Yosemite's Level of Detail
"When I think about realism in a computer game, I always think of Yosemite National Park in California - huge mountains and thousands of trees in a richness and variety that makes any visitor feel tiny in comparison. I don't think we will be reaching that level of complexity in some time. So, it's a perfect example to show how resolution and level-of-detail (LOD) techniques work.
Let's imagine for a second that you actually tried to do a game set in Yosemite with the tools we have seen so far. You do aggressive clipping, which, because your camera has an aperture of 60 degrees, reduces complexity to one-sixth of the total triangles. Then, some serious culling chops that in half to reach one-twelfth. Also, you have a dream-like occlusion calculator. Let's daydream for a second and imagine that after the tree stages, you have reduced the visible triangle counts to about one-fiftieth.
Now how many triangles does Yosemite have? I know it might sound absurd, but follow me through this exercise because it will be a true eye-opener. Let's assume you do a 20km x 20km square terrain patch, with terrain sampled every meter. If you do the math, you will see we are talking about a 400 million triangle map. Not bad, but let's not stop there. Now comes the fun part: trees. Trees are hard to get right. From my experience, a high-quality, realistic should be in the 25,000 triangle range (yes, each one). Assuming one tree for every 20 meters, that yields 1 million trees for the whole park, or even better, 25 billion triangles per frame. At a very reasonable 30 frames per second, the triangle throughput of our engine is… well, you get the idea, right?"
Core Techniques and Algorithms in Game Programming p 373-374