How do I do a realistic color change on a realistic fractal map ?

[Deadshade]

My skills aren't that great, but for what it's worth, I use Selection / Feather tool a lot when I want to blend. It can't be used in every scenario, but works great when you want to remove the edge from a block of colour.

I am doing that a lot in Wilbur to avoid too sharp cliffs too. However the problem is that I don't really want just to blur the edges (this then looks like a smudge separating 2 textured areas) ) but rather create a transition where the proportion of pixels of a given color gradually changes from 100% to 0% when one goes across.
The reason I so intensely look for a solution is because this is the only way to do a very realistic map. In the nature when one goes from a jungle to the desert, the textures and colors gradually change - there are no edges, no blurs and no sharp boundaries (well to be accurate, the sharpness depends on the scale and the nature of the biomes - ice/tundra is rather sharp but jungle/Savannah isn't.)

So this is a very general problem as soon as one wants to do a very realistic map on continental scales - the shapes/textures and colors for a biome are given but then one has to manage a believable and gradual color and texture transition from one to the other.

[waldronate]

It sounds like you want to do a fractalized blend instead of a basic linear blend. One way to accomplish this is to start with the blurred layer mask as a transition and then multiply that mask by a noise image that runs from a value of about 64 to a value of about 192 (0.25 to 0.75) [alternatively, add noise to the layer mask and then blur that noise slightly; repeat at different, decreasing blur and noise values to get a fractal effect). Note that the below images were done with Photoshop, but the concept of layer masking should be similar enough to be applicable.

Untitled-3.jpg
two solid-color layers with a blurred mask that has noise added (followed by a slight noise and levels adjustment)

Untitled-4.jpg
two two-color cloud layers with the same layer mask as above.

Putting in more than a few minutes work on the mask would probably give much better results.

[Deadshade]

It sounds like you want to do a fractalized blend instead of a basic linear blend. .

Yes that is exactly what I want to do - another way to say diffusion pattern ! Thanks Waldronate - the second one is probably on the right track but it should look less like spilled coffee.
I am really newbie in using GIMP masks and even less "blurred" masks so will have to figure this out first.
I suppose that this is a similar (identical ?) method like what Freodin wrote.

[waldronate]

The first step in making the mask is the same sort of thing that Freodin described (make a mask and blur it), but it's the adding noise step plus blur and then scaling and/or thresholding of the layer mask that makes it look less like a broad, smooth transition.

The first image I showed is literally two solid colors, yellow on top and green below, blending through a noisy gradient mask (the mottled green area is the effect of the mask modulating the yellow onto the green). It's when the same mask is used to transition between the yellowish and greenish noise textures below that things look much better.

[Freodin]

Next attempt:

fade2.jpg

Now this is a little more complicated... I don't claim to be a MasterGimper myself, so it is possible that there is another / more simple way to do what I did here.

There is a layer mode that you can set for single layers called "dissolve". What it basically does is using the opacity of the layer as factor to randomly show or hide a pixel... just what we want here.

So it is - as I did it here - a two-step process:

1. Create your (alpha) mask. This can be done by all of the processes already mentioned here.
Start with a transparent layer over a BLACK background layer.
a) in my first example, I handpainted a harsh white on transparent division that I then heavily blurred... resulting in a smooth transition between white and full transparency.
b) You can use selections, and the "Feather..." option from the "select" menu... and then fill with white. This also results in a smooth transition. (This is what I used in this image here: simple circular selection, feathered 200 and filled with white.)
c) Do it by hand. Set the opacity of your chosen paint-tool to a lower value (10-20%) and start to paint up. The more often you paint over an area, the brighter it gets. That might not result in such a perfect transition as the other two methods, but here you can exactly define where you want to have how much opacity.

2. Set the layer mode of the white/transparent layer to "dissolve". You should now have a more or less random distribution of white pixel, denser at the white parts, more sparse at the black parts.

Create a new layer "from visible". (I found that this is the only way to "fixate" the results of a dissolve layer mode.)
This new layer will now be used as the layer mask. Call it "mask".

I think it looks nicer with a little blur... play around a little.

So, now you need your "grass" layer and your "desert" layer on top. Add a layer mask for the top layer, "copy" your "mask" layer and paste it into the layer mask.

That should give you the result as in the example.

[Deadshade]

Well thanks Freodin.
This last example is almost exactly a diffusion pattern I have been thinking of. So it looks indeed like a fractal transition between 2 régions. Of cours it has to be irregular but your circle was just an example.
So now I have to tame the functions you described and transform it in a reproductible workflow.