I'm not a huge football fan by any measure, but I watch a game occasionally if it is on while I'm in a room. As a geek, one of the things that's really fascinated me about football coverage these days is that yellow line they place on the screen showing where the 1st down line is located. In watching it, it looks like it is actually on the field, but of course I know it is generated via computer. It is really impressive when you consider that it looks like it is on the field and that the players seem to move over the top of it.

One of the geek web sites had a link to this video which includes a technical explanation of how they produce the yellow line.

It is an interesting video. Check it out.