PolylineKit.Core 0.1.0-alpha.2

PolylineKit.Core

Area, intersection area and IoU of 2D polygons and paths: numbers only, with no external runtime dependencies. Supports .NET Standard 2.0 and .NET 10.

dotnet add package PolylineKit.Core --prerelease

Compare two outlines

using PolylineKit;

Point2[] first = [new(0, 0), new(2, 0), new(2, 2), new(0, 2)];
Point2[] second = [new(1, 0), new(3, 0), new(3, 2), new(1, 2)];
var overlap = PolylineArea.CompareRegions(first, second);
// IntersectionArea = 2; UnionArea = 6.
double? iou = overlap.IntersectionOverUnion; // 1/3

Paths close implicitly. NonZero is the default fill rule; pass PathFillRule.EvenOdd for parity filling. IoU is null when union area is not positive. The examples use C# 12 collection expressions; older compilers can use explicit Point2[] arrays.

Polygons with holes

Supply the exterior and holes by role; their original directions do not matter for valid simple polygon boundaries:

Point2[] exterior = [new(0, 0), new(10, 0), new(10, 10), new(0, 10)];
Point2[] hole = [new(2, 2), new(6, 2), new(6, 6), new(2, 6)];
var polygon = PreparedRegion.FromPolygon(
    exterior, new System.Collections.Generic.IReadOnlyList<Point2>[] { hole });
double area = RegionArea.FilledArea(polygon); // 84 under NonZero and EvenOdd

FromPolygons also accepts disconnected polygon parts. FromRings keeps raw ring directions: these same two rings passed to it give 100 under NonZero and 84 under EvenOdd. The factories do not infer nesting or repair geometry. See the region guide and the tested NetTopologySuite conversion example.

PolylineKit, Clipper2 or NetTopologySuite?

Recorded workloads give a guide, not a universal ranking: the building evaluator was 3.05× faster than its configured NTS backend and effectively tied with Clipper2; the complete county/district coverage took 24–25% less time than reusable Clipper2. At 1,024 rings per operand, warm Core comparisons were 18–34% slower than Clipper2. Selected warm geometry calls allocated 0 B; preparation, first calls, growth and oversized workspaces can allocate. These are historical measurements, not timings of alpha.2. See the measured scopes and limits.

Contracts and limits

• Coordinates are planar and areas use squared coordinate units. There is no implicit projection, normalization, alignment or decimal rounding grid.
• Every nonempty ring needs at least three vertices after consecutive and closing duplicates are removed. Null, too-short or nonfinite input throws; valid collinear rings can have zero area. Empty prepared regions are valid.
• Calculations use double. Exact geometric decisions do not make intersection positions or final areas exact; tiny features near large coordinates can lose precision. Self-intersecting rings are interpreted by the chosen fill rule, without geometry repair. Signed-area orientation cannot establish shell/hole topology for an arbitrary self-intersecting walk.
• Prepared regions copy coordinates and can be reused across calls and threads. Preparation does not cache own areas or intersections. Coverage of a zero-area region is undefined; summing overlapping objects' coverages can double-count.

The package also provides normalization, scaling, resampling and similarity alignment as separate operations. For a trusted simple ring's own area alone, a shoelace sum is cheaper than general fill-aware processing.

The assembly retains its PolylineKit.Winding.dll name for binary compatibility; all public types use the PolylineKit namespace. Core never references Clipper2. See the area guide, transformations and MIT license.