RectClip produces incorrect result

[egarzago]

Thank you for Clipper2!

Woking with RectClip found some instances that produces a valid polygon, but with the incorrect result (a sliver is present). I was able to recreate the issue on a simple case when intersecting a (rotated rectangle) with a Rect64.

Here's the code to reproduce (C++20 on MSVC, using the conan package for clipper2/2.0.1:

#include <clipper2/clipper.core.h>
#include <clipper2/clipper.h>
#include <iostream>

namespace clipper = Clipper2Lib;

int main()
{
    clipper::Rect64 rect(19669914, -390330085, 24669914, -385330085);

    clipper::Paths64 polygon;
    polygon.push_back(
        {{8821540, -379573701},
         {27274660, -333900624},
         {72947737, -352353745},
         {54494616, -398026821}}
    );

    auto result = clipper::RectClip(
        rect,
        polygon
    );

    for (const auto& path : polygon)
    {
        std::cout << "Initial path:\n";
        for (const auto& pt : path)
        {
            std::cout << "(" << pt.x << ", " << pt.y << ")\n";
        }
    }

    for (const auto& path : result)
    {
        std::cout << "Clipped path:\n";
        for (const auto& pt : path)
        {
            std::cout << "(" << pt.x << ", " << pt.y << ")\n";
        }
    }

    return 0;
}

Output:

Initial path:
(8821540, -379573701)
(27274660, -333900624)
(72947737, -352353745)
(54494616, -398026821)
Clipped path:
(24669914, -390330085)
(24669913, -385976859)
(23069090, -385330085)
(24669914, -385330085)

Here are the plotted Inputs/outputs

And this is the sliver that gets created:

Zooming in, it seems that the intersection with the boundary of the Rect64 is one unit off (inwards):

Unsure if this is of any help, but I suspect the root cause is in rounding when getting the intersection between the input polygon and the Rect64. Perhaps one could ensure that in GetIntersection (

Clipper2/CPP/Clipper2Lib/src/clipper.rectclip.cpp

Line 118 in f9c5eb6

inline bool GetIntersection(const Path64& rectPath,

) and GetSegmentIntersection the resulting intersection point is guaranteed to touch the boundary of the Rect64 based on the input Location, i.e. if Location::Left -> ip.x = rectPath[0].x, and so on.