I've been working on the terrain and camera control for my “Lunar Lander on a round planet” project.
The planet generator now creates a BSP representation of the terrain in addition to the boundary representation, to be used for collision and contact.
I downloaded the latest version of the Box2D physics engine to use as a reference. The released version has only convex polygons and circles as its physics primitives; I would need to compose a bunch of convex polygons to make the terrain. Apparently there is an unreleased version with an edge-list primitive, which could also work for the terrain.
Box2D and Chipmunk use spatial hashing rather than BSPs. I'm not sure yet which would be best for my planet; I'm going forward with a BSP for now. I have an idea for computing contact with BSP leaf nodes using the split planes to implicitly define the convex polygon around the leaf. That ought to use memory fairly efficiently.
When I started the BSP implementation I tried to set it up such that the deepest splits would identify which of their leaves were inside and which were outside (based on the surface normal). The idea is to avoid having any storage for the leaves; only for the splits. I think that only works if your splits always lie on the surface somewhere, though. I ended up using two sentinel values to represent inside and outside leaves (instead of the one sentinel value you'd need for the other scheme) and it simplified things considerably.
Implementing ray intersection against the BSP terrain has allowed me to experiment with some camera framing algorithms. Framing the ballistic trajectory is good for some situations. One problem is that the impact point can change distance instantaneously which makes the camera jump around. I also tried shooting a bunch of rays in all directions to form some sort of idea of the locally visible terrain, but this also tends to change in jumpy ways. Currently I'm working on an algorithm that is based primarily on the specific mechanical energy, since that changes smoothly.
Generating terrain with lots of tunnels made me realize that I needed to enhance my gravitation formula to work inside the planet. The force of gravity falls off linearly as you work your way from the surface to the center.