I have always been amazed at how accurate the computer-generated first-down line is and also how it has improved to where it almost never overlaps players and referees. According to an article at howstuffworks.com, "The idea to paint a first-down line across the football field on people's TV screens sounds so simple. As it turns out, implementing this is incredibly complex. It takes a tractor-trailer rig of equipment, including eight computers and at least four people, to accomplish this task!" The second page of the article is where they really explain how it works, down to small details like how they consider the curve of the football field.