Any solution on how to create an accurate soldermask from a custom copper layer?

I am currently trying to achieve the result in the attached picture.
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. (Result)

Is there any accurate solution for this problem like inverting the coordinates from the .kicad_mod file?

It’s just a quick Idea, and I have not tried it myself, but you could experiment with:

  1. Create a set of gerber files.
  2. Open the gerber files in KiCad’s Gerber viewer.
  3. Export your copper layer to a solder mask layer into a new PCB.
  4. Close the project, Open the PCB Editor in “Standalone Mode”.
  5. Open the original PCB.
  6. PCB Editor / File / Append Board and then append the board you created from the Gerber Viewer.

If what you want to achieve is for the Copper to show through vacated fill’s in the Mask, this may help…

Some Graphic by imported DXF (how you get the graphic doesn’t matter) onto a Copper Layer (example is on Front Copper).

Copy the Graphic (or sections of interest), Set the Active layer to desired Mask layer, Paste the copied graphic.

To aid in accuracy of placement, set the Grid to reasonable value to avoid snapping to very-nearby grid-point.

Images show Graphic of Cup and Two Lines that are copied then pasted to Front-Mask (and double-clicked and layer changed).

You can see the Uncovered Copper peeking through the Mask and the very dark copper of the remaining masked lines. The Purple-ish lines are the Pasted Lines (purple is the color of the Front Mask).

If you copy a graphic on a Copper Layer, you need to double-click it after pasting it, and change the Layer to desired Mask layer.

ADDED: It occurred to me I drifted from your focus on the ‘Image’… So, as follows:
I loaded a .PNG into Inkscape and tweak it using the Filters. I tried about a dozen and hit on one that didn’t cause 3D-viewer to choke (from too much data). The result is pretty good and I intentionally used an image with lots of texture. So, it was mostly about getting the most usable Filtered Image then, adjusting the Threshold in Kicad’s Image Converter. Last image below…

You may have a look at: