Over the years I've written a bunch of things in the orbit of retrocomputing, the largest of which was an incremental game based on a C64 emulator. Somehow I've never written anything substantial for the C64 itself; this post documents my learning while implementing a graphical effect in assembly language, over the course of twelve thousand words and three digressions into side quests.

Let me know if this was entertaining, useful or even both.

Fun fact: a bouncing ball was one of the first programs that the C64 User Guide taught you: https://archive.org/details/commodore-64-user-guide/page/n57...

Yes, that's right kids: the C64 came with a manual that didn't just teach you where to flip the power switch of your computer but actually how to program it!

Nice!

Why not just go full character mode?

The smooth motion is nice, but a bit of overkill, also it adds complexity to the collision code.

I suspect this can be done using a variation of Bresenham's line algorithm for the trajectories (keeping delta X and delta Y for each ball) and avoid most/all of the complex trig and math. Just addition and subtraction and a few sign changes.

The delta can be kept at a higher resolution than the grid (i.e. 16x so a mask can be used instead of division), so you can fudge the collision with small delta changes on impact using the SID as a RNG.