Comment by duskwuff
20 hours ago
It's a bit more than that. The list of X coordinates is cumulative - once an X coordinate has been marked as an inversion, it continues to be treated as an inversion on all Y coordinates below that, not just until the next Y coordinate shows up. (This manifests in the code as D3 never being reset within the NOTRECT loop.) This makes it easier to perform operations like taking the union of two disjoint regions - the sets of points are simply sorted and combined.
Uhm can you better explain that? I don’t get it. D3 doesn’t get reset because it’s guaranteed to be 0 at the beginning of each scanline, and the code needs to go through all “scanline blocks” until it finds the one whose Y contains the one specified as argument. It seems to me that each scanline is still self contained and begins logically at X=0 in the “outside” state?