menu-> colors-> Levels sets the white and black for the whole picture.
However, the lighting is not uniform over the whole page, the "white" is evolving in a gradient from the top to the bottom. When searching for a good "white" the bottom of the page is overexposed, while the top is still under exposed. I'd love to end up with an evenly white background, kind of applying the effect as a gradient
Masks don't seem to work here.
How should I proceed?