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by giovannibajo1 371 days ago
Yeah those are the horizontal spans I was referring to.

It’s a sorted list of X coordinates (left to right). If you group them in couples, they are begin/end intervals of pixels within region (visibles), but it’s actually more useful to manipulate them as a flat array, as I described.

I studied a bit the code and each scanline is prefixed by the Y coordinates, and uses an out of bounds terminator (32767).
1 comments
duskwuff 370 days 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.
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giovannibajo1 370 days ago
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?
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duskwuff 369 days ago
> 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.

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