OpenNest/docs/superpowers/specs/2026-03-20-nfp-bestfit-strategy-design.md

NFP-Based Best-Fit Strategy

Problem

The current best-fit pair generation uses RotationSlideStrategy, which samples Part2 positions by sliding it toward Part1 from 4 directions at discrete step sizes. This is brute-force: more precision requires more samples, it can miss optimal interlocking positions between steps, and it generates hundreds of candidates per rotation angle.

Solution

Replace the slide-based sampling with NFP (No-Fit Polygon) computation. The NFP of two polygons gives the exact mathematical boundary of all valid positions where Part2 can touch Part1 without overlapping. Every point on that boundary is a guaranteed-valid candidate offset.

Approach

Implement NfpSlideStrategy : IBestFitStrategy that plugs into the existing BestFitFinder pipeline. No changes to PairEvaluator, BestFitFilter, BestFitResult, tiling, or caching.

Design

New class: NfpSlideStrategy

Location: OpenNest.Engine/BestFit/NfpSlideStrategy.cs

Implements: IBestFitStrategy

Constructor parameters:

• double part2Rotation — rotation angle for Part2 (same as RotationSlideStrategy)
• int type — strategy type id (same as RotationSlideStrategy)
• string description — human-readable description
• Polygon stationaryPoly (optional) — pre-extracted stationary polygon to avoid redundant extraction across rotation angles

GenerateCandidates(Drawing drawing, double spacing, double stepSize):

1. Extract perimeter polygon from the drawing inflated by spacing / 2 using PolygonHelper.ExtractPerimeterPolygon (shared helper, extracted from AutoNester)
2. If polygon extraction fails (null), return empty list
3. Create a rotated copy of the polygon at part2Rotation using PolygonHelper.RotatePolygon (also extracted)
4. Compute NoFitPolygon.Compute(stationaryPoly, orbitingPoly) — single call
5. If the NFP is null or has fewer than 3 vertices, return empty list
6. Convert NFP vertices from polygon-space to Part-space (see Coordinate Correction below)
7. Walk the NFP boundary:
   • Each vertex becomes a PairCandidate with that vertex as Part2Offset
   • For edges longer than stepSize, add intermediate sample points starting at stepSize from the edge start, exclusive of endpoints (to avoid duplicates with vertex candidates)
   • Skip the closing vertex if the polygon is closed (first == last)
8. Part1 is always at rotation 0, matching existing RotationSlideStrategy behavior
9. Return the candidates list

Coordinate correction

ExtractPerimeterPolygon inflates by halfSpacing and re-normalizes to origin based on the inflated bounding box. Part.CreateAtOrigin normalizes using the raw program bounding box — a different reference point. NFP offsets are in polygon-space and must be mapped to Part-space.

Correction: Compute the offset between the two reference points:

programOrigin = (program.BoundingBox.Left, program.BoundingBox.Bottom)
polygonOrigin = (inflatedPerimeter.BoundingBox.Left, inflatedPerimeter.BoundingBox.Bottom) → (0, 0) after normalization
correction = programOrigin - polygonOrigin

Since both are normalized to (0,0), the actual correction is the difference between where the inflated perimeter's bottom-left sits relative to the program's bottom-left before normalization. In practice:

• The program bbox includes all entities (rapid moves, all layers)
• The perimeter polygon only uses non-rapid cut geometry, inflated outward

PolygonHelper will compute this correction vector once per drawing and return it alongside the polygon. NfpSlideStrategy applies it to each NFP vertex before creating PairCandidate offsets.

Floating-point boundary tolerance

NFP boundary positions represent exact touching. Floating-point imprecision may cause PairEvaluator's shape-intersection test to falsely detect overlap at valid boundary points. The PairEvaluator overlap check serves as a safety net — a few boundary positions may be filtered out, but the best results should remain valid since we sample many boundary points.

Shared helper: PolygonHelper

Location: OpenNest.Engine/BestFit/PolygonHelper.cs

Static methods extracted from AutoNester:

• ExtractPerimeterPolygon(Drawing drawing, double halfSpacing) — extracts and inflates the perimeter polygon
• RotatePolygon(Polygon polygon, double angle) — creates a rotated copy normalized to origin

After extraction, AutoNester delegates to these methods to avoid duplication.

Changes to BestFitFinder.BuildStrategies

Replace RotationSlideStrategy instances with NfpSlideStrategy instances. Same rotation angles from GetRotationAngles(drawing), different strategy class. No ISlideComputer dependency needed.

Extract the stationary polygon once and pass it to each strategy to avoid redundant computation (strategies run in Parallel.ForEach):

private List<IBestFitStrategy> BuildStrategies(Drawing drawing)
{
    var angles = GetRotationAngles(drawing);
    var strategies = new List<IBestFitStrategy>();
    var type = 1;

    // Extract stationary polygon once, shared across all rotation strategies.
    var stationaryPoly = PolygonHelper.ExtractPerimeterPolygon(drawing, 0);

    foreach (var angle in angles)
    {
        var desc = $"{Angle.ToDegrees(angle):F1} deg NFP";
        strategies.Add(new NfpSlideStrategy(angle, type++, desc, stationaryPoly));
    }

    return strategies;
}

Note: spacing inflation is applied inside GenerateCandidates since it depends on the spacing parameter, not at strategy construction time.

No changes required

• PairEvaluator — still evaluates candidates (overlap check becomes redundant but harmless and fast)
• BestFitFilter — still filters results by aspect ratio, plate fit, etc.
• BestFitResult — unchanged
• BestFitCache — unchanged
• Tiling pipeline — unchanged
• PairsFillStrategy — unchanged

Edge Sampling

NFP vertices alone may miss optimal positions along long straight edges. For each edge of the NFP polygon where edgeLength > stepSize, interpolate additional points at stepSize intervals. This reuses the existing stepSize parameter meaningfully — it controls resolution along NFP edges rather than grid spacing.

Files Changed

File	Change
OpenNest.Engine/BestFit/NfpSlideStrategy.cs	New — IBestFitStrategy implementation
OpenNest.Engine/BestFit/PolygonHelper.cs	New — shared polygon extraction/rotation
OpenNest.Engine/Nfp/AutoNester.cs	Delegate to PolygonHelper methods
OpenNest.Engine/BestFit/BestFitFinder.cs	Swap RotationSlideStrategy for NfpSlideStrategy in BuildStrategies

What This Does NOT Change

• The RotationSlideStrategy class stays in the codebase (not deleted) in case GPU slide computation is still wanted
• The ISlideComputer / GPU pipeline remains available
• BestFitFinder constructor still accepts ISlideComputer but it won't be passed to NFP strategies (they don't need it)