Export STEP

Write AP214 / AP242 STEP files via STEPControl_Writer and STEPCAFControl_Writer for assemblies.

STEP is the de-facto interchange format for parametric CAD. OCCT exposes two writers depending on whether you need an assembly tree.

Single shape — STEPControl_Writer

import type { TopoDS_Shape } from '@taucad/opencascade.js';
import { getOc } from './ocjs-init';

export const shapeToStep = async (shape: TopoDS_Shape): Promise<Uint8Array> => {
  const oc = await getOc();
  using writer = new oc.STEPControl_Writer();
  using progress = new oc.Message_ProgressRange();
  oc.Interface_Static.SetIVal('write.step.schema', 5); // AP214

  const transferStatus = writer.Transfer(
    shape,
    oc.STEPControl_StepModelType.STEPControl_AsIs,
    true,
    progress,
  );
  if (transferStatus !== oc.IFSelect_ReturnStatus.IFSelect_RetDone) {
    throw new Error('STEP Transfer failed');
  }

  const path = `/out_${Date.now()}.step`;
  const writeStatus = writer.Write(path);
  if (writeStatus !== oc.IFSelect_ReturnStatus.IFSelect_RetDone) {
    throw new Error('STEP Write failed');
  }

  const raw = oc.FS.readFile(path) as Uint8Array;
  oc.FS.unlink(path);
  return new Uint8Array(raw);
};

Two return-status checks are mandatory — OCCT silently produces a zero-byte file otherwise.

Multi-shape assembly — STEPCAFControl_Writer

For assemblies with named parts, per-shape colors, or PBR materials, write the shape into an XCAF document first, then use STEPCAFControl_Writer.Perform. Never use the empty-filename Transfer overload — it silently activates multi-file mode and emits an empty STEP body.

import { getOc } from './ocjs-init';

export const assemblyToStep = async (shapes: Array<{ name: string; shape: TopoDS_Shape }>) => {
  const oc = await getOc();
  using docName = new oc.TCollection_ExtendedString('doc', true);
  const doc = new oc.TDocStd_Document(docName);
  const shapeTool = oc.XCAFDoc_DocumentTool.ShapeTool(doc.Main()).get();

  for (const { name, shape } of shapes) {
    const label = shapeTool.AddShape(shape, false, false);
    using nameStr = new oc.TCollection_ExtendedString(name, true);
    oc.TDataStd_Name.Set(label, nameStr);
  }

  using writer = new oc.STEPCAFControl_Writer();
  using progress = new oc.Message_ProgressRange();
  oc.Interface_Static.SetIVal('write.step.schema', 5);

  const path = `/asm_${Date.now()}.step`;
  const ok = writer.Perform(doc, path, progress);
  if (!ok) throw new Error('STEP assembly write failed');

  const raw = oc.FS.readFile(path) as Uint8Array;
  oc.FS.unlink(path);
  return new Uint8Array(raw);
};

Schema selection

Code	Schema	Use when
1	AP203 (config control)	Legacy mech CAD; avoid for new work
4	AP214 (auto industry)	Default — broad tool support
5	AP214 (alt revision)	Recommended for new projects
6	AP242 (managed model-based)	If you need PMI / PBR materials

AP242 emits the same geometry as AP214 plus product manufacturing information attached to the model. Most consumers (Fusion 360, SolidWorks, FreeCAD) auto-detect schema and treat 5 and 6 interchangeably for pure geometry.

Bytes out, file in

Browser: URL.createObjectURL(new Blob([bytes], { type: 'application/step' })) and trigger a download via a hidden <a> tag.
Node: fs.writeFileSync('out.step', bytes).
Cloud: pipe directly to S3 / GCS as application/octet-stream.

Common pitfalls

Check IFSelect_RetDone after both Transfer and Write — partial failures are silent otherwise.
Always unlink the MEMFS path after FS.readFile to free the wasm heap.
Copy bytes out of MEMFS (new Uint8Array(raw)) — the original view becomes detached after unlink.

Single shape — STEPControl_Writer

Multi-shape assembly — STEPCAFControl_Writer

Schema selection

Bytes out, file in

Common pitfalls

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