line on football

In many outdoor ball sports, goal line refers to the line which a team is attempting to advance the ball towards. In particular, see
Football pitch for usage of the term (and the associated term byline) in soccer Goal line (American football) for usage in American and Canadian football Goal line (ice hockey) for usage in ice hockey. 1st & Ten is the name for a computer system that generates and displays the yellow first down line that a TV viewer sees during a live broadcast of a college or professional American football or Canadian football game. The line, which is not physically present on the field and is seen only by the television audience, spans the width of the football field and indicates where the first down marker is located. The purpose of the line is to make it easier for television viewers to follow play on the field. Some television football broadcasts change the color of the line from yellow to red on 4th down, or show a second computer-generated line (usually blue in color) that marks the line of scrimmage.
The system is based on chroma key or "green screen" technology, which has been used in other applications for many years prior to the invention of 1st & Ten.
The idea of creating an on-field marker to help TV viewers identify 1st down distances was conceived by ESPN Programmer Gary Morgenstern, and executed by ESPN's NFL Coordinating Producer, Fred Gaudelli. The 1st & Ten line is made available by a private company called Sportvision and was developed in 1998. A similar graphics system was developed by Princeton Video Image, which competes with Sportvision.
Each football field has a unique crown and contour (not perfectly flat) so a 3D model is made of the field prior to the game. It also has a unique color palette, typically various shades of green, depending on the type of surface (i.e. real or artificial grass) and the weather (e.g. bright, shady or even snowing).
There are a number of cameras on the field (3 on the 50 yard line, and 1 on each twenty yard line). Each one has a computer within it that monitors how the camera is used during the game (pan, tilt, zoom, focus and extender) and transmits that info live to the Sportvision production truck 30 times per second. This information helps keep the yellow 1st & ten line in the proper place without being distorted whenever the camera follows the players or the ball.
There are seven computers in total and a crew of 4 people. Two crew members, one inside the stadium and one in front of a computer, communicate the position of the real first down line to make sure everything is working. Another crew member is a troubleshooter. The last crew member monitors the various colors that will make up the color palette onto which the line is drawn.
The first computer in the truck gathers all the separate readings from the cameras and transmits a single, consolidated data stream to the central computer.
The central computer takes these readings, the 3D field model & color palette, the knowledge of which camera is on the air, and together using a geometrical calculation determines which pixels in the video frame would make up the first down line. All pixels that are obstructed by a player, a referee, the ball or any other object are identified and not included in the calculation. This will ensure that the 1st & Ten line will be projected only onto the field.
The only pixels that should change are the ones that are the same color as the field, typically several shades of green. As a result, there are two situations that are difficult. One is when the player's uniform color nearly matches that of the field (for example, the Green Bay Packers' jersey on a bright, sunny day). The other is when the field itself changes, like during a rain/snow storm or if the grass field becomes very muddy. In those cases, the field's color palette would need to include brown and/or white shades.
The data collection and computation also requires time. The audio feed goes to an audio delay to be synchronized with the delayed video. The total delay for the viewer from the live feed ends up being about 2/3 of a second.
After the central computer has determined which pixels represent the 1st & Ten line, the final computer takes that pixel information and draws the yellow line in video format at 60 times per second.
In recent years the system has been upgraded to add more features. During Fox broadcasts, the line on 4th down becomes red and down and distance appears on the field inside of an arrow pointing toward the offensive team's end zone. On CBS, a color-coded chart showing a field goal kicker's success at various distances may appear.
This technology is also the basis for showing ads where they may not appear (i.e. behind home plate in baseball during national broadcasts), and Race F/X in which images can be displayed on the race track, and info can follow a specific car, no matter what the camera does. This technology is used by Fox, ESPN, TSN, RDS, NBC, and TNT.