You'd need to reverse engineer it and make your own map viewer/editor from scratch.
I was thinking that the code already exists (in the game source files) for reading the data files, composing the map, occluding tiles, applying lighting effects, applying fog-of-war, applying field-of-view. If this function or module were replicated to a standalone tool and then much of it commented out (FOW, FOV, lighting) it could loop through all maps (including vents); read a map; "draw" it - but instead of the output going to a screen, it would be dumped to a file.
screenshot the areas as you go
This is certainly an effective fallback solution, but it is labour intensive and it might miss transition zones, secret doors / rooms, vent systems, etc. Plus although it's fine if people save the screenshots lossless (eg as PNG) from the start, but if they don't know
JPEG is lossy then they've done all that manual effort and ended up with a degraded result with bleeding edges (and not the good kind of "bleeding edge"!)
Should already be possible to open the png in paint and edit it
That's true, however there is a big difference in efficiency between:
- raster images (eg BMP, PNG) which are a grid of coloured pixels
- vector images (eg SVG) which contain geometric shapes (rectangles, arcs, lines, arrows) and text
I had thought you had made the file as SVG and exported it to PNG. Working with vectors is much less effort to rearrange lots of blocks to change the document shape from landscape to portrait. Multiple shapes can be rotated to 40 degree angle at once.
For your next version, check out the open source cross platform
Inkscape and see if it makes your process of building up the map easier. It might take a little bit of getting used to that the software knows about rectangles, lines, etc and resizing them itself, but I think after a brief adjustment it will be a relief that the software does more for you.
