February 13, 2024

Using the Gimp to fix colors on a faded scanned map

I have a scanned file (from a public website) of a scanned map. The background is a yellow/brown faded color. I want to make the background white and enhance contrast for other colors that remain after making the background white.

I type "gimp toughnut_westside.pdf" and am pleased to see that the gimp will input a PDF file. The import dialog tells me that it was scanned with 300 pixels per inch and is:

The file is 8797839 bytes (just under 9 megabytes).

Fix the background

I use Colors -- Levels and near the bottom pick and eyedropper with a white square next to it. I click the eyedropper on a part of the map that is the background orange and "shazam" -- it does a nice job of making the background white across the image.

Pump up saturation

The aging that made the paper "yellow" also faded the original colors. We want to try to fix that somehow. Sort of at random I try Colors -- Saturation. It offers me a slider labeled "Scale". I slide it all the way right and get about 2.2, but I find I can just type in values. After fighting with the entry widget I find I can input up to 10.0

And play with curves

There is also Colors -- Curves and I use this to do some final corrections that make me pretty happy.

Save the result

I use File -- "Export as" and it is clever enough to set things up to export a PDF. Note that you do not want to use "Save as".

But we don't get an entirely happy outcome. I save to "fix.pdf" and get a huge 51 megabyte file. Various utilities indicate that the size in inches is still correct. The gimp import dialog actually gives me the best information, confirming that the size in pixels matches the original file and that 300 dpi is still indicated in the saved file.

This actually seems to have worked out with little frustration and wasted time and even if the file is now 5 times bigger than it was originally, who cares in this day and age.

Feedback? Questions? Drop me a line!

Tom's Digital Photography Info / tom@mmto.org