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[OpenGLDrv.OpenGLRenderDevice]
FrameRateLimit=120
Translucency=True
VolumetricLighting=False
ShinySurfaces=False
Coronas=False
HighDetailActors=True
DetailTextures=False
UseBGRATextures=True
UseAA=False
UseSSE=2
UseTexIdPool=True
DynamicTexIdRecycleLevel=100
UseTexPool=True
SwapInterval=0
OneXBlending=True
TruFormMinVertices=0
TruFormTessellation=3
UseTruForm=False
NumAASamples=0
RequestHighResolutionZ=True
MaskedTextureHack=False
UseVertexProgram=False
UseCVA=False
UseMultiDrawArrays=False
AAFilterHint=0
TexDXT1ToDXT3=False
BufferClippedActorTris=False
SinglePassDetail=False
ColorizeDetailTextures=False
DetailClipping=False
UseDetailAlpha=True
RefreshRate=85
MaxTMUnits=0
NoFiltering=False
UseFilterSGIS=False
MaxAnisotropy=0
UseTNT=False
Use4444Textures=False
UseS3TC=True
UseAlphaPalette=False
AutoGenerateMipmaps=False
UseTrilinear=False
UsePrecache=False
AlwaysMipmap=True
ShareLists=False
UsePalette=True
UseMultiTexture=True
UseZTrick=False
MaxLogTextureSize=8
MinLogTextureSize=0
MaxLogVOverU=8
MaxLogUOverV=8
GammaCorrectScreenshots=True
GammaOffset=0.400000
LODBias=0.000000
DescFlags=0
GammaOffsetBlue=0.000000
GammaOffsetGreen=0.000000
GammaOffsetRed=0.000000
Description=AMD Radeon
CacheStaticMaps=False
Use16BitTextures=False
Half the problem was a program called f.lux, which apparently has to be closed with OpenGL but not D3D9. The other half seems to be with ATI:
Source: http://www.cwdohnal.com/utglr/newsarchive.html8-29-2005
Numerous versions of ATI's drivers up to and including the current version 5.8 have issues that may affect gamma correction for UT in OpenGL mode, and various other games and applications. This isn't anything I can fix. If gamma correction doesn't work, try moving the in game gamma slider back and forth as this will sometimes fix it.
Moving the slider or taking a screenshot corrects the gamma problem but needing to do this every time UT is run seems like a hassle. Figured twelve years later there would be a solution by now.