I am currently trying to achieve the result in the attached picture. https://imgur.com/a/6I9NlLZ
The goal is that the solder mask only covers the copper layer and thus the inner core of fibreglass is visible. To achieve this I needed a positive version of the image as F.CU (front copper layer) and an inverted version of it as F.Mask (front soldermask). I already converted the positive and negative vector image with Kicad’s image converter many time but the results never quite line up due to conversion tollerances. I have also tried Inkscape and the svg2shenzhen plugin but the same problem there.
https://imgur.com/a/xIlgtOq (Result)
Is there any accurate solution for this problem like inverting the coordinates from the .kicad_mod file?