filens-step

Read STEP/STP geometry locally in a browser with a Rust WebAssembly core. The package has no runtime dependencies, CLI, Python, server or CAD installation. Parsing, units, assembly placement and tessellation run inside WASM. The TypeScript adapter supplies an isolated in-memory WASI host and typed mesh data.

Install the release candidate from the npm next tag:

npm install filens-step@next

import { readStep, createStepReader } from 'filens-step';

const mesh = await readStep(await file.arrayBuffer(), { tolerance: 0.1 });
// Non-indexed triangles, in millimetres; three Float32 coordinates per vertex.
console.log(mesh.positions, mesh.normals, mesh.triangleCount, mesh.warnings);

// Optional URL/bytes/precompiled-module override for a custom asset host or CSP.
const reader = createStepReader({ wasm: '/assets/filens-step.wasm' });
const another = await reader.read(await file.arrayBuffer(), { signal });

The default WASM URL resolves relative to the installed ESM module. Serve the WASM asset emitted by your bundler with the JavaScript. The module is fetched and compiled only on the first read, then reused. Every read creates a separate WASM instance and virtual filesystem. Failed fetches can be retried. The default tolerance is 0.1 mm; input is limited to 64 MiB. AbortSignal cancels asynchronous loading, but synchronous tessellation itself cannot be interrupted on the same JavaScript thread. Large files can be loaded in a caller-owned worker.

With Three.js, use the returned arrays directly:

geometry.setAttribute('position', new THREE.BufferAttribute(mesh.positions, 3));
geometry.setAttribute('normal', new THREE.BufferAttribute(mesh.normals, 3));
// No index buffer is needed.

This is an approximate preview, not a certified CAD geometry conversion. The tested 44 files with embedded solids all produce meshes without missing faces; 43 pass the recorded geometry screen. HEAD_BACK.stp retains a trimmed-sphere region/orientation disagreement with OCCT. Tightening tolerance is not a known fix for it. Source colors, names and the assembly tree are omitted. External assembly references need their part files; multi-file assembly loading is not implemented. Empty geometry and incomplete faces are reported as errors rather than silently accepted as a complete model.

The bundled WASM is 947,729 bytes raw, 334,212 bytes with gzip level 9, or 261,101 bytes (254.98 KiB) with Brotli 11. Those are WASM-only sizes, not the npm tarball or complete download. JavaScript, TypeScript declarations, provenance and dependency notices are also included. See the size and geometry evidence.

Mesh transport uses a compact binary Float32 payload, preserving the previous reader's exact vertex and normal values. On the tested as1-oc-214.stp assembly, browser reading falls from 278.9 to 224.2 ms with unchanged tessellation and tolerance; this excludes downloads, WASM compilation and rendering.

Build inputs, the pinned upstream commit and exact WASM hash are shipped in dist/filens-step-manifest.json; dependency license texts are in dist/filens-step-THIRD-PARTY-NOTICES.txt. FiLens first-party code is MIT. The patched truck core is Apache-2.0; see upstream PR #141. In the FiLens workspace, npm run build:step builds the browser package without a Rust toolchain; npm run build:step:wasm rebuilds the pinned Rust core.