You can patent an algorithm. For example, marching cubes algorithm was patented so people had to think of ways to develop something similar (like marching tetrahedrons). Also there was a tool developed by some students from one of the European universities called Planta. That tool ought to be a cheaper alternative for vegetation rendering, compared to SpeedTree (and also free for non commercial use). But after initial release, they had to stop any development because IDV warned devs that they will be sued because basically IDV patented a lot of things related to vegetation rendering. Unfortunately people at patent offices are typically too dumb to reject such (generic) patent claims.