Football is a complicated sport. Offensive players can move around before the quarterback calls hike, but only certain ones at certain times in certain directions. A defender can rough up a receiver within five yards of the line of scrimmage, but only if he remains in front of the receiver and the contact is continuous; after that, the defender can still make some contact, but only as long as it does not “significantly hinder” the receiver from catching the ball—whatever you interpret that to mean. And that’s without getting into what constitutes a “catch,” a seemingly basic question that the NFL rule book turns into a matter of great metaphysical complexity. At least one thing in football is not complicated—that is, if you’re watching on TV: the yellow first-down marker. The virtual line, convincingly projected onto the field during every major football telecast through augmented reality, makes the sport immediately more digestible. Football, the cliché goes, is a game of inches, and even the most die-hard fans benefit from the clarity and drama that the …