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.
Utilization comparison by part type
| Part type | Rectangular nesting | True-shape nesting | Typical gap |
|---|---|---|---|
| Rectangles, squares | 85–92% | 87–93% | 2–5% |
| Simple L-shapes, angles | 72–80% | 84–90% | 10–12% |
| Complex brackets, gussets | 58–68% | 80–88% | 18–24% |
| Organic / freeform shapes | 45–60% | 74–85% | 25–30% |
| Mixed jobs (variety of shapes) | 65–74% | 82–91% | 15–20% |
These ranges are consistent with published research on 2D bin packing (Bennell & Oliveira, 2008, European Journal of Operational Research) and with real-world data from nesting software vendors.
For a shop cutting purely rectangular stock (flat bars, standard plate sections), rectangular nesting is adequate. For any shop cutting irregular parts - which is most laser and plasma shops - true-shape nesting recovers material that rectangular nesting structurally leaves on the table.
A concrete example
Consider an L-shaped bracket: 120 mm wide, 100 mm tall, with a 60×50 mm notch cut from one corner. The bounding box is 120×100 mm.
With rectangular nesting, each bracket occupies 12,000 mm² of sheet area. The actual part area is 7,800 mm² (12,000 minus the 4,200 mm² notch). The nesting efficiency per part is 65% - 35% of the rectangle each part occupies is empty space.
With true-shape nesting, the optimizer can interlock two brackets - rotating one 180° so its notch faces the notch of its neighbour. Two brackets that interlock this way occupy roughly 14,800 mm² instead of 24,000 mm² (two bounding boxes). Efficiency per part pair: 87%.
That gap - 65% vs 87% - is 22 percentage points on a single part type. Across a full job with 100 brackets, it’s the difference between needing 3 sheets and needing 2.4 sheets.
When rectangular nesting is sufficient
There are real scenarios where rectangular nesting is adequate or preferable:
Parts with no significant concavities. If your parts are mostly rectangles, trapezoids, or convex shapes with minor chamfers, the gap between rectangular and true-shape nesting is small. The added computation time of true-shape nesting may not be worth it.
Grain direction constraints. On anisotropic materials (grain-direction-critical aluminium, directional rolled steel), parts often can’t be freely rotated. If all parts must run in the same orientation, the interlocking advantage of true-shape nesting diminishes. Rectangular nesting with fixed orientation may be within a few percentage points of true-shape nesting.
Very simple parts at high volume. A shop cutting nothing but 200×100 mm rectangles all day doesn’t need true-shape nesting. Rectangular packing gets to 90%+ on uniform parts.
Prototyping and one-off jobs. If you’re cutting a single prototype part and wasting the rest of the sheet regardless, the nesting method doesn’t matter.
Outside these cases, true-shape nesting is the right choice for any job where material cost is a meaningful factor.
The rotation question
True-shape nesting’s advantage over rectangular nesting depends substantially on rotation freedom. More rotation options mean more packing configurations, which means higher utilization.
Free rotation (any angle) produces the best utilization. For irregular organic shapes, the difference between 4-angle rotation and free rotation can be 5–10 percentage points.
Fixed angles (0°, 90°, 180°, 270°) is the most common practical setting. It respects grain direction and material properties while still allowing the interlocking that rectangular nesting can’t achieve.
No rotation eliminates most of the interlocking advantage. True-shape nesting without rotation still avoids the bounding-box penalty, but the utilization gains are smaller.
For most laser and plasma jobs with mild steel or acrylic, free rotation is the right default. For structural steel plate with directional mechanical properties, fixed-angle true-shape nesting is a reasonable compromise.
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.
Ready to try Lapas?
Start free - no credit card