threeironteeshot

This is a rather long and complex answer. The short answer is essentially we have sensors on the cameras that tell the computer where the field is. That gives us our drawing board. The field acts as a chroma key (aka "green screen") and we key on the green grass to make it so that the line doesn't draw on players (green uniforms or grass stains on jerseys make it very hard to not draw on players). once the virtual field is set up properly, it is as simple as clicking on the screen to get the line to appear. Again, this is the very simple explanation.

This answer requires a lengthy response. So there are primarily three "game" cameras (1,2,3). They are situated at the 25 yard lines and at midfield (left to right). You will also see other cameras with lines outfitted on some shows. Those are camera 4 (high end zone) and skycam. So if the replay doesn't have the line it is because a different camera is being used for the replay.

As for the zoom and how cameras sense: Essentially the sensors tell the computer where the field is in relation to the camera position using XYZ axes. They do this because voltage from the camera is converted into a digital value and relayed to the computer. So when the camera is zoomed in 40% and looking a certain number of degrees left (pan) and down (tilt) there is a voltage associated with that. We calibrate the computer the day before the game going through all the zoom values, pan values, and tilt values that would be used during the game to get the digital field setup. There is one computer per outfitted camera and we calibrate them all individually. The accuracy of the coordinates gets worse the farther away from the camera the action is. For example, if camera 1 (left 25) is shooting something at the right side red zone, the yellow line won't be super accurate. That is why the director takes the game camera that is most directly on top of the action when scrimmage begins. So we have to set the line of scrimmage and first down line on the camera the director is about to take as the "game" camera to maximize the accuracy of the line.

Hope that answered it.

Ninja edit to make the OP look like he eats tons of boogers! Well played.

We have a list of nightmare places to go because of issues with keying on players. Boise and Oregon are hell when they wear uniforms the same color as the field. When that happens we do the best we can but ultimately we end up drawing on the players. It is unavoidable when it is that alike in color.

I can share a lot of stuff actually, but I would ask that you be a bit more specific. I don't want to talk to stuff you aren't interested in.