(02-04-2018, 08:31 AM)rich2005 Wrote: Another day, another way. Without having to go out of greyscale. Maybe this is PS equivalent.
Copy-paste-anchor an image copy into a layer mask.
Colour -> Invert the layermask.
Much simpler to do Layer>Add layer mask, and tick "Init with grayscale copy of layer", and "invert mask".