Try zooming in on Russia, Alaska, Brazil, or Egypt and you find a lot of empty tiles. Still, if you can render the whole thing in 4 days I suspect caching what people actually look for would be good enough as people probably hit max zoom on manhattan 1000x as much as they do on some random village in the amazon. The advantage being you can just invalidate tiles as you get new information vs trying to render the whole thing every time you get new street data for Ohio.
True, and countries that are empty except for random river squiggles can save a good bit of storage (at the highest zooms so those river squiggles are isolated), but, again, if you have to start with the lowest level first, the described approach isn't saving a whole lot in processing time, even with big empty space.
One thing I forgot to say in my top post, though, is the presentation mentions compositing layers together on the fly, and that is one of the key tools they use, but the presentation drops that thread. I'm curious if originally it had more on that front and what they do with that.
