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I use a scanned B&W image in Gimp. I use bucket fill to fill the white spaces. What are the best settings for Bucket Fill if I want to best preserve the black border lines in the scanned image? Currently, the border lines seem to get lighter when I use the bucket fill and I don't like the looks. My settings were where I wanted them in a previous version of Gimp, but I had to upgrade to 2.10 and I don't remember the settings I had. I typically filled with a 90.0 threshold. Thank you!

Plain bucket-fill isn't so correct because there are pixels of mixed colors on the edges that may not be "similar" enough to be painted over, and this leaves a faint halo of the previous color.

In 2.10 the bucket-fill has a "Fill by line art detection" that may be what you want.

Otherwise a good technique is to remove the backrgound (Colors>COlor to alpha) and either:

• paint on a additional layer under the layer with the lines
  
• paint in Behind mode in the same layer