It definitely does work like a BSP tree. The issue to me is that a tree is a data structure, but the meshes in I, Robot are encoded as literal subroutines of m

It definitely does work like a BSP tree. The issue to me is that a tree is a data structure, but the meshes in I, Robot are encoded as literal subroutines of machine instructions. There is no tree to traverse, the Mathbox simply executes the instructions as it encounters them. These aren't even "interpreted" (fake) instructions for a software VM (like Sweet16). They're literal instructions for the custom Mathbox microcode. The Mathbox executes the instructions, and outputs polygons to the display list as told. It's a lot like a compiler optimization that unrolls a loop inline... Can you even call it a loop once it's unrolled? In this case, it's as if a BSP was created, then turned into literal instructions for a custom machine. Many times the polygons were duplicated in the code (in-lining polys works faster than branching back). It blurs the line between code and data. I'm really blown away by the sophistication of the I, Robot Mathbox... The microcode was exceedingly complex, much more than just some fast matrix math (like Battlezone etc). It executed instructions to compute dot products, which would potentially result in X/Y/Z points being multiplied by a rotation matrix, projected, and written as polygons to the display list, for later rasterization by a different processor altogether. Anyhow it's fun to revisit this stuff :)