[Richard]

DOOM 3 has a type of light called "ambient light". Because D3 was supposed to be dark and stuff it was never used extensively in the game and perhaps the reason why this might have not been noticed. I first heard about this from id a while ago:

On ATI and geforce 5xxx (and below) cards, ambient lights will light a surface evenly (because we use a solid blue normalization cube map), but on 6xxx cards, the ambient keyword is ignored (because nvidia replaces our cube map lookup with normalization through math).

and

The 6800 has a zero cost normalize function which they are probably substituting for our normalize-with-a-cube-map method. That works on all surfaces except ambient ones where our cubemap isn't a normal cube map.

I also believe this was only introduced in the 61.32 drivers. I don't have a NV4x to test myself but recently I released a custom D3 map set outdoors which makes heavy use of this feature and I got feedback from a few people saying the level looked too dark. I was then reminded about this nVidia optimisation. The purpose of this thread is two-fold: primarily raise awareness for an issue of a specific optimisation worsening image quality and secondly a call to anyone willing to test this further. Thank you.