Physically Based Rendering
Implementing Physically Based Rendering (PBR) in Doom 3 is a challenge and comes with a few compromises because the Doom 3 content was designed to work with the hardware constraints in 2004 and that even meant to run on a Geforce 3.
The light rigs aren't made for PBR but it is possible to achieve good PBR lighting even with the old content by tweaking the light formulars with a few good magic constants. However I also want to support the modding scene to allow them to create brand new PBR materials made with Substance Designer/Painter or other modern tools so multiple rendering paths have been implemented.
PBR allows artists to create textures that are based on real world measured color values and they look more or less the same in any renderer that follows the PBR guidelines and formulars.
RBDOOM-3-BFG only supports the standard PBR Roughness/Metallic workflow.
Adding PBR is a requirement to make the new content look the same in RBDOOM-3-BFG as in Blender 3.x with Cycles or Eevee and Substance Designer. PBR became the standard material authoring since 2014. With RBDOOM-3-BFG modders can work with modern tools and expect that their content looks as expected.
The PBR implementation is restricted to standard PBR using the Roughness/Metallic workflow for now. Specialized rendering paths for skin, clothes and vegetation will be in future releases.