I wanted to actually understand collision detection — not "call the library function," but understand it well enough to write it myself. So I built a small 2D physics engine in C++ and SDL2, no external physics library, and picked the Separating Axis Theorem (SAT) as the core algorithm. Here's how it works, and the gotchas I ran into turning "shapes are touching" into "shapes bounce correctly." Why not just bounding circles? Circle-vs-circle collision is one line: compare the distance b...