In this image of my render, the top and bottom layers are unclosed, unfilled lines/paths (like circuit traces). I converted one of these path sets to ProofGrade engrave to see how it looks, and it has changed it to the equivalent filled, closed path.
A few folks with laser experience had to unlearn some things about this. I was starting from scratch so I was able to learn the basics. Even after a year of forum activity and lots of file dissection, I still had a few cases where I didn’t quite get if figured out. Your example is superb though and something that folks can learn from. Thanks for posting.
Neat! So, the overlapping fills that resulted in alternating fills of burn are demonstrative of the even/odd fill rule oft used in graphics algorithms, it seems! Or, in this case, the GFUI is filling based on the # of closed paths that are stacked. Which is a bit weird, but reasonable.
Potentially more directly related. May be fallout from the way the SVG rendering engine is implemented. Interesting.