A lot of this has changed since then, of course. No grass swatches anymore! We have a lot more computer horsepower to throw at the problem now, plus a lot of gained experience.

We have a bay in our lab with a to-scale football field where we can run indoor tests, but on the field it is computer vision that is doing it all. The cameras are instrumented to get attitude, zoom, and so on (we have to modify the lens to get the accuracy we need), and then of course you have to compensate for lens distortion. If you look closely, the "line" is not a line, but a curve. If we drew a line it would look stupid, and not reg well in most parts of the image.