Many thanks again.
Especially what you are showing in the Wilbur case is exactly what I would like to do. The problem being that as I didn't start with the black and white masks, I created the isles directly by hand in FT3 by loading the PNG map as is. Reading your pdf I am also more and more under the impression that Wilbur is better adapted to editing/transforming while FT3 is relatively easy and focused on creating random maps.
Of course most people who are interested by creating fantasy maps start to do so by hand and generally have an idea what they want so that the ability to edit a prexisting world is more important than creating random worlds which obviously will never be what they need.

So my world now only exists as a ftw file and I didn't find the ftw format in Wilbur and I can't load it there. As I have still the trial version of FT3 I can't create a PNG (or BMP) version readable by Wilbur so am stuck with FT3.

As I promised, I will resist to ask a question untill Tuesday but would like to make a comment which may interest people generally curious about marketing (e.g analysis of potential customers needs).
A new map software Customer will be basically like me last week. I draw maps, illustrate games, create game worlds. All this happens by hand because pen and paper is still the fastest and easiest creation tool.
Then one day one wants to increase the realism - have self similarity in the design. That leads to fractals.
5 minutes of Google and one finds 3 things : Wilbur, FT3 and X. The first is free, the second has a reasonable cost and the last is much too expensive for just a hobby.

OK let's try Wilbur first. After a half an hour of research no manual can be found. A fast check of the UI confirms that it is hopeless to do anything without a manual. One doesn't even understand the words on buttons. Exit Wilbur.
FT3 yields strangely the same result. Even if one has to pay for a complex and difficult software, there is no manual. There are some videos, documents and vague tutorials that show perhaps 5% of the functionnalities but nothing that would even remotely look like a manual. This is puzzling. How can one sell a complex software without a manual ?
X seems perfect. There is a manual, tutorial and the description of functionalities seems to cover the needs that one has. Unfortunately the price is showing that this is for professionnals who don't pay it out of their pocket.
So a person motivated enough (like me) will find that FT3 COULD be a solution provided that one is ready to invest a huge amount of time in forum mining and looking for every available scrap of knowledge.
A less motivated person simply gives up.
But even with that I am still hesitating to buy FT3 and if I don't then the main, almost unique reason will be that there is no manual. I know that there is the "help" function but it explains nothing in 95% of cases so doesn't provide any help either.
Just an example : there are mega tons of parameters to be set and one not only doesn't know what they do but what is their numerical range min,max. There could be for example 2 pictures showing the result with value X and the result with value Y. It is fast and easy to do that for someboy understanding well FT3 and to put it in a manual. I estimate a good FT3 manual at 100-150 pages with 1/3 pictures and 2/3 text. How long could it take ?
The same is of course true for Wilbur but as this is a free software, I can understand that there is not enough free time and thus the software stays reserved for a quasi professional "elite".

To finish this comment I would say that people running Profantasy Software would certainly sell many more copies of FT3 if they invested some (one shot) time in writing a real manual.