How do they do that? Football first-down markers

[Jason]

I've been pondering this for awhile and just came across it randomly in an email I received from my power company (of all places!):

http://www.duke-energynews.com/service/document.cfm?id=3460&userid=486083

How Do They Do That? The Digital First Down Marker in NFL Football Games

The 1st and TenTM line, the famous yellow line that shows television viewers how far the team with the ball must go to make a first down, has become part of the culture of watching professional or college football on TV. The 1st and TenTM line is made available by a private company called Sportvision and was developed in 1998. Sportvision supplied much of the text and the graphic image below for this article. Princeton Video Image developed a similar graphics system, but is no longer in business. Sportvision's first product was the highlighted hockey puck that for a time appeared during televised NHL games.

The virtual yellow 1st and TenTM line makes use of Sportvision's patented video overlay technology to create the illusion that a yellow first-down line is painted on the field, allowing players to cross over and stand on it. Of course, the line is only visible on TV; the players don't see it.

How is it done? First, a laser is placed in the center of the field to collect data on elevation points. A football field is not flat. It has a 12"–18" crown down the middle, the entire length of the field resulting in different elevations. That information is used to draw a 3D computer map of the contours of the field, and the map is adjusted and overlaid onto the camera's view of the actual field. Anywhere from three to five separate broadcast cameras are outfitted with custom Sportvision sensors and encoders to capture camera movement data. As cameras pan, tilt, and zoom, this data enables the virtual line to follow suit, staying in perspective and getting larger and smaller, as needed.

According to Wikipedia, there are at least seven computers in total and a crew of four 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 locate it properly on the TV screen. A third crew member is a troubleshooter. The fourth crew member monitors the various colors that will make up the color palette onto which the line is drawn.

Drawing the yellow line so that it appears to be painted on the field underneath the players is accomplished through a sophisticated process of color keying that allows the operators to tell the computers what colors to draw on (grass, dirt) and what not to draw on (skin, uniforms). 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.

And finally, after adding the precise location of the first down marker to the system, voila ... The yellow line appears like magic on your TV screen. In recent years the system has been upgraded to add more features. During Fox broadcasts, down and distance appears on the field inside 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. Some television football broadcasts show a second computer-generated line (usually blue or red in color) that marks the line of scrimmage.

For motorsports, Sportvision's RACEf/xTM technology is able to super-impose a "pointer" that points directly to any of the cars, and tracks the car's actual movement with its movement on the screen. The pointer consists of an arrow pointing to the car, as well as a car number badge, driver name, real-time position, and one or two data items (i.e., speed and time off leader). Using what's called differential carrier phase GPS, the RACEf/xTM system is able to accurately measure the position of each car on the track accurate to two centimeters. Each race car contains a DAPS (data acquisition and positioning system) unit as well as a telemetry and GPS antenna installed in the roof. Antennas are set-up around the track to collect information from each car five times per second. This information is relayed to the Sportvision production truck where the data is aggregated and processed by an array of computers running proprietary software developed for the RACEf/xTM system. The company has also developed a KZone™ System for baseball, a HOOPSf/xTM system for basketball, as well as a GOLFf/xTM system.