Btw. Why is lighting handled differently in Fitzquake (ostensibly also DP) and regular GLQuake-based engines
The original quake (software) engine used overbright lighting, which means the lightmap brightness can go up to 200%. So, light.exe creates BSP files with lightmaps that go up to 200%.
In glquake, since it was such a rush-job (I think carmack said he wrote it in a weekend) there is no overbright lighting, so every part of the lightmap that goes above 100% is flattened to equal 100%. This is why glquake looks just fine in darker rooms, but in bright areas the lighting looks very flat.
Fitzquake and darkplaces restore the overbright feature of the software renderer, so that lighting hot-spots will actually be as bright as intended.