Comment by duskwuff

> D3 doesn’t get reset because it’s guaranteed to be 0 at the beginning of each scanline

There's no such guarantee. The NEXTHOR loop only inverts for points which are to the absolute left of the point being tested ("IS HORIZ <= PT.H ? \\ NO, IGNORE THIS POINT").

Imagine that, for every point, there's a line of inversion that goes all the way down to the bottom of the bounding box. For a typical rectangular region, there's going to be four inversion points - one for each corner of the rectangle. The ones on the bottom cancel out the ones on the top. To add a second disjoint rectangle to the region, you'd simply include its four points as well; so long as the regions don't actually overlap, there's no need to keep track of whether they share any scan lines.