I don't know GIMP, but in general it should not be too complicated, you should just play with layers' transparency and use soft brushes, then fiddle until you find a result you like.
I've been trying to figure out for a while now, to no avail, how to merge two types of terrain. Can't find anything related myself so I'm asking for help here.
By merging two terrain types I mean making a smooth transition from one terrain type to the other, say from snow to grassland and then desert or whatever.
GIMP is my program of preference.
I get a feeling it's something quite simple that's been staring at me this whole time. Point me in the right direction if there's a tutorial of a sorts but spare me the torment if the answer is too complex and beyond the skills of a simpleton like myself.
I don't know GIMP, but in general it should not be too complicated, you should just play with layers' transparency and use soft brushes, then fiddle until you find a result you like.
You can also use "feathering" to create natural transitions between textures. I did a little tutorial HERE. It's for PSP8, but should be easy enough to translate to GIMP.
My: maps ~ mapping freebies
If you put your terrains on two layers, overlap them a bit, add a blank layer mask to the topmost layer and use a grungy brush set to low-mid opacity to stroke the outer edge of the overlap (on the layer mask) to soften the transition... that should work in the GIMP. It'll work in any painting program that has layers and layer masking capabilities. The other option, which is more unforgivable if you change your mind but works without masking, is to put the terrains on two layers and use a grungy brush eraser on the topmost layer to the same effect. There's probably an uncountable number of ways to achieve a similar effect... the wonder of digital media. This way is a little more hands-on.