I think it could be usefull in-game if u want to make it brighter or darker in order to see textures better. If it could be added in the options menu it would be great.

I've noticed that there are some shaders which are better for seeing certain textures, some which highlight certain aspects of textures, along with some that completely alter the colours of textures - maybe this could be woven in some how, who knows.

Would be nice to have a contrast function. What parts of the screen would you propose be altered for this?

in options menu a slider for brightness and contrast