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RectClip produces incorrect result #1095
Description
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 (
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.
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