I wrote Free Rider (and Free Rider 2) about 20 years ago. It has been played over one billion times since I wrote it.

When I was little I used to be made to go on walks in the Chiltern hills with my mum and dad. Sometimes I would distract myself from the terrible boredom and suffering of being made to go for a walk by wondering how nature managed to simulate all of the fractal complexity of rocks and tree stumps and flints and clay under my feet. Eventually I started imagining a tiny bicycle bouncing over this terrain and wondered whether I would ever be able to realize such a thing in the computer.

Progress started with learning Verlet Integration, which was pretty fun for simulating particles on springs, double pendula, stretchy chains and stuff like that. The most interesting problem was how to efficiently compute line/circle collisions on an infinite canvas at 30fps. Once you have those two pieces it’s not a huge leap to get bicycles rolling around on almost arbitrarily complex terrain.

My friend John came up with the free rider guy’s iconic hat – without which I think the game would have almost certainly failed.

I’ll admit that I’m slightly embarrassed about Free Rider - it’s something I made when I was about 18 - but what I do really like is that young people still seem to enjoy using it to make drawings (mostly meticulously detailed natural landscapes) and that there is still an active community around these drawings.