(05-03-2020, 08:58 PM)Ofnuts Wrote: Can you post the the XCF (cropped to the sample above) as well as the corresponding exported PNG?
I think I solved it. It has to do with newer versions of Gimp using linear colour space to blend layers. If I switch to legacy gamma colour space, it blends as it's supposed to do.
The same problem can be recreated by painting some semi-transparent black patterns on a white layer, use the colour-to-alpha command to get rid of the white and then add a white background layer: you'll get a different image than the one you started with.
I'm not expert about this matter - I actually knew nothing about it untile yesterday evening - but I guess that the original sketchbook exported image worked nicely in a linear colour space, so it was displayed correctly in Gimp, then when I export it in PNG it gets gamma corrected and, when opened again in Gimp, it doesn't blend correctly with newer layers.