SpeedTree

A tree is a nasty rendering problem: thousands of leaves, transparency everywhere, and a forest needs tens of thousands of them. SpeedTree, from US company IDV, tackled it with procedural modelling — trunk, branches and leaves generated from rules and endlessly variable — and a runtime that handles level of detail all the way down to flat billboards and impostors in the distance, plus the wind that makes the whole thing sway.

Oblivion was an early showcase, and Avatar proved the same tool was good enough for Hollywood. Unity bought IDV in 2021. If game forests all look a bit alike, you're not imagining it.