Modding the lighting and specular as in idtech

[Radiance]

You don't have access to any of the C++ deeper capabilities, and in fact, you don't even have access to any functions outside of what is declared in the hps_api.hps file (or in other script files that also ultimately point back to that one file).

Why there isn't access of the C++ deeper capabilities?

That being said, maybe this isn't what you were asking. Was there a particular effect or feature in the Doom engine that you were interested in replicating?

Only specular bump map and lighting rendering.

What about particle system ?