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I've been taught this method of removing gray background in illustrations by Donovan, when I was working on the Aesop's Fables project. It worked relatively well for the images that I had that looked like this at the start:


And in the end after applying the method they looked like this:


I was using the GIMP program to achieve this. Step-by-step instructions:

  1. Open the original, make a duplicate with Ctrl-D
  2. In the duplicate, use the dropper tool to pick a new background color from that speckled paper, somewhere that's in the middle, greywise
  3. Select ALL of the duplicate image and clear it to that new medium Grey, then copy that entire grey image (Ctrl-A Ctrl-. and Ctrl-C) (that second is Ctrl-period)
  4. Now, go back to the original image, and paste the grey blob over it. Just leave it selected, and then bring up the Layers Dialog (Ctrl-L) . ... (you now have two layers in the original, the background and the floating pasted layer)
  5. Okay, Notice there is a Mode (Normal) and Opacity (100) by default there. Change the mode to Divide, and leave the opacity at or near 100. It should look a little lighter, a lot cleaner, still a bit speckly here and there
  6. Hit the Anchor icon on that layers dialog to anchor your changes. (really it's a merge down to single layer)
  7. Now pull up Layers | Colors | Levels, and adjust it. (I usually hit auto, then sometimes manually set a white point) Which btw you can pick from the image with those droppers ... notice the hover titles?
  8. If "auto" doesn't seem to have changed much, tweak it around a little bit. See the Level Adjustment section in Guide to Image Processing for some guidance.