This looks a lot like the quincuncial projection applied to a cube map render, with some custom logic to subtly warp geometry in a stable way so as to make cert

This looks a lot like the quincuncial projection applied to a cube map render, with some custom logic to subtly warp geometry in a stable way so as to make certain details pop based on distance. You can try for yourself by downloading blinky (formerly fisheye quake) [1] and experimenting with the projections. That said, it's probably not just a projected cube map. Doing so will introduce noticeable sampling artifacts, so I am guessing they also do something more to address such issues. [1] https://github.com/shaunlebron/blinky https://github.com/shaunlebron/blinky