OpenNest/docs/geometry/lead-in-placement.md

Lead-in placement at corners

Straight (LineLeadIn) lead-ins at closed internal contour corners use the inward angle bisector instead of the normal of whichever edge was selected. With the default 90-degree approach angle, a rectangular cutout therefore gets a diagonal lead-in into the scrap, rather than one lying along the other edge. This applies to automatic assignment and manual placement. The manual preview uses the same Core calculation, including when the lead-in style changes while the cursor is stationary.

ContourCuttingStrategy.ComputeLeadInNormal combines the two adjacent inward unit normals. It handles either winding, either selected edge, rotated parts, and line/arc junctions using their local normals. Summing vectors avoids angle wraparound and edge-length weighting. Open contours, disconnected endpoints, zero-length edges, and cusps without a unique bisector retain the entity normal.

The configured approach-angle offset is still applied relative to the computed normal; 90 degrees follows the bisector. Lead-in length is unchanged. Mid-edge points, external contours, circles, curved/composite lead-in styles, and lead-outs keep their existing placement rules.

This is a local direction correction, not a whole-path clearance guarantee. An excessively long lead-in or an approach angle rotated away from the bisector can still leave a small cutout. General non-circular containment/length clamping and sharp-corner handling for curved/composite lead-ins remain separate work.

Outside perimeter corners

A straight (LineLeadIn) lead-in at a convex corner of the outside perimeter (interior angle under 180 degrees) extends the edge cut first: the pierce sits on that edge's line, behind the corner, and the torch travels straight into the corner and keeps cutting along the same line. Which of the two edges was picked (auto assignment or the manual cursor) does not matter; the cut direction never changes. The approach angle is ignored at such a corner.

The straight lead is used only while its pierce stays at least CuttingParameters.PierceClearance (or the pierce zone radius, when larger) from the contour and the lead crosses the contour nowhere but at the corner. Very flat corners (about 165 degrees and over for a 0.25 lead with 0.0625 clearance) fall back to the normal lead-in, perpendicular to the first-cut edge, so tessellated curves do not get straight leads. Reflex perimeter corners (the inside corner of an L) use the notch bisector, like cutout corners.

A LineLeadOut mirrors this: at a convex perimeter corner it runs straight on past the corner along the last-cut edge, with the same clearance fallback to the last-cut edge's normal and a bisector at reflex corners. A tabbed perimeter gets no run-out: every lead-out style leaves from where the trimmed cut actually ends, on that end's normal, so an arc lead-out starts on its own radius and the tab gap stays uncut. ContourCuttingStrategy.ResolveLeadIn/ResolveLeadOut own these rules; program generation and the manual preview share them. Other lead-in styles are unchanged.

Rounded corners in Plan Cutting

Plan Cutting prefers to start a contour at a convex corner. The tool returns to the start after it has put heat into the plate, so a start partway along a straight edge can leave a step. A start at a corner hides it. An edge midpoint is a fallback, used only when no corner on a side facing the next cut fits.

A corner rounded by a small fillet counts as a corner. The fillet must be tangent to straight edges at both ends, turn convexly through less than a half circle, and have a reach (radius * tan(sweep / 2)) no longer than either edge. Each of its two tangent joints is then a convex corner entry (PreparedContours.FilletReach). Ranking counts the joint on both sides of the corner it rounds, as it would a sharp corner. Longer curves, half-circle ends and inside (reflex) fillets keep their tangent joints, which rank level with edge midpoints. The lead-in at a fillet's joint is the normal one for its edge, since emission still sees a smooth joint there.

Curved leads

Every curved lead (Arc, Line + Arc, Clean Hole, the arc lead-out) meets the cut tangentially and in the cut direction. Its arc center lies along the contour normal, away from the material, so the arc turns with a cutout's winding and against an outside perimeter's; ContourCuttingStrategy.LeadTurn gives that direction, and program generation, the pierce-zone check and the manual preview all pass it to the lead. A Line + Arc approach angle is measured back from the normal on the side the arc turns from, so the arc takes the short way round under either winding.

An Arc lead-in's SweepAngle (degrees, in (0, 180], default 180) sets how much of a circle it is: 90 is a quarter circle, 180 a half circle that pierces two radii out on the normal. Settings and cut pins saved before the sweep existed read as 180.

At a convex corner of an outside perimeter, an Arc, Line + Arc or Clean Hole lead-in uses the normal of the edge cut first, and an Arc lead-out that of the edge cut last, whichever of the corner's edges was picked; the arc then lies wholly on the scrap side of that edge. Reflex corners and cutout corners keep the picked edge's normal: a tangent arc there would cross the part.

Cutouts too small for their lead-in

Text, narrow slots and small holes often have no entry where the configured lead-in fits. When Plan Cutting has tried every entry of a cutout and none fits, it erodes the cutout instead (ErodedPierce, OpenNest.Core):

1. The cutout's pole of inaccessibility gives its inscribed radius, the deepest any erosion can go.
2. The cutout is offset inward (Clipper.InflatePaths, negative) by the pierce clearance (GetEffectivePierceClearance) when it has room for it, else by nine tenths (ErodedPierce.DepthFraction) of its inscribed radius, plus a margin for flattening, round joins and Clipper rounding. Shallower levels, down to half that depth, back it up.
3. Each of the cutout's ranked entries gets the nearest point of the deepest level that reaches the wall beside it: a letter stroke narrower than the cutout's widest place is still pierced on its own centre line next to the entry, so the route can enter and leave a letter where it needs to.
4. The lead-in runs straight from the pierce to that entry, or else to the cutout's native point closest to the pierce (PreparedContours.ErodedPierceEntries, a ContourChoice whose Pierce is set). A cutout so pierced has no lead-out: a feature too small for its lead-in has no room for one either.
5. A cutout too small to erode is pierced at its pole.
6. LeadPathValidator checks each candidate like any other planner-chosen entry, pierce zone included. When none is clear, the "No tested lead-in fits" finding says why.

Cutouts where a configured lead-in fits keep it, so plans of other parts are unchanged. Hole entry selection, the preferred hole path and contour recovery all use the fallback; recovery's reading-order sweep also pierces a letter this way at the side it leaves from. Only Plan Cutting uses it: the emitter's own automatic assignment and the manual lead-in preview keep the configured lead.

Regression checks

Run dotnet test OpenNest.Tests/OpenNest.Tests.csproj --filter "FullyQualifiedName~CutoutCornerLeadInTests|FullyQualifiedName~PerimeterCornerLeadInTests|FullyQualifiedName~ErodedPierceTests". The cutout tests exercise generated part programs, default automatic placement, every rectangular corner with both adjoining edges and windings, part rotation, acute and obtuse angles, reflex corners, line/arc corners, preview agreement, and unchanged/fallback behavior. The perimeter tests cover every square corner under both windings and a rotation, auto and manual placement from either edge, preview agreement, approach-angle handling, the flat-corner clearance fallback, reflex notches, and straight lead-outs. ErodedPierceTests covers the eroded pierces of a slot and a small round hole, a cutout that keeps its configured lead, the containment of text cutouts that sit a micro-inch apart and, with the optional SmallFeatureLeadNestPath input, a plate of stencil-text tags planned in reading order. ArcLeadInTangentTests checks curved-lead tangency, direction and sweep on perimeters and cutouts under both windings, and at every perimeter corner from either edge.

Windows manual check: import a part with small rectangular cutouts and use Plate > Plan Cutting. Confirm diagonal lead-ins at the cutout corners. Under Plate > Place Lead-in, select the part, lock a cutout, and hover a corner: the preview should point into the cutout and the committed lead-in should match. Hover a mid-edge point and confirm perpendicular placement is unchanged. Windows visual interaction is not verified by the Linux cross-build.