I have been pondering how is it that those folks at NFL made such a stable Yellow line for the first-down line. I guess this homoraphy tracking system is one way about it. Interested readers may like to know that htere's an article in teh Spectrum from IEEE last month regarding the awesome looking yellow line on the football field and teh 2 years and 2 million dollars they spent to create such a system.

Potentially, initialization can be done by having the camera face a perfectly planar surface. Using the same point correspondence algorithm over time, it can be determined automaitcally, via neighborhood correlation matching or some such measure, that the observed is planar, and the poitns can be taken as is to compute an initial H.