Meshing & drawings

Three tools cover the mesh-and-drawing pipeline: generate_mesh tessellates a B-rep body and returns quality metrics; simplify_mesh decimates that mesh with QEM and writes a .stl or .obj; and generate_drawing produces a multi-view ISO 128-30 DXF — a standard part sheet from a single body, or a general-arrangement assembly sheet with a parts list and balloons from two or more. Both servers support all three tools.

_{🖱️ Drag to orbit · scroll to zoom · auto-rotating. (Model exported via export_scene → glTF.)}

Step 1 — Tessellate with quality metrics

Call generate_mesh first to inspect surface quality and decide whether you need a tighter mesh or a decimated export.

linearDeflection (mm) is the dominant knob: smaller values produce more triangles and a more faithful mesh. angularDeflection (radians) caps the angle between adjacent triangle normals and matters most on curved faces. Omit outputPath if you only need the metrics.

{
  "bodyId": "housing",
  "linearDeflection": 0.05,
  "angularDeflection": 0.3
}

{
  "triangleCount": 9214,
  "minEdgeLength": 0.04,
  "maxEdgeLength": 2.1
}

To write a fine mesh at the same time, add outputPath:

{
  "bodyId": "housing",
  "linearDeflection": 0.05,
  "angularDeflection": 0.3,
  "outputPath": "/tmp/housing_fine.stl"
}

{
  "triangleCount": 9214,
  "minEdgeLength": 0.04,
  "maxEdgeLength": 2.1,
  "outputPath": "/tmp/housing_fine.stl"
}

Use returnGeometry: true if the client needs the vertex and triangle data inlined in the response rather than written to disk.

Step 2 — Decimate with simplify_mesh

simplify_mesh runs QEM decimation (OCCTSwiftMesh / vendored meshoptimizer) and always writes a file — outputPath is required. Target the triangle budget with targetReduction (fraction to remove) or targetTriangleCount (absolute cap); both can be set together.

Use maxHausdorffDistance as a quality gate: the tool aborts if the achieved surface error exceeds the threshold. preserveBoundary locks seam edges; preserveTopology prevents genus changes.

{
  "bodyId": "housing",
  "outputPath": "/tmp/housing_lod.obj",
  "linearDeflection": 0.1,
  "targetReduction": 0.75,
  "maxHausdorffDistance": 0.5,
  "preserveBoundary": true
}

{
  "inputTriangles": 9214,
  "outputTriangles": 2304,
  "hausdorffDistance": 0.31,
  "outputPath": "/tmp/housing_lod.obj"
}

Check hausdorffDistance against your tolerance budget. If it is close to maxHausdorffDistance, loosen targetReduction or tighten linearDeflection on the upstream tessellation step.

Step 3 — Single-part technical drawing

generate_drawing writes a DXF via DrawingComposer. Pass bodyId for a single-part sheet; the spec object controls the sheet size, title block, and which ISO projection views to render.

{
  "bodyId": "housing",
  "outputPath": "/tmp/housing.dxf",
  "spec": {
    "sheet": "A3",
    "title": "Housing — Rev B",
    "views": ["front", "top", "right", "iso"]
  }
}

{
  "outputPath": "/tmp/housing.dxf",
  "views": ["front", "top", "right", "iso"],
  "bodyCount": 1
}

The DXF is the output — open it in any CAD viewer or convert to PDF with a DXF-aware tool. spec also accepts sections and dimensions sub-objects (see OCCTSwiftScripts/Sources/DrawingComposer/Spec.swift) for section cuts and driven dimensions on part sheets.

Step 4 — General-arrangement assembly sheet

Pass bodyIds (two or more) instead of bodyId to produce an assembly sheet. generate_drawing renders a shared multi-body view and auto-generates a parts list with one balloon per body. bodyIds takes precedence over bodyId when both are supplied. Per-view sections and dimensions in the spec are ignored on assembly sheets.

{
  "bodyIds": ["housing", "cover", "shaft", "bearing"],
  "outputPath": "/tmp/assembly_ga.dxf",
  "spec": {
    "sheet": "A2",
    "title": "Gearbox Assembly — GA Sheet",
    "views": ["front", "top", "iso"]
  }
}

{
  "outputPath": "/tmp/assembly_ga.dxf",
  "views": ["front", "top", "iso"],
  "bodyCount": 4
}

Body names shown in the parts list come from the manifest. Use rename_body before generating the drawing if the default ids are not human-readable.

Putting it together

A typical workflow for a new part:

1. Build or import the body (execute_script / import_file).
2. generate_mesh — verify triangle count and edge length distribution.
3. simplify_mesh — produce a lightweight .obj for the game engine / web viewer.
4. generate_drawing with bodyId — produce the engineering DXF for the fabricator.

For an assembly:

1. Load or build each component; confirm body ids with get_scene.
2. generate_drawing with bodyIds — one call, shared views, parts list, balloons.