Hartmut Kaiser wrote:
Brandon Kohn wrote:
There are many buggy floating point boolean op libraries out there. Why do we need another?
I would like to ask you to refrain from spreading FUD with regards to the library under review without any concrete proof.
Hartmut, please see Barend's reply to my question about robustness: "
4. If the non-self-intersecting invariant is violated at the input of the polygon pair intersection algorithm what does the algorithm do?
This depends on how the polygon self-intersects, and how the self-intersection intersects with the other polygon. If the self-intersection is untouched by the other polygon, you'll get exactly that self-intesection back (for a union) or it is discarded (for an intersection). The self-intersections are not even tried to be detected. This is on purpose because it saves another call of get_intersection_points, which is the most time-consuming in the current implementation. If the self-intersection interferes with the other polygon, you can get an invalid polygon back. As the input was already considered as invalid, this is what is to be expected. We've researched this recently because it occured in practice. The algorithm will find an invalid path, detect that and skip that output built up so far. " and "
7. When intersection points are snapped to the floating point grid at the output of the polygon pair intersection algorithm edges may move laterally some small distance which may introduce self intersection artifacts. How is this handled by the polygon intersection algorithm?
This is not handled explicitly. On segment-segment intersection in the epsilon-regions, the intersection will fall between the endpoints of both segments. Even if it is rounded, it will not surpass those endpoints. Therefore, self-intersections will be unlikely in most cases. I'll not say that it is impossible, but it is unlikely. Our implementation is not yet checked on 100% numerical robustness. If using floating point (IEEE-single), there might be errors in the 1e-3x regions. If using double precision, there will be less errors. That is to say: in close-to-100% of the use-cases: no errors. But in the regions of 1e-3xx differences, errors might be expected, and in some cases they might even cause self-intersections, I'll not deny that now, we've to do more research here. " I might add that the robustness problem with line segment intersection is worse than that caused by snapping to floating point grid, since many bits of precision are lost during the line segment intersection point computation. I am not arguing for rejection of the library. I am arguing that it should be accepted when it meets requirements of algorithmic correctness, performance and numerical robustness and provides sufficiently generic interfaces for performing multi-polygon intersection operations. I'd like to add that the work to support infinite precision floating point coordinates in the intersection should be done before it is accepted, since it turns out that isn't actually supported yet. Basically I'm arguing that the library should be finished before becoming a part of boost, and that we accept in on condition that Barend finish it and do the things he says he plans to do before it is released. Thanks, Luke