I found this solution on the getpaint.net web site:
- I use the Bucket (fill) with a full transparent color.
- Set the foreground to white and fully transparent, i.e. RGB all = 255, Transparency – Alpha = 0
- Set the background to white and not transparent, i.e. RGB all = 255, Transparency – Alpha = 255
- You have to change the tool parameters from “Normal blend” to “Overwrite” in advance, or you won’t see any effect.
- Tool = Bucket Fill
- Flood Mode = Contiguous
- Fill = Solid Color
- Tolerance = 17%
- Antialiasing = Enabled
- Overwrite
- You can access the parameters after choosing the bucket. Press the little chemical bottle icon, in the right to the fill tolerance slider.