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Underexposed on top, overexposed on bottom I have to restore a document from a picture in GIMP for printing. The picture taken from the document is of moderate quality. The document is just black text on a white sheet of paper.

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?

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Your color change is mostly Blue because the value drop is higher in the Red and Green channel:

enter image description here

So if you only need a monochrome image you can just extract the Blue channel. The easiest way to do this is to open the Channels list, and drag the "Blue" channel to the canvas. This creates a layer which is a copy of that blue layer.

enter image description here

Another technique (your sample is too small to test it):

  • Filters > Enhance> Wavelet decompose
  • Replace the Residual layer by its average (Filters > Blur > Pixellize with a block size as big as the layer).
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  • The trick with equalizing the color on the Residual layer after the Wavelet decomposition did the trick! Amazing! Thanks. – lode Jun 22 at 7:15
  • On the original size drawing, the blue channel isolation didn't do it (enough)... ;) – lode Jun 22 at 19:33
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    A combo of both methods may work even better. Also if you use Wavelet Decompose, you can sharpen the image by duplicating layers at the top levels (1 and 2). – xenoid Jun 22 at 21:09
  • Thanks, very inspiring! – lode Jun 23 at 1:37

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