Map borders on GIMP

Question

I'm trying to create a map for Risk. I found an image scouring the Internet that fits what I want perfectly, except instead of having borders, each province is just a different color. I would like to add borders to each province. (The first image in this album is what I am describing here).

So, using GIMP, I tried going to Select > Border and creating a 1px one for the first territory. It worked fine (see the second).

But then adding a border to an adjacent one creates what looks like a 2px border between territory 1 and territory 2 (see the third).

Is there any way for GIMP to create a border around a selection such that if there already exists pixels of that color, it'll just ignore it?

(I realize that a possible solution is to blow up the image 2x with no interpolation, create borders around everything which will be each 2px, and then downscale it to the original size. The size I want for the image (14000px by 5500ish px) is a bit big for that, though, so I'd like to avoid it if possible.)

Thanks very much!

Answer 1

Just use one of the filters in filters/edge-detect - with some extra actions until you get what you want.

What I did here: duplicated the layer, cut out the oceans (I don't know if it is your intention) by clicking on one by one with the magic-wand, right click on the layer in the layers dialog, add alpha channel, then edit->cut.

After that, , select->None , filters->edge detect->edge... , and picked a suitable algorithm from the list - the oceans border will be thinner, if you don't want that, just fill then with white instead of cutting them out to transparency.

Now you have your countries in black, and colorfull edges: Use the select by color tool, click on a black part of the map, and edit->cut; to turn the edges black use either colors->threshold, or colors->curves` flatlining the curve to zero.

Add a backgorund layer on the color of your choice and export the image.