Raytracers are really cool! On the other side of the (not so minimal or efficient spectrum) I made my own javascript-based raytracer using canvas and webworkers:

https://github.com/jleppert/raytracing.js

It was based off the excellent introduction Ray Tracing in One Weekend (http://in1weekend.blogspot.com/2016/01/ray-tracing-in-one-we...).