Fill by line art with perfect edges on a B&W image or vice-versa

[PixLab]

No method is wrong, but some time I see convoluted methods to bucket fill with line art detection when there is a BackGround, where it can be extremely simple to bucket fill with line art detection and get perfect edges even with a BG.
Thus I'll show you one simple way without deleting any BackGround, no selection, no mode change in your tool settings.

GIMP has a layer mode in which we are interested, it's the Multiply mode for white BG or its opposite Screen mode for black BG

Get you image in GIMP, then:

1. Create a new white layer (if the BG is white), and put it at the bottom
   
2. Create a transparent layer above the white layer
   
3. Put the top layer (original image) in Multiply mode (Screen mode for black BG)
   
4. Bucket fill by line art the transparent layer
   

Done, perfect edges!