Learning to cast a fly rod well is famously hard.
Tiny errors in the stroke get amplified by the rod, then the line, then the leader. The whole thing can look fine for 80% of the motion and still collapse into a tailing loop. Sunglasses at this stage are more about not putting a hook in your eye than glare.
I’ve been building a physics sim of that system.
Rod blank under load is not the path a rigid stick would trace. The line is a long flexible body with drag. Material specs are messy and often trade secrets. The human side is multi-joint choreography. But unlike a lot of “feel” skills, a good cast has a defined, measurable target: fly leg rides over rod leg and the two never meet, loop tight, leader turns over, nothing comes near your head.
The path that prevents you from wearing an eye-patch the rest of your life is the loaded tip path. Convex is safe. Straight is the target. Concave tails every time.
Once the rod and line are simulated well enough to score that, the problem stops being “how do I cast” and becomes “what tip path produces the best scored cast, then what body motion can produce that path.”
That’s the part I’m running in the spirit of
@karpathy’s autoresearch loop: generate a tip-path candidate, simulate the cast, score it, keep or discard, repeat. Thousands of permutations. Then work backwards onto a simplified 3D body holding the rod.
And all of this for a cozy video game I’m making with my son 😆