Work on the physics engine is progressing remarkably quickly! Last week I implemented a fairly nice, albeit basic, collision resolution system. To test it, I set up a sizable asteroid field (~1000 asteroids) and a simple ship. I gave the ship the ability to spew small pellets, which serve to test the collision engine. I'm really, really happy with how painless it is to setup the physics engine: it just takes one initialization call, one "Add" call for each object in the system, and then a "FindAllCollisions" call in the update step. Everything else is automatic - the engine keeps track of the objects, their collision meshes and density fields, and figures out when objects are colliding.
Everything on the screen is collidable, so there are no special cases or exceptions. If asteroids are included in the physics engine, then all asteroids can collide with all other asteroids. If bullets are included, bullets can collide with bullets as well as with asteroids, etc. Everything functions just as you would expect. Finally, the part that really matters - the performance - is really, really good!!! I was amazed by how fast the engine runs. I can get about 1000 asteroids + several thousand bullets on screen at once while maintaining 60fps. That's a lot of colliders! It's really cool to watch bullets collide multiple times off of different asteroids.
There's still work to be done. The system still has a few glitches: bullets are occasionally piercing asteroids, the system could be even faster, and the physics could be more realistic (Newton would be crying if he saw my collision response equations). The next step is probably to implement spatial signatures, just to see if I can squeeze some more performance out of the system. When everything gets tightened up, I'll definitely be recording a video demonstration of it.
All in all, I couldn't have hoped for better luck with the physics engine I'm quite proud of it! I think the engine will serve the inhabitants of my reality well.