The "1st & Ten" first down line appears as if it's drawn on the field with yellow chalk, because it doesn't interfere with the action on the field.
In fact, it's placed over the video and drawn around the players using virtual imaging technology, and by measuring the attitude of the on-air camera's pan, tilt, zoom and focus.
The first part of process is easy. Whenever a football team progresses down the field and earns a first down, a production assistant in the press booth of the stadium, using binoculars and an intercom, lets the control room know where the next first down marker will be placed, so Sportvision knows where to key the line.
Now comes the hard part. A central computer examines every frame of video in real time (i.e. 30 times per second) and determines which pixels to change to yellow. These are all the points in the image where an actual painted-on-the-field first down line would be visible, such as grass along the line that is not obscured by a player or referee.
The computer determines which pixels to change based on information about the camera's view, a 3D model of the field, which camera is on air, and a palette of colors for the field and another palette for players.
Technicians must also take weather conditions into account, which means if it's raining, snowing, or cloudy outside, the colors on the field palette must be changed to compensate for brown mud, white snow, or shadows.
And then there's the broadcast equipment on the field. Each camera in the system is instrumented with very precise encoders for pan, tilt, zoom, focus and extender (1x or 2x doubler). A computer at each camera reads the encoders and transmits these readings to the control room 30 times per second.
The final computer has only one simple but crucial task, draw the yellow line in video 60 times per second and superimpose the yellow onto the program video.
There are eight computers, three sets of special encoders and abundant wiring dedicated to generating the virtual first down line in video format. The data collection and computation requires time, and the virtual first down line must be superimposed on the program video at exactly the correct field every 1/60 of a second, requiring substantial video and audio processing.
The game's program feed without the 1st & Ten line comes from the primary production truck into Sportvision's 1st & Ten equipment. To synchronize the computed virtual first down line with the program feed, the 1st & Ten system delays program video and audio a fixed number of frames.