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I have a few images that have been anti-aliased onto a white background that I want to put on a transparent background.

Just selecting it with the -wand tool/fuzzy select tool/select your terminology of choice- and deleting the background tends to leave a ring of off-white pixels around the image, or eat into the image depending on the tolerance setting.

Is there some better way to do this, preferably an automatic tool? (I'm on Linux)

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up vote 2 down vote accepted

Using Gimp, you can further refine the selection using Select -> Feather. I generally use the wand tool to select the white background, Select -> Grow Selection by 1 pixel, Feather by 2 pixels, and then remove the background.

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Have you tried imagemagick? You can specify or change the background color of an image:

mogrify foo.png -background transparent

changes foo in place, while:

convert foo.png -background transparent new.png

doesn't touch foo.png.

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