I use the Gimp plugin. I don’t make a duplicate image. I mask with red ff0000 (different colours are possible) - essentially I paint with red what should be impainted. The brush has to be 100% hard, no soft edges.
Afterwards I apply the filter with the wanted settings. For example:
The steps in detail:
- I make a copy of my original layer
- take a hard brush and paint everything I want to be impainted with the same colour I will choose in Gmic filter - standard is ff0000.
- I start Gmic-filter and use it with my favourite settings. I usually choose a new layer for the results, to see what I have masked before. If the result is too far away from my expectation, I try it with different settings. Anyway most of the time some afterwork is necessary.