One solution with Gimp 2.10:
* Filters>Enhance>Wavelet decompose and decompose to maximum level (7).
* In the 'Residual' layer, sample the color (sample average with a marge radius)
* Bucket-fill the layer with the sample color (so tha it becomes a uniform layer)
A "softer" but a bit more labor-intensive version is to paint the problem areas of the Residual layer with a large and soft brush.
A more perfect version would also do something similar on the nect to last layer (level 7).
A completely different solution is to do two scans after rotating the paper by 180° in between. Then in Gimp you align the two images and set the top one to "Lighten only". Caveat: this requires an accurate scanner, the scanners of combined printer-scanners are rarely accurate enough.
* Filters>Enhance>Wavelet decompose and decompose to maximum level (7).
* In the 'Residual' layer, sample the color (sample average with a marge radius)
* Bucket-fill the layer with the sample color (so tha it becomes a uniform layer)
A "softer" but a bit more labor-intensive version is to paint the problem areas of the Residual layer with a large and soft brush.
A more perfect version would also do something similar on the nect to last layer (level 7).
A completely different solution is to do two scans after rotating the paper by 180° in between. Then in Gimp you align the two images and set the top one to "Lighten only". Caveat: this requires an accurate scanner, the scanners of combined printer-scanners are rarely accurate enough.