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I am trying to run Deus Ex on something newer than my ancient laptop and I immediately encountered the problem of Deus Ex being too dark and being unable to modifiy the brightness. I did a little digging and found that switching to OpenGL should allow for brightness to be adjusted without any problem. Switching over does fix the brightness issue, but it introduces new problems for textures and lighting. Textures have a nasty tendency to flicker, and the lighting on any given patch of ground looks godawful, often glitching out as I move around a room. I can't find anything about this on the forums, so I'm not sure if it's just my hardware throwing a hissy fit or not.

EDIT: looks like the renders linked to in the mods thread fixed the problem fairly decently.
Post edited July 04, 2016 by Jonesy89
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I'm having the same problem with the game being unplayable and the brightnes not changing.
Using 3dfx glide mode is pretty much the only way to play this on modern machines. (The openGL mode is a joke and direct3d is dark. Like, way dark.)
Thankfully the GOG version comes with a glide wrapper (a thingy that tricks the game into believing you have a 3dfx card) pre-installed.