three-iges-loader

Entity implementation guide

Use this document when adding or fixing an IGES entity decoder. It is written for human developers and AI assistants.

File checklist

For entity type NNN:

1. packages/iges-core/src/entities/decoders/typeNNN.ts — decode RawEntity → geometry type
2. packages/iges-core/src/entities/registry.ts — register decoder, add to GEOMETRY_ENTITY_TYPES if drawable
3. packages/iges-core/src/types.ts — add XxxGeometry interface if new shape
4. src/three/toThree.ts — tessellate to Three.js (if drawable)
5. test/fixtures/<name>.iges — minimal fixture
6. packages/iges-core/test/*.test.ts — assertions on parseAndResolveIGES

Decoder contract

import type { DecodeContext } from "../decodeContext.js";
import type { SomeGeometry } from "../../types.js";

/** Entity type NNN — <name from IGES spec> */
export function decodeTypeNNN(ctx: DecodeContext): SomeGeometry | null {
  const { entity, transform, warnings } = ctx;
  const p = entity.params; // ParamValue[]
  // Use paramNumber(p, index), paramInt(p, index)
  // Return null + warnings.push(...) if unsupported form
}

• Do not import three in iges-core.
• Do not apply transforms in decoders; resolveReferences() applies world transforms.
• Parameter indices in spec are 1-based; paramNumber uses 0-based indices (first PD field after entity type = index 0).

Parameter index mapping

IGES spec tables list parameters starting at index 1 (entity type is separate).

Spec index	Name	paramNumber(p, ?)
1	First parameter	0
2	Second	1
n	…	n - 1

Implemented entities

Type 116 — Point

PD index	Field	Type
1–3	X, Y, Z	Real

Type 110 — Line

PD index	Field
1–3	Start X,Y,Z
4–6	End X,Y,Z

Type 100 — Circular arc

PD index	Field
1	ZT
2–3	Center X,Y
4–5	Start X,Y
6–7	End X,Y

Radius = distance(center, start) in XY.

Type 106 — Copious data

Use form number from DE and IP (first PD integer):

Form / IP	Meaning
12 / IP=2	(x,y,z) triples
40	Witness line, common Z
63	Closed planar polyline

Type 126 — Rational B-spline curve

See decodeType126.ts for knot/weight/control point layout (K, M, flags, knots, weights, CPs, t0, t1).

Evaluation: math/nurbs.ts → sampleNurbsCurve().

Meta entities (no mesh)

Type	Role
124	Transform — parsed in resolve/parseTransform124.ts
314	Color palette
402	Associativity (visibility, groups)
406	Property

Spec references

• Eclipse IGES wiki
• IGES 5.3 PDF
• Wikipedia IGES

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