Might not be a simple as some people think though.
While the line looks simple on TV, the technology behind it is very complex. Sensors were placed on the three main game cameras (at midfield and the two 20 yard lines), capturing the pan, tilt and zoom movements of those cameras 30 times a second. A three-dimensional, virtual model of each field had to be constructed, with the exact measurements of the crown of the field (the center of the field is always higher, for drainage, than the sides and ends, but the precise levels vary in each venue). An exhaustive color palette had to be produced on the fly as the game progressed, so that the green pixels of grass the yellow line replaced would be exactly the "right greens" even as shadows crossed the field and changed the grass hues -- an essential feature to assure replacing only the green blades of grass and not the green threads of a Packers or Eagles jersey.
The operation of the system was also extremely complex and had to integrate in a sophisticated and complete way with the TV broadcast. We were fortunate to have, in Jerry Gepner, the best person in the industry at pulling off this detailed integration and coordination, but it was an all or nothing proposition. Either the line would appear to stick like paint to the grass, and remain in the right spot throughout the play, or it would not go to air. Being a yard off, or having it appear on a player's jersey, or shudder slightly -- these were not options.
