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: theSuper 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 afirst 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.

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 yourtelevision 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 iscomputer generated, representing the exact spot that the offense must reach for a first down.

Sportvision公司总部位于纽约,推出“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 the1st and Ten systemworks.

The Technology

One of the quirkier aspects of computer-generated video effects is the vast amount of effort it takes to do seemingly simple things. The most basic concepts can sometimes take a gigantic amount of effort to implement (seeHow Centropolis FX Creates Visual EffectsandHow Industrial Light & Magic Worksfor interesting background).

Painting avirtual first-down lineon a football field is an excellent example of this process; the concept of painting a first-down line across the field on people's TV screens certainly sounds simple. As it turns out, implementing this is incredibly complex. It takes a tractor-trailer rig of equipment, including eight computers and at least four people, to accomplish this task

Here are some of the problems that must be solved in order for this system to work:

The system has to know theorientation of the field with respect to the cameraso that it can paint the first-down line with the correct perspective from that camera's point of view.
The system has to know, in that same perspective framework, exactlywhere every yard line is．
Given that the cameraperson can move the camera, the system has to be able tosense the camera's movement(tilt, pan, zoom, focus) andunderstand the perspective changethat results from that movement.
Given that the camera can pan while viewing the field, the system has to be able torecalculate the perspectiveat a rate of 30 frames per second as the camera moves.
A football field is not flat -- it crests very gently in the middle to help rainwater run off. So the line calculated by the system has to appropriatelyfollow the curve of the field．
A football game is filmed bymultiple camerasat different places in the stadium, so the system has to do all of this work for several cameras.
The system has to be able tosense when players, referees or the ball crosses the first-down lineso it does not paint the line right on top of them.
The system also has tobe aware of superimposed graphics网络可能overlay on the scene.

A key piece of hardware used in the system is aspecial camera mountthat holds the television cameras. This mountencodesall of the camera's movement (such as tilt, pan, zoom and focus). The data the mount produces helps the computers understand exactly what each camera is doing in real Another key piece is acomputerized 3-D modelof the field. The computers know exactly where the cameras are located in the3-D modeland can orient the virtual first-down line on the field accordingly. The model also accounts for things like the crest of the field and the location of the yard lines on the field.

Color palettesare also critical to the system. The computers must be able to distinguish betweengrass, on which the line should be painted, and everything else (players, referees, the ball, etc.), on which it should not. Color palettes solve this problem. You can see the palettes at work in this frame:

The player does not have the line painted over his jersey because of the color palettes.

All counted, there areeight computersused in the system:

FourSGI computers
OnePC
Three special computers used in conjunction with the television cameras

These special computers' sole task is to record aspects of each camera's movement 30 times per second from the camera mount, and then send that data back to the production truck for analysis and use.

Up next, how the line is actually drawn.

Drawing the Line

In order to determine where the line should go, acentral computerutilizes several pieces of information:

Thevirtual fieldmodeled from measurements of the actual field (taken before the game), and thedata from the camera mountsshowing each camera's range of view
Theraw video feed from the camera that's currently on-air(determined by a separate computer in the Sportvision production truck)
Two distinct color palettes, one representing the on-field colors that should be changed to yellow to represent the first down line, and another representing colors that should not be changed (like those in the players' and officials' uniforms -- this allows a player to appear to "obscure" the line, making the line appear as if it were really painted on the field).

Once the computer determines exactly which pixels should be colored yellow, this information, along with the raw video feed of thetallied (on-air) camera, is sent to a computer that draws the yellow line60 times per second．然后发送到一个线性键控线超级impose the yellow line onto the program video. Since it takes time for all this to occur, the program video is sent through several frame delays so that the generated yellow line and delayed program video can besynchronizedand turned into what you see on your TV screen.

On game day, it takes four people to run the system:

A测位仪and anoperatorwork together to manually input the correct yard line into the system. The spotter is in the press box and the operator is in the production truck physically keying in the correct number.
Two other Sportvision operatorsare on hand to make any adjustments or corrections necessary during the game. These adjustments might include adding colors to the color palettes due to changing field conditions, such as snow or mud.

Altogether, the process of creating a first down line for viewers at home is far from simple. Any football fan watching at home would tell you, however, that it's well worth the effort.

For more information on the 1st and Ten system and related topics, check out the links on the next page.

How the First-Down Line Works

The Technology

Drawing the Line

Lots More Information

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