In order to determine where the line should go, a central computer utilizes several pieces of information:
- The virtual field modeled from measurements of the actual field (taken before the game), and the data from the camera mounts showing each camera's range of view
- The raw 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 the tallied (on-air) camera, is sent to a computer that draws the yellow line 60 times per second. The line is then sent to a linear keyer to superimpose 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 be synchronized and turned into what you see on your TV screen.
On game day, it takes four people to run the system:
- A spotter and an operator work 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 operators are 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 below.