OpenNest/docs/breakoffs.md

Breakoffs

A breakoff is scrap drawn on a part, such as a forming backgauge tab, that must stay attached through cutting and forming and is snapped off afterwards. In the CAD Converter's Breakoffs panel, Add Breakoff, then click the line that joins the part and the scrap. Lines or arcs that end partway along another visible line or arc first split it there (the first piece keeps its entity ID), so a joining line can end exactly where it meets the outline. The clicked entity then grows into the chain that continues as a single path. Its ends must touch, within 0.001, one closed outline drawn around both part and scrap, and the chain must run inside it: either a separate line drawn across the outline, or a section of the part outline with the scrap's outside edges drawn separately.

The smaller side is the scrap; Flip (or "Larger side is scrap" in the dialog) swaps it. The dialog sets the tab count (both end tabs included, at least 2) and tab width; tabs start evenly spaced and can be dragged along the joining line. Edit re-spaces the tabs only when the count or width changes. The view shades the scrap, draws the cuts and marks where each cut is pierced.

Geometry contract

BreakoffBuilder (OpenNest.Core/Breakoffs) resolves each Drawing.Breakoffs entry against the entities being cut:

• The two sides are the outline from one end of the joining line to the other, closed back along the joining line. They must add up to the outline within 1%. Tab positions are measured from the joining line's end with the smaller X (then Y).
• The outline is kept, with every joint snapped exact (lines move their endpoint; a short connecting line is added between arcs), because the planner closes contours within 1e-6 and the program is re-chained from any entity.
• The joining line between consecutive tabs becomes open cuts on the Break layer (LayerType.Break, SpecialLayers.Break), each travelling with the scrap on its left. The joining line itself is not cut at the tabs; end tabs keep cuts off the outline.

Cutting break cuts

Break cuts cut like Cut motions but are not material: SpecialLayers.IsMaterial, material capture, overlap checks, nesting and area ignore them. ShapeProfile and ConvertGeometry pass them through in order without re-chaining or re-winding.

Both Plan Cutting (PreparedContours, emitted after scribes and before contours in every full or prefix emission, never in single-contour probes) and Assign Lead-ins (ContourCuttingStrategy.Apply/ApplySingle) emit them through ContourCuttingStrategy.EmitBreakCuts:

• Each cut is entered from its scrap side with the internal lead-in; NoLeadIn pierces on the line. There is no lead-out, since the cut ends at a tab.
• It is entered at whichever end has more scrap before the part's contours along the scrap direction (BreakCutEntry), because the scrap narrows to a wedge near the outline. Entering at the far end cuts it in reverse.

Checks treat them as intentional open cuts retained by tabs: ReleasedContourState never makes them rapid obstacles or "open fragment" warnings, but reports a missing lead-in like any contour. LeadPathValidator lets a lead into a break cut run through its own part's material, and still refuses one that touches any of the part's boundaries or another part. The posts cut them: Gravograph IS maps Break to its cut settings; CI Fiber and Cincinnati treat any non-lead, non-scribe layer as cut.

Breakoffs that do not resolve (hidden or missing joining line, overlapping tabs) are listed with the reason and not applied, but stay on the drawing. CadImporter.BuildDrawing applies breakoffs and copies them to the drawing; SourceEntities keep the original geometry so a reopened drawing can be edited. Split pieces get no breakoffs.