SixLabors.PolygonClipper 1.0.0

SixLabors.PolygonClipper

SixLabors.PolygonClipper provides high-performance polygon clipping and stroking in C#. Boolean operations (union, intersection, difference, xor) are implemented with a Martínez-Rueda sweep-line pipeline for complex polygons with holes and multiple contours. Contour normalization is handled by a dedicated Vatti/Clipper2-inspired pipeline (PolygonClipper.Normalize) that resolves self-intersections/overlaps into positive-winding output. PolygonStroker can optionally run that normalization pass on emitted stroke geometry.

Features

• Works with non-convex polygons, including holes and multiple disjoint regions
• Handles edge cases like overlapping edges and vertical segments
• Preserves topology: output polygons include hole/contour hierarchy
• Deterministic and robust sweep line algorithm with O((n + k) log n) complexity
• Includes PolygonClipper.Normalize (Clipper2-inspired) for positive-winding contour normalization
• Includes PolygonStroker for configurable geometric stroking (joins, caps, miter limits)
• Uses double precision geometry without coordinate quantization

Usage

The API centers around Polygon and Contour types. Construct input polygons using contours, then apply Boolean operations via PolygonClipper:

Polygon result = PolygonClipper.Union(subject, clipping);

Boolean operations can process self-intersecting inputs directly. Use normalization when you want canonical positive-winding contours (for example, before export or rendering pipelines that rely on winding semantics):

Polygon clean = PolygonClipper.Normalize(input);

Normalize uses a fixed positive-winding normalization path.

Stroking

Use PolygonStroker to generate filled stroke polygons from input contours:

Polygon stroked = PolygonStroker.Stroke(input, width: 12);

Configure join/cap behavior through StrokeOptions:

StrokeOptions options = new()
{
    LineJoin = LineJoin.Round,
    LineCap = LineCap.Round,
    MiterLimit = 4,
    ArcDetailScale = 1
};

Polygon stroked = PolygonStroker.Stroke(input, width: 12, options);

Algorithm References

This project draws on the following algorithm and implementation references:

6. Martínez et al., "A simple algorithm for Boolean operations on polygons", Advances in Engineering Software, 64 (2013), pp. 11-19.
   https://doi.org/10.1016/j.advengsoft.2013.04.004

2. 18. Vatti, "A generic solution to polygon clipping", Communications of the ACM, 35(7), 1992, pp. 56-63.
       https://dl.acm.org/doi/pdf/10.1145/129902.129906

21re, rust-geo-booleanop (Rust Martínez-Rueda implementation reference).
https://github.com/21re/rust-geo-booleanop

Angus Johnson, Clipper2 polygon clipping library (reference implementation for the normalization pipeline).
https://github.com/AngusJohnson/Clipper2

License

Six Labors Split License. See LICENSE for details.