In Gimp, I frequently fall into this trap where I've already done a very complicated selection and then realize that the Feather edges Radius was not set to the desired value yet and then I end up needing to do the complex selection again after adjusting the radius. In this simple example, I had the Feather edges Radius mistakenly set to 20 instead of 0. I only realized that after I pressed delete which gave me this too feathered delete result. So I did an Undo and while the selection was still active I went ahead to adjust the Radius to 0 and then try to delete this selected area again, but the new Radius 0 setting has no effect, it still uses the Radius 20 setting. Am I missing something? In my opinion that is not quite user friendly so I'm wondering am I missing something where I need to press a key or click somewhere to update the radius so the new value is effective in the selection? Or is that currently not supported and I need to do the selection again? Sometimes the selection if very complicated and I end up needing to redo the selection with the new setting. That is quite frustrating as I do this mistake quite often.
Gimp has no knowledge of the origin of the selection(*). The selection is just a "channel", a grayscale image, and the feathering is essentially a blur applied to that grayscale image. And as with regular images if you find you don't like the blur you have to go back to the initial image, not apply a new blur over the already blurred image.
This means that you should have gone one undo step further and undo the feathering before doing a new one.
However, undoing isn't always practical, but you can avoid this once you know that you can save a selection. If you have a selection:
You can save it to a channel with
Select > Save to channel:
And if you remove the selection (
Select > None or else) you can come back to that selection again by right-clicking it and clicking
Channel to selection.
You can of course save the selection at multiple times, but it becomes necessary to rename the channels to keep track of things.
These "channels" are part of the image (even if they have no effect on what you export to JPG/PNG) and are saved in the XCF, so you can save the XCF, and reuse these channels much later when you rework the image.
So for your specific question:
- you create the "raw" selection
- you save it to a channel
- You feather it and test
- If this isn't right you re-obtain the selection from the saved channel before trying a new feathering.
- If necessary, you can also save the feathered selection
Caution: when you play with channels, they become the active "drawable" (ie, the target of paint and transform operations), so before continuing, make sure your layer is selected again, otherwise you will alter the channel.
For more tricks, see this.
(*) which could be very complicated since it could be a combination of various selections made with various tools with various levels of feathering