First of all , thank you for your quick answers , I should have added an example as English is not my mother language .
Here what I am trying to do , just note that I just made it as an example , and really quickly , so let's all pretend I have well cut my layer here Albert Hoffman (RIP ) .
I just pasted it on a field background for the purpose of the test , I am not caring here about the realistic aspect of the scene , the difference of lighting source origins ...
Even if cut my layer with great precision using a great zoom ,the edges of my layer will always seem too sharp .
In the solutions you gave me the Soften Edges was the closest solution to what I expected .
(In fact I had already downloaded this plugin but as I had downloaded as much plugins as I could , it was pretty hard to know where to find the good ones )
I am pretty sure it can meet most of thee needs , but I would prefer not a global effect on the layer but much more a tool like paint brush ...
The sort of thing you have in Gimp for the blur/sharp tool ...
If you know or have tips or how to achieve this task only on certain parts of the layer , that would be great !
And by the way thank you all again !