Creating an "opposite" image

[Krikor]

(03-31-2022, 08:16 AM)PixLab Wrote: 100% and still having a gap, good to know

About squeezing, I did squeeze even more    
In the latest method I did, no need anymore to Select ➤ Grow 1px
Instead, the step of the fuzzy tool put Select By ➤ LCh Lightness , then just delete (all LCh/HSV works but they are a lot more slow)
By doing so it will select also everything between opaque and transparent, thus no need anymore to grow 1px or more

PixLab, considering that the outside will be removed anyway, I still prefer the method in your post #23 - (Select By ➤ LCh Lightness causes some annoying slowness).

Another very simple option that I tested now:

1- With Select by color, select the transparent areas;
2- With the Fuzzy select tool, in the Subtract from selection mode, select the external area;
3- New layer, Select - Grow =3, fill it with black;
4- On the initial layer, alpha for selection;
5- In the new layer, delete (Edit - clear).

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BTW:
Considering the topic covered in this thread, I recommend reading: https://www.gimp-forum.net/Thread-Backgr...r-graphics .