I also designed my own 3D engine but after attacking the physics engine, I quickly (and, never regrettably!) switched to the open source Bullet engine instead a

I also designed my own 3D engine but after attacking the physics engine, I quickly (and, never regrettably!) switched to the open source Bullet engine instead after going through its code. I would never have the stamina to write that, and all that code and bunch of algos are really necessary for doing physics in "free world" games like mine. Btw Bullet has continuous collision detection. It's not fundamentally magical - you extrude a hull of the shapes to do CCD on along their velocity tangents and collide the hulls instead of the shapes. This effectively solves the problem of bullets or other fast moving objects going through walls etc. (this is why the engine is called Bullet btw) Actually the most difficult problem I had with the physics was the player physics, as its usually not handled by letting the player be a normal physics body (for gameplay reasons). The player shape can easily get caught up on infinitesimal seams between polygons and edges in the world around you. :)