The PS1's GPU does not support perspective correction at all; it doesn't even receive homogeneous 3D vertex coordinates, instead operating entirely in 2D screen space and leaving both 3D transformations and Z-sorting to the CPU [1]. While it is possible to perform perspective correct rendering in software, doing so in practice is extremely slow and the few games that pull it off are only able to do so by optimizing for a special case (see for instance the PS1 version of Doom rendering perspective correct walls by abusing polygons as "textured lines" [2]).

[1]: https://github.com/spicyjpeg/ps1-bare-metal/blob/main/src/08... - bit of a shameless plug, but notice how the Z coordinates are never sent to the GPU in this example.

[2]: https://fabiensanglard.net/doom_psx/index.html

The README mentions that it uses both (new) fixed point as well as soft floating point.

Unless I'm mistaken, the PS1 just plain doesn't support perspective correction. All texture mapping is done in hardware using a very not-programmable GPU; there'd be no way to do perspective correction, decent frame rate or not, outside of software rendering the whole thing (which would be beyond intractable).

The common workaround for this was, as suggested, tessellation - smaller polygons are going to suffer less from affine textures. Of course that does up your poly count.

it's not possible to have either subpixel vertex precision or perspective correct mapping with the PS1 GPU, as it only takes 2D whole-pixel coordinates for triangle vertices. (contrary to popular belief, N64 also uses exclusively fixed point for graphics btw, it just has subpixel units.) better tessellation can mitigate the perspective issues by a lot, but the vertex snapping is unsolvable, and it is indeed present here. look closer and you might see it.