It may be just that people's monitors have different brightness or contrast settings. You could change the brightness and contrast using OpenGL. It would be near-enough. I think gamma is a bit more complicated because you have to brighten the image but at the same time leave the black areas black - ie spread the brightening accross the levels. Probably there is a way to do that in OpenGL too using some kind of multiplication operation, blending perhaps, or the accumulation buffer.
Gamma control
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