/3/ - 3DCG

Back when I used to make shitty Unreal Tournament maps, texturing was trivial.
>use subtractive BSP/CSG to build out blocky level geometry
>literally just click on polygons to assign textures
>no need to handle unwrapping, scale and orientation is always correct, seamlessly tiles if adjacent polygons have the same texture, etc

Let's suppose I want to do something similar in Blender (or Modo, or any other programmable 3D package for that matter): Apply image textures to given polygons without having to manually unwrap them.
What exactly was Unreal doing behind the scenes to generate the correct UVs? Maybe it only worked due to the restrictive CSG system, and couldn't extend to arbitrary meshes?