A basic native macOS app to previews 3D model files, like .step files.

Supported formats:

STEP stores B-Rep geometry (parametric curves and surfaces) that must be tessellated into triangles before it can be shaded — which realistically requires OpenCASCADE.

• If FreeCAD is installed (/Applications/FreeCAD.app), the app shells out to its freecadcmd tool to tessellate the STEP file into a solid mesh (FreeCAD bundles OpenCASCADE). This is the normal path and renders STEP files as proper shaded solids. Tessellation deflection is scaled to the model's size.
• If FreeCAD is not installed, the app falls back to a wireframe preview: a minimal parser extracts CARTESIAN_POINT, VERTEX_POINT, and EDGE_CURVE entities and renders them as a point cloud plus straight-edge wireframe. Curved edges appear as chords.

Model loading runs off the main thread with a loading spinner, so the slower FreeCAD path never freezes the UI.

make           # builds build/STEP File Viewer.app
make run       # builds and opens the app
make clean

Requirements: macOS 13+, Xcode command-line tools (swiftc). No Xcode project — the Makefile drives swiftc against the sources in Sources/STEPFileViewer/ and assembles a code-signed .app bundle.

The window has a standard macOS title bar (close / minimize / zoom buttons); the current filename is shown as the window title. Below it, the content area is a translucent light "Quick Look" background that the desktop blurs through, filled by the 3D viewport with a small statistics HUD docked bottom-right.

A color well in the top-right of the viewport recolors every geometry material (the diffuse channel). The chosen color is persisted to UserDefaults as objectColor.rgba (four sRGB components) and restored on the next launch.

Bottom-right overlay shows:

• tris, quads, ngons — face counts by topology (quads are only preserved when the source file declared them, e.g. OBJ; SceneKit triangulates everything on import, so we tally counts at MDLAsset level)
• lines, points — non-mesh primitives (used by STEP wireframe preview)
• verts — vertex count
• meshes — submesh count when > 1
• x, y, z — bounding box dimensions in source units

It also has a wireframe toggle button at the bottom of the HUD, which switches the viewport between solid shading and wireframe rendering (SCNView.debugOptions = .renderAsWireframe).

Sources/STEPFileViewer/
  App.swift              # @main, AppDelegate, NSWindow configuration
  ContentView.swift      # Root SwiftUI view — viewport + HUD + drop target
  SceneViewport.swift    # SCNView wrapper — orbit camera, color, wireframe
  ModelStore.swift       # Observable model state + file picker
  ModelLoader.swift      # Format dispatch + SCNGeometry builders
  ModelStatistics.swift  # Face / vertex / bounds accounting
  StatisticsHUD.swift    # Bottom-right statistics overlay + wireframe toggle
  STLLoader.swift        # Binary + ASCII STL
  ThreeMFLoader.swift    # 3MF (ZIP + XML)
  STEPLoader.swift       # STEP: FreeCAD solid mesh, wireframe fallback
  FreeCADBridge.swift    # Shells out to FreeCAD to tessellate STEP
Tests/
  test_loaders.swift     # Standalone CLI test runner for loaders
  fixtures/              # Committed sample model files for the tests
Resources/
  Info.plist             # Bundle metadata + document type associations
.github/workflows/
  ci.yml                 # Build + test + artifact upload on macOS
Makefile                 # swiftc → .app bundle + codesign; `make test`

make test

This compiles the non-UI loader sources together with Tests/test_loaders.swift into a small CLI runner and executes it against the committed sample files in Tests/fixtures/ (cube.stl, cube_ascii.stl, tri.obj, quad.3mf, tiny.step, block.step). The "STEP solid via FreeCAD" test self-skips when FreeCAD is not installed.

.github/workflows/ci.yml runs on every push to main, on pull requests, and on manual dispatch. On a macos-latest runner it:

1. builds the app with make,
2. runs the loader tests with make test (the STEP-solid test skips, since FreeCAD is not on the runner),
3. uploads the built STEP File Viewer.app as a workflow artifact.