I bought a cheap medion scanner which turned out to be a bad idea since the light in it is not evenly distributed and gives a kind of vignette How can I correct it in an open source program that runs on linux (I'm thinking gimp or darktable or maybe something else?). My guess would be to do a scan without a negative inside and then somehow add the empty scan as a kind of mask to the scanned image and then tell the program to reverse the colors. I'm using astrodmx for capturing data from the scanner (couldn't find anything better for linux, maybe there's some software that could do the correction at the moment of capturing). Scanner model is medion MD86601.