Creating an "opposite" image

[Ottia Tuota]

I think nobody has suggested using a mask. But perhaps this equivalent to some other method, I don't know.

1. Make a white layer.
2. On top of it, make a black layer.
3. To the black layer, add a white mask.
4. Select the original picture. Press ctrl-C.
5. Select the black layer's mask. Press ctrl-V. Anchor the floating selection.

At this point we have:

   

What is left to do, is to make the outside regions white. Do as in PixLab's post #23:

6. New from visible.
7. Fuzzy select the outside regions (I used threshold 50).
8. Grow selection by 3 pixels.
9. Delete.
10. Select > None.
11. Make unvisible all other layers except the top layer and the white layer.