Basically it's done using Wilbur. The most important tutorial is the
Fun with Wilbur Vol 1.
First I draw the basic outline of the mountains in Gimp. This is what the most recent version of the mountains for this particular map looks like (pre-Wilbur):
Attachment 84566
This map is then exported as a PNG file and uploaded into Wilbur. Note that I made the map smaller (5000x2500 in this case) because Wilbur has difficulties with processing large maps, so I'll have to do different sections separately for a 6000x4100 map like this one.
Once I have the basic outline in Wilbur, I use a mix of Precipitation Erosion, Percentage Noise, Incise Flow and Fill Basin to create a more realistic topography. The process is covered in detail within the Fun with Wilbur Vol 1 tutorial (by Waldronate). The usual process is something like this:
- Span the height Range (Filter->Mathematical->Span), for example Low: -1 and High: 4000
- Percentage Noise followed by a few cycles of Precipitation Erosion
- Fill Basins followed by more Noise and Erosion
- Fill Basins followed by Incise Flow (adjust the settings until you create large valleys)
- Erosion, then Fill Basins again and Incise Flow (this time I try to create more narrow river channels instead of wide valleys)
Finally, I turn the result into a height map (Texture-> Gray Maps-> Height Map) and save it as a PNG file.
This is what the "Wilburified" mountains look like:
Attachment 84567
Then simply add your mountains into the main map (Open as Layers). At this point you have what you seek: a black-and-white height map. Then simply colour it with whatever gradient you want (I use a simple tan -> dark brown gradient) and choose the appropriate Mode for the layer (Burn in my case). The basic process of doing this step (Gradient Map) is covered in Arsheeh's excellent
Eriond tutorial. The Eriond tutorial also has an alternative way to do the mountains, which you may find interesting.
Anyway, here's what the final result looks like (so far, still WIP):
Attachment 84569
Note: The mountains look slightly different (in terms of colour) from the previous version largely because I used a different Span this time around (-1 to 6000 instead of -1 to 4000).