Meshing & Export

A B-Rep solid is exact analytic geometry. To render it, 3D-print it, or hand it to a mesh pipeline you tessellate it into triangles; to round-trip it through CAD you export the exact B-Rep. This page covers both.

Tessellating a shape

mesh(linearDeflection:angularDeflection:) triangulates the shape. Linear deflection is the max chord deviation (mm) — smaller = finer; angular deflection caps the angle between adjacent facet normals (radians):

let box = Shape.box(width: 10, height: 5, depth: 3)!
guard let mesh = box.mesh(linearDeflection: 0.1) else { return }

mesh.vertexCount          // Int
mesh.triangleCount        // Int
mesh.vertices             // [SIMD3<Float>]
mesh.normals              // [SIMD3<Float>] (per-vertex)
mesh.indices              // [UInt32] — every 3 = one triangle
// indices.count == triangleCount * 3

The Mesh also exposes boundingBox, size, center, raw interleaved vertexData/normalData (ready for a GPU buffer), and trianglesWithFaces() — per-triangle access that carries the source B-Rep face index and a per-triangle normal, so you can map a picked triangle back to its face.

For fine-grained control, mesh from a MeshParameters:

var params = MeshParameters.default
params.deflection = 0.05
params.inParallel = true       // multi-threaded
let fine = box.mesh(parameters: params)

Deflection is a quality/size trade-off:

Use case	linear deflection
Quick preview	0.5
FDM print (0.2 mm layers)	0.1
Fine FDM (0.1 mm)	0.05
SLA / display-quality	0.02

Mesh → shape

A triangle mesh can be lifted back to a B-Rep (a shell of planar faces). The weld tolerance must scale with the model size — too tight leaves the mesh unwelded:

let shape = mesh.toShape(weldTolerance: 1e-6)   // raise for large-coordinate meshes

The result is a shell/compound of planar facets — not necessarily a valid solid; run healing if you need one.

Exporting

Two families: tessellated formats (STL/OBJ/PLY — triangles, take a deflection) and exact B-Rep formats (STEP/IGES/BREP — full analytic geometry). All Exporter.write… calls throw.

// tessellated (deflection controls facet density)
try Exporter.writeSTL(shape: box, to: stlURL, deflection: 0.05)        // binary by default
try Exporter.writeSTL(shape: box, to: stlURL, deflection: 0.05, ascii: true)
try Exporter.writeOBJ(shape: box, to: objURL, deflection: 0.1)
try Exporter.writePLY(shape: box, to: plyURL, deflection: 0.1)

// exact B-Rep
try Exporter.writeSTEP(shape: box, to: stepURL)
try Exporter.writeIGES(shape: box, to: igesURL, unit: "MM")
try Exporter.writeBREP(shape: box, to: brepURL)                        // native OCCT, full precision

// glTF / GLB (mesh + materials, for the web / model-viewer)
try Exporter.writeGLTF(shape: box, to: glbURL)                 // GLB (binary: true by default)
try Exporter.writeGLTF(shape: box, to: gltfURL, binary: false) // text .gltf

Instance methods mirror the statics where handy: box.writeSTL(to:deflection:), box.writeSTEP(to:modelType:), box.writeIGES(to:unit:).

Importing

let step = try Shape.load(from: stepURL)            // STEP (also Shape.loadSTEP)
let iges = try Shape.loadIGES(from: igesURL)        // loadIGESRobust sews/heals tolerance issues
let brep = try Shape.loadBREP(from: brepURL)        // exact + triangulation if exported with it
let stl  = try Shape.loadSTLRobust(from: stlURL, sewingTolerance: 1e-6)   // auto sew + heal

For mesh formats prefer the robust loaders (loadSTLRobust) — they sew and heal the seams a raw triangle soup carries. Loaders throw ImportError on failure.

STEP round-trip

try Exporter.writeSTEP(shape: model, to: stepURL, modelType: .asIs)
let reimported = try Shape.load(from: stepURL)
// reimported.isValid == true

For multi-part assemblies with structure/colors, export the document, not a flattened shape — see XCAF Assemblies and Exporter.writeSTEPAssembly(_:to:). To shrink a STEP with shared geometry, Exporter.optimizeSTEP(input:output:) deduplicates it.