True-Shape Nesting
A nesting method that places parts according to their exact geometric outline — including curves, holes, and irregular edges — rather than fitting each part into a rectangular bounding box.
What is true-shape nesting?
True-shape nesting (also called irregular nesting or arbitrary-shape nesting) is a method of placing parts onto a sheet material so that each part occupies only the space its actual geometry requires — not a rectangular box around it.
Rectangular nesting vs. true-shape nesting
In rectangular nesting, each part is enclosed in its bounding box (the smallest rectangle that fits around the part), and those boxes are arranged on the sheet. This is fast to compute but wastes significant material when parts have irregular outlines, concave curves, or cutouts.
In true-shape nesting, the actual part geometry is used. Two L-shaped parts, for example, can be rotated and interlocked so they fit together almost like puzzle pieces — dramatically increasing sheet utilization.
Why it matters
| Scenario | Rectangular nesting | True-shape nesting |
|---|---|---|
| 20 identical L-shapes on a 2500×1250mm sheet | ~52% utilization | ~89% utilization |
| 50 mixed irregular parts | ~60% utilization | ~82% utilization |
| Simple rectangular parts | ~88% utilization | ~91% utilization |
The difference is most pronounced for curved, L-shaped, T-shaped, or complex parts — common in sheet metal fabrication, laser cutting, and composites.
How true-shape nesting works
True-shape nesting algorithms use computational geometry techniques — most commonly the No-Fit Polygon (NFP) method — to determine all valid positions where one part can be placed relative to another without overlap. The optimizer then searches through possible arrangements to find the one that minimizes wasted area.
Modern metaheuristic approaches (genetic algorithms, simulated annealing) explore a large number of possible placements to find near-optimal solutions within reasonable time.
True-shape nesting in Lapas
Lapas performs true-shape nesting by default. It supports all standard DXF geometry types — LWPOLYLINE, ARC, CIRCLE, SPLINE, and ELLIPSE — and handles concave and convex polygons, parts with internal holes, and nested INSERT/BLOCK entities.
The optimizer offers two engines:
- Heuristic engine — fast (seconds), good quality for most jobs
- Metaheuristic engine — deeper search, higher utilization, takes longer for complex jobs
Both engines perform true-shape, not rectangular, placement.
Industries that benefit most
True-shape nesting produces the greatest improvement over rectangular nesting when parts are:
- Non-rectangular — L-shapes, T-shapes, triangles, trapezoids
- Curved — rounded brackets, flanges, radiused corners
- Nested into each other — concave parts can interlock with convex parts
- Available at many rotations — more orientation freedom = more interlocking opportunities
| Industry | Typical part geometry | TSN benefit |
|---|---|---|
| Sheet metal fabrication | Brackets, panels, flanges | High |
| Laser cutting (custom parts) | Mixed irregular shapes | High |
| Plasma cutting (structural) | Gussets, clips, tabs | Medium–High |
| CNC woodworking (furniture) | Shelf pins, brackets | Medium |
| Apparel / upholstery | Fabric pattern pieces | Very high |
| Glass cutting (rectangular) | Rectangles only | Low |
For shops cutting rectangular parts exclusively, rectangular nesting and true-shape nesting produce nearly identical utilization. The investment in a true-shape tool only pays off when part geometry is irregular.
Common misconceptions
“True-shape nesting means parts can overlap” — No. Parts never overlap. True-shape means the collision detection uses the actual part outline, not a bounding box around it.
“It always finds the optimal arrangement” — No. True-shape nesting is NP-hard. Even the best algorithms find near-optimal solutions within a time budget. The deep optimizer runs longer and generally finds better solutions, but cannot guarantee the global optimum.
“True-shape nesting is only for complex parts” — For simple convex polygons, true-shape and rectangular produce similar results. The difference grows with part complexity (concavity, curved edges, internal holes).
True-shape nesting vs. strip nesting
Strip nesting divides the sheet into horizontal strips and fills each strip with parts of similar height. It’s faster to compute and easier to implement but produces lower utilization than full true-shape nesting.
Strip nesting may be preferred in specific cases — guillotine-cut sheet (where all cuts must run edge-to-edge) or certain glass applications where the cutting sequence matters. For general CNC cutting, true-shape nesting is almost always the better choice.
FAQ
Does rotation improve true-shape nesting results?
Yes, significantly. Free rotation (any angle) allows the algorithm to find interlocking orientations that aren’t possible at fixed angles. Compared to no-rotation, free rotation typically improves utilization by 8–20% for irregular parts. The tradeoff is longer compute time and, occasionally, grain direction requirements that forbid free rotation.
How is true-shape nesting different from automatic nesting?
These aren’t mutually exclusive — they describe different aspects. “Automatic nesting” means the computer optimizes placement rather than a human doing it manually. “True-shape nesting” describes how collision is detected (by actual geometry, not bounding box). Virtually all modern automatic nesting software uses true-shape placement.
Can true-shape nesting handle parts with internal holes?
Yes. A part with an internal hole (e.g., a circular flange with a center hole) is treated as a polygon with a cutout. The nesting algorithm can also, in some cases, nest smaller parts inside the hole of larger parts — this is called “part-in-part” or “hole nesting.” Lapas supports this.
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