Mesh format readers

The native mesh-format types (STL, OBJ, PLY) are public enums you can call directly when you already have the bytes and want to skip extension dispatch. The glTF and 3MF paths are handled by internal MeshIO adapters over SwiftGLTF / ThreeMF — reach them through MeshIO.load / MeshIO.write. All are pure Swift, no OCCT.

Topics

• STL · OBJ · PLY · glTF / GLB · 3MF

STL

Native STL reader/writer (ASCII + binary). The reader auto-detects ASCII vs binary by layout.

public enum STL {
    public static func read(data: Data, weldEpsilon: Float = 1e-4) throws -> Mesh
    public static func binaryData(_ mesh: Mesh) -> Data
    public static func asciiString(_ mesh: Mesh, name: String = "mesh") -> String
}

• read welds vertices on import (STL splits all triangles). Throws MeshError.empty / .notRecognized.
• binaryData writes binary STL; asciiString writes ASCII with an optional solid name.
• Example:

  let mesh = try STL.read(data: try Data(contentsOf: url))
  let bytes = STL.binaryData(mesh)
  

OBJ

Native Wavefront OBJ reader/writer — geometry only (v + f). Faces of any vertex-ref form (v, v/vt, v/vt/vn, v//vn, negative-relative indices) are fan-triangulated.

public enum OBJ {
    public static func read(data: Data, weldEpsilon: Float = 1e-4) throws -> Mesh
    public static func string(_ mesh: Mesh) -> String
}

• read throws MeshError.empty / .notRecognized; welds when weldEpsilon > 0.
• string writes an OBJ document (1-based v / f).

PLY

Native PLY reader/writer. Reads ASCII and binary-little-endian PLY (the vertex element’s x/y/z and the face element’s index lists, fan-triangulated). Writes ASCII.

public enum PLY {
    public static func read(data: Data, weldEpsilon: Float = 1e-4) throws -> Mesh
    public static func string(_ mesh: Mesh) -> String
}

• read throws MeshError.unsupported("PLY big-endian") for big-endian binary; .empty / .notRecognized otherwise.
• string writes an ASCII PLY document.

glTF / GLB

Handled by MeshIO’s internal SwiftGLTF adapter — call via MeshIO.load / MeshIO.write.

• Read delegates the hard decode (JSON/GLB container, buffers, external .bin, accessors) to SwiftGLTF; the adapter walks the scene graph, decodes POSITION + index accessors, and bakes node transforms. Triangle primitives only.
• Write is native and minimal: positions (float32 VEC3) + indices (uint32 SCALAR) packed into one buffer. .gltf embeds the buffer as a base64 data URI (self-contained); .glb writes the binary container (12-byte header + JSON chunk + BIN chunk).

let mesh = try MeshIO.load(contentsOf: gltfURL)   // resolves external .bin relative to the URL
try MeshIO.write(mesh, to: glbURL, format: .glb)

3MF

Handled by MeshIO’s internal ThreeMF adapter — call via MeshIO.load / MeshIO.write.

• Read uses ThreeMF’s flattened LoadedModel (build items expanded into placed mesh instances with accumulated transforms), so multi-object / instanced models are positioned correctly.
• Write emits a single-object 3MF package.

let mesh = try MeshIO.load(contentsOf: threeMFURL)
try MeshIO.write(mesh, to: outURL, format: .threeMF)