I read that the S5 screen can go pretty bright, but only in certain situations and it's not controllable by the user.
Is there a hack that can allow me to set the screen brightness to be at its true max?
It is only that bright in direct sunlight. Normal max brightness is 351 nits. I believe it's 698 nits in direct sunlight. I believe OP is asking if there is a way to make it stay closer to the 698, via software hack.
To answer your question, even if it is the brightest phone out there, it still sucks in sunlight. In my honest opinion.
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