A simpler way: You can avoid sampling neighbours by clamping the ray-march step so it never moves further than the current cell's boundary (plus a tiny epsilon). That way, you only cross into adjacent cells at the edge and avoid the 8 to maybe 26 extra SDF samples. (This only works if the geometry is entirely contained within each cell.)

Cool language! What language and library is this?

This seems to be the article's author's own language Bauble[1], "a toy for composing signed distance functions in a high-level language (Janet), compiling them to GLSL, and rendering them via WebGL"[2].

[1]: https://ianthehenry.com/posts/bauble/building-bauble/ [2]: https://github.com/ianthehenry/bauble

Looks like a lisp? Here's the library I think they're using (and wrote): https://github.com/ianthehenry/bauble

What's an SDF?

Signed Distance Field

Huh