Exactly. My mask is just black and white and I planned on using WM to add features/hills/mountains etc while not going outside the boundaries of my outline. I have tried what sfriedberg suggested, but I lack the knowledge about how to go about it using Gimp. I got close to what I needed though by shrinking my selection my 10 pixels each time and changing its colour from black to white gradually. One of my earlier attempts had elevation data on the mask, but it looked horrendous in WM and I found it hard to select just one mountain out of it all and apply erosion and then select a different hill and apply a different terrain type. One of my biggest problems is knowing what order I have to do things. There is a significant lack of tutorial videos about that are remotely like what I am trying to attempt. All the video’s I see are either single small islands with one type of terrain, or mountains/valleys with no coastline.
I have tried getting my main land level, just above water level and planned on adding hills/mountains where I need them and then planned on moving to smaller details like rivers/lakes/valleys/desert canyons/desert rock-pillars etc, but I can’t seem to get past the main land stage. I don’t want beaches all around with smooth gradients up to the main land, only in some places and the places where there are no beaches, I planned on having a mixture of sheer cliffs and steep rocky areas. I am just finding it hard getting to this point and like I said, there are no tutorials out there that show me how to do this. Or at least, none that I can find.
I have managed to get hold of some templates people have used to try and understand how things work together and I have tried adapting my island to their template, but even that is proving tedious and doesn’t give me the results I am looking to create.