Path tracing ideas:
* https://raytracing.github.io/books/RayTracingInOneWeekend.html
* [smallpt](https://www.kevinbeason.com/smallpt/) (100 line c implementation of path tracer)
* [three path tracer](https://github.com/gkjohnson/three-gpu-pathtracer)
* http://blog.hvidtfeldts.net/index.php/2015/01/path-tracing-3d-fractals/
* [Global Illumination and Path Tracing](https://www.scratchapixel.com/lessons/3d-basic-rendering/global-illumination-path-tracing/global-illumination-path-tracing-practical-implementation)
* [Path Tracing, by Steve Rotenberg](https://cseweb.ucsd.edu/classes/sp17/cse168-a/CSE168_08_PathTracing.pdf)
