Mesh-to-STEP keeps planar faces

A STEP file can contain planar faces reconstructed from a mesh. This example writes a tetrahedron and inspects the resulting entities to distinguish a successful container conversion from recovery of a CAD design. The source shape is already planar, so no curved surface is needed.

How this was measured

Generate the four-triangle, ten-millimetre tetrahedron and run the local core STEP writer. Count the reported faces and output bytes, then inspect the text for cylindrical, spherical, and B-spline surface entities. The output has four planar faces. These project-created bytes were generated locally without invoking the hosted upload endpoint.

Recorded output from this experiment
Measurement	Value
step Bytes	4586
faces	4
has Curved Surface Entities	false

What the result does not establish

A tetrahedron cannot demonstrate recovery of curves because it has none. Converting a tessellated cylinder with a faceted writer would retain planar approximations, not recover its source sketch, dimensions, constraints, or modeling history. The hosted STL-to-STEP route has a separate consent and limits boundary.

Try these files

These samples were created by this project and released under CC0. They contain no visitor files. Sizes and SHA-256 hashes identify the exact bytes measured in this experiment.

Ten-millimetre tetrahedron; STL itself stores no unit. (STL, 284 bytes)
SHA-256
7d98ff53aa7c0ac15350e51fc0adf77ebd39df597cac09f07269e1ff3fff9aa4
Four planar faces exported from the tetrahedron mesh, without feature history. (STEP, 4,586 bytes)
SHA-256
9ed8671d93d0c3639d9b15214d7430941a47aa466e0ed0f9134bc1f7720138f0

Use this result

Why an STL converted to STEP still has facets

Versions and reproduction

Evidence generated 2026-09-05T21:13:29.140Z. The checked-in generator rebuilds the sample collection and measurements with one command:

node --experimental-strip-types scripts/build-workbench-evidence.ts

CAD source generation: scripts/build-workbench-cad.py with cadquery-ocp 7.9.3.1.1; checked-in analytic cylinder outputs. The STEP and IGES outputs are checked in so ordinary evidence rebuilds do not require Python.

@astrojs/cloudflare: 14.3.0
@cloudflare/containers: 0.3.7
astro: 7.3.1
fflate: 0.8.3
jose: 6.2.12
occt-import-js: 0.0.23
rhino3dm: 8.32.2
three: 0.185.1

Geometry source and generator SHA-256 hashes:

core/affine.ts: cff986695025f835e54c530ef654a3b7b19862d5f5feba08a361f2db708fbe65
core/bytes.ts: 850a1863a411549bbaf73e85d339dd3109fb57aca4e89e1cbeb0a06cdb3cc924
core/convert.ts: e9e96ecbb498338b85bc122a7e328ae9d96666b2dadce581e4f86e8b7b59689d
core/deflate-validation.ts: 7427464082422b2a484cb4b845e21a41dbb015ffddb83a4d1f938b6c32cb40f1
core/detect.ts: 4c64ca45732c4339749fed372b20e6996f17e1f4d307046ad71e047723ccf3e9
core/formats.ts: f037f0543d19f5ffebe97ea40450c5b646d6d5164ae4e816bf3dce6cf3685479
core/index.ts: 6dcf467a7b1952cf270fd834927404b19eb58bc6d1f2542d8c420bda90d4e3f7
core/inspect.ts: 44100ac51622e986e3ab63e49d6704dde539141454e273b76d5be70b5115a1ec
core/model.ts: bef860184a185c098d72d30a12cfaa7fe3199b96dbde0e65428b8f3bfe6c79a5
core/read/3dm.ts: f26a7d22c86dc6780ce218822eafa4cc5dbf76f35289a6266f6f2161bd84127e
core/read/3ds.ts: 0142d9f82937566220d6616c52800ffa856fff500bab3e26e90c0cdc0c6f7e32
core/read/3mf-package.ts: 11ad7a82d1dc5698412479d0c6351c5b88cfbc308ebee4a5d91fdc71d969af51
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core/read/amf.ts: b68b2de1c27d3c374ce4e2ce4215c8a6edcfbccac3e9a7ed0f623956cfe4afde
core/read/dae.ts: acf671ba4ea931ca4e958a67886978e61da9c7cb936cb835265f86d3b271b2c5
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core/read/stl.ts: ac53b792bd9cb8bff4366331fa0bb49844c7d6fcf9df47819cc75544e812dec9
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core/read/wrl-preflight.ts: 2a4ef48b020ef98b381ab33aa34266bb5c43de716e2e38256598d69996d235ac
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core/write/step.ts: 05b888318d999a952aa5d9b2bddc47a79ef9d361b6828058df4b834c6ca886fd
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core/write/usdz.ts: 4cd8d9bf0d1aa1755c374980e5fb863b3045979e8201f8d79fc8e0857c09e962
core/xml-namespaces.ts: 10820125e1331ee730f577319faea4f279f1dd5a7b29a4bb7ec56475b7a9b0d8
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scripts/build-workbench-evidence.ts: 68716afd03c0d6e79a051eac57cc7a37a26218b9b119857f35dfb1663ac8770a
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test/core/workbench-writer-transforms.test.ts: 03e8b9524d2b952f94c2ffef015d3e9b6d3a2f1882f16cb2138e96eeb5b85af0
test/core/workbench-wrl.test.ts: eb2f2188c418080e14a4695f3b3025aaeb507a7ab5ea11be55cd7e17c4af76e4
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