Klip

An F# library for fast and robust polygon clipping.

Klip is a partial port of Clipper2 covering the general polygon boolean operations - intersection, union, difference, and XOR. Offsetting, rectangle-only clipping, and triangulation are not included.

It runs on .NET and JavaScript via Fable, so the same source serves Rhino, Revit, and browser apps. To make it suitable for JS runtimes it is in many parts derived from the TypeScript port clipper2-ts. All original tests pass, along with many new tests for unions of almost-aligned polygons.

The key difference from Clipper2: Klip uses float coordinates throughout instead of int64. Clipper2 snaps every coordinate onto an integer grid before clipping; Klip removes that step and computes directly on the unrounded input. Intersection points are kept at full floating-point precision rather than snapped to the grid, so the exact input positions are preserved.

Coordinate precision

Because the engine computes on unrounded float coordinates, point coincidence, colinearity, and horizontality use small per-instance tolerances instead of exact equality. The defaults absorb floating-point noise without fusing genuinely distinct points:

• Point coincidence - two coordinates are the same point when they differ by less than a small absolute distance.
• Colinearity - three points are colinear when the turn angle between their edges falls below a small scale-free angle tolerance.
• Horizontality - an edge is horizontal when its slope falls below a small scale-free angle tolerance, rather than an exact topY = botY test. This keeps a shared near-horizontal edge that is a hair off exact (e.g. a top at 37 vs 37.000001) from landing its ends on distinct scanlines and sealing an open notch into a phantom hole.
• Adjacent-edge joins - the near-top guard scales with local edge height, and the perpendicular join distance defaults to the point-coincidence tolerance.

Contours that share a seam are merged: horizontal seams join when their X-ranges overlap (a real seam's overlap far exceeds float noise), and sloped/near-vertical seams join via the tolerance-gated adjacent-edge checks. Contours that touch at a single point (e.g. the two lobes of an XOR) remain separate, as in Clipper2.

You do not need to scale coordinates before clipping - use your source units directly. If results are off for your coordinate magnitude (e.g. seam-sharing pieces come out separate, or slivers survive), the tolerances don't fit your inputs: set them all from one absolute tolerance with the Tolerance property (see Tolerances and scaling) rather than rescaling your input.

The Snap module can optionally pre-snap almost-aligned coordinates (see below).

Types

Original documentation: https://www.angusj.com/clipper2

Types keep their original C# names. The ..64 suffix historically meant 64-bit integers; in Klip the XY coordinates are float, and there is no separate ..D API because the regular path types already preserve floating-point coordinates.

• Path64<'Z>: a single contour. X and Y are stored in a flat interleaved ResizeArray<float> as x0, y0, x1, y1, ....
• Paths64<'Z>: a ResizeArray<Path64<'Z>> - multiple contours, such as an outer polygon and its holes.
• PolyTree64<'Z>: a tree output that preserves parent-child contour relationships (holes inside outers).
• ZCallback64<'Z>: a callback assigning user-defined 'Z metadata to vertices created at intersections.

Generic 'Z metadata

'Z is an optional generic type parameter for user-defined metadata attached to vertices, defaulting to unit. (In the original Clipper2 the optional Z value is always int64.) 'Z values are metadata, not a 3rd coordinate.

If you do not use 'Z, use the no-Z helpers such as Path64.createFrom and Paths64.createSingle, which produce Path64<unit> / Paths64<unit> values. The 'Z-aware helpers live in the parallel ...Z functions and the KlipperZ module.

Path helpers

The Path64 and Paths64 modules provide construction and utility helpers:

• createFrom, createFromSeq (on both modules) copy coordinate data into new buffers.
• createDirectly reuses the supplied ResizeArray buffers directly (coordinates are not rounded).
• createFromXYMembers / createFromxyMembers accept objects with X/Y or x/y members.
• enableZ / enableZWith attach metadata buffers and reject paths that already have Z values.
• mapXY, iterXY, mapZ, iterZ, orientation helpers, and signedArea cover common inspection and transformation tasks.

Boolean operations

Closed-polygon wrappers

The Klipper.* wrappers always treat input as closed polygons:

• intersect clip subject - intersection of subject and clip.
• union clip subject - union of subject and clip.
• unionSelf subject - resolves self-intersections within a single subject.
• unionSelfChecked subject - reorients all subjects to positive orientation before unioning.
• difference clip subject - regions of subject not inside clip.
• xor clip subject - regions in subject or clip but not both.
• removeSelfIntersectionsPositive subject / removeSelfIntersectionsNegative subject - resolve one self-intersecting path using the matching directional fill rule.

For a custom ClipType, FillRule, or PolyTree64 output:

• booleanOp (clipType, subject, clip, fillRule) - returns Paths64<unit>.
• booleanOpPolyTree (clipType, subject, clip, fillRule) - returns a PolyTree64<unit> preserving the parent-child hierarchy.
• polyTreeToPaths64 polyTree - flattens a PolyTree64<unit> back into Paths64<unit>.

Each function has a counterpart in the KlipperZ module that takes an option<ZCallback64<'Z>> (first argument for the wrappers, trailing zCallback argument for booleanOp / booleanOpPolyTree) to attach 'Z metadata.

open Klip

let subject =
    Paths64.createSingle [ 0.0; 0.0; 10.0; 0.0; 10.0; 10.0; 0.0; 10.0 ]

let clip =
    Paths64.createSingle [ 5.0; 5.0; 15.0; 5.0; 15.0; 15.0; 5.0; 15.0 ]

let union = Klipper.union clip subject
let intersection = Klipper.intersect clip subject

let nonZeroDifference =
    Klipper.booleanOp (ClipType.Difference, subject, clip, FillRule.NonZero)

Open vs closed paths

Open/closed is not inferred from coordinates (a trailing vertex equal to the first is just stripped) - each path is tagged when added to the engine. Rules, inherited from Clipper2:

• Subject paths can be open or closed; clip paths are always closed.
• For Intersection, Difference, and Xor: open and closed subjects are processed independently - closed subjects are ignored for the open-path solution, and vice versa.
• For Union: open subjects are clipped wherever they overlap any closed path (subject or clip).

The Klipper.* and KlipperZ.* wrappers always treat input as closed. To clip open paths (polylines / line segments), use Clipper64 directly and call AddOpenSubject:

let c = Clipper64<unit>()
c.AddOpenSubject(openLines)   // polylines - endpoints stay endpoints
c.AddSubject(closedPolygons)  // optional, closed
c.AddClip(clipPolygons)       // clip is always closed
// Execute returns a (closedSolution, openSolution) tuple;
// openSolution is null when no open subjects were added.
let closedSolution, openSolution = c.Execute(ClipType.Intersection, FillRule.EvenOdd)

Calling AddPaths with PathType.Clip and isOpen = true is invalid. ExecutePolyTree follows the same open-output convention as Execute.

Direct Clipper64 options

Use Clipper64<'Z> directly for open subjects, repeated execution with the same input, or lower-level tuning:

• PreserveColinear: keep removable colinear vertices in closed solutions.
• Tolerance: sets all five scale-dependent tolerances from one absolute tolerance - the distance below which points are considered identical and lines touching. The value is used as-is, not as a multiplier of the defaults; see Tolerances and scaling.
• ReverseSolution: reverses output orientation.
• ZCallback: computes metadata for vertices created at intersections.

The individual tolerance properties (point coincidence, adjacent-edge joins, colinearity, horizontality, the near-top join guard, and the sliver culls) remain functional as expert overrides but are marked [<Obsolete>] and hidden from editor completion - the Tolerance property is the supported tuning surface. Each is documented in detail on the member itself in Src/Engine.fs.

Tolerances and scaling

The distance tolerances are absolute and do not auto-scale - the engine does not normalize coordinate magnitude. Set them all from one absolute tolerance with c.Tolerance <- t - the distance below which points are considered identical and lines touching: the four distance tolerances become t, and the area-valued split tolerance becomes t² (valid range 0.0 .. 1e12; 0 makes the comparisons exact). The value is used as-is, not as a multiplier of the defaults - those are calibrated for integer-Clipper2-style inputs of coordinate magnitude ~1e6 and do not correspond to any single call of this method.

Every tolerance comparison in the engine is dimensionally homogeneous, so clipping is scale-equivariant: scaling all inputs by s together with the tolerance yields the identically scaled solution. The angle tolerances are scale-independent and never need adjusting.

Snap preprocessing

Optionally call Snap.xAndY tolerance pathGroups or Snap.xAndYSingle tolerance paths to snap nearly-equal x and y coordinates to their respective averages. This is an in-place mutation done before adding paths to Clipper64. Call it on all paths at once so the same shared coordinate is used across subject and clip.

Building

For .NET:

dotnet build
dotnet test Test/FSharp/Tests/Tests1/Tests1.fsproj
dotnet test Test/FSharp/Tests/Tests2/TestsZ.fsproj

For JavaScript:

cd Test/TypeScript
dotnet tool restore
npm install
npm run clean   # clean previous Fable output
npm run build   # F# → JavaScript via Fable, then vite build
npm test        # vitest --run, against the compiled bundle
npm run buildts # optional: F# → TypeScript via Fable, then tsc and vite build
cd ../..

The JavaScript bundle ends up in Test/TypeScript/_dist/Klip.mjs and is what the Vitest suite imports - rebuild before testing after any F# source change. The TypeScript/Fable build emits a separate bundle under Test/TypeScript/_distTS/Klip.mjs.

Performance

On .NET, the local benchmark harness is roughly on par with Clipper2 C#. In JavaScript, the latest local run is about the same as clipper2-ts and about 80% slower than clipper2-wasm on average.

See Test/TypeScript/bench/README.md and Test/README.md.