the virtual line stops at the right place. Finally, the 3-D model must account for the crest or slope of the field, which would also affect the line in relation to the location of the field [2].
Now that the computers know where the cameras are in relationship to the playing field, they must also know how the cameras move in all three axes as well as how much they zoom. This is done using a special mount that "encodes all of the camera's movement (such as tilt, pan, zoom and focus)" [2]. This new information is then compared with the information contained in the 3-D model so that the line remains in the same position regardless of the movement of the camera [3].
There is one final component to this intricate issue. The mounts discussed earlier can account for not only the movement of the camera with respect to its tripod, but also the movement of the tripod itself. This is actually a rather common occurrence. Camerman could unintentionally move the stand, or if it were a rather windy day, the tripod cold easily sway. Also, stadium vibrations caused by a particularly rowdy crowd could be transferred into the camera, causing noise in the mount position data. To account for all these factors, Fiber Optic Gyro's (FOG's) are used. These finely tuned sensors account for any wiggling the camera may undergo. They relay information "100 times per second and feed the information to a computer, which calculates how to make the line appear stationary" [4]. The FOG's are so precise they can detect an angular rotation as small as .002 degrees. Even the military is looking to use these with their devices that must magnify images on vibrating platforms [5].

Color Palettes

With the yellow line accurately placed on the field, the computers must now accommodate for players crossing over the line. This is done using a common computerized special effect known as "chroma-keying." A computer is used to replace a particular color with something else - in our case, a yellow line [3]. This is similar to the technique that allows the weatherman to stand in front of the Doppler map. The weatherman is actually standing in front of a green screen while a computer is programmed to replace all the green with the desired image. Similarly, only colors associated with the playing surface are replaced with the yellow line, the applying surface acting much like a virtual green screen. Since the players and referees are not the same color as the field, the yellow line is not superimposed on them. In fact, the colors of the players and referees are also entered into the computer, but with a command that prevents their colors from being replaced by the yellow line. This sounds simple enough, but what happens when the field gets ripped up toward the end of the game? What if a shadow appears halfway across the field or if the sun goes down completely? What if it starts snowing? All of these factors change the color of the field, requiring attention from the crew [3].

The Crew

About four people are required to run the virtual first-down line, one spotter along with three operators. The spotter physically watches the game and uses a radio to communicate with an operator who sits inside the equipment trailer containing all the computers. The spotter tells this operator what yard line the first-down line is located on. The operator then enters this information into the computer. Two other operators reside in the trailer working with the computers on various issues. One of their key jobs is constantly monitoring the field conditions, making adjustments to the color pallet as the game progresses. Other issues involve calibrating and refining the 3-D map.


Every once in a while when the ball is really close to the first-down line, the referees call the sideline officials out to the center of the field to check to see if really is a first down or not. This process seems futile to those watching it on television since the virtual first down lines is so accurate. In a sense, people at home have a much better perspective on the game then the referees! As referees have just started relying on instant replay technology to review controversial plays, it is a reasonable prediction that someday virtual first down technology will be used to assist officials in this call. This would allow the game to proceed faster and would also help the right call to be made more often. The opportunities are limitless when it comes to technology and sports. As long as the audience continues to tune in faithfully, modern technology will continue to play a large role in viewing experience.


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