cad-preview

Extract the embedded preview/thumbnail image from CAD and 3D files — in pure JavaScript, on any machine. No SolidWorks, no eDrawings, no native libraries, no cloud service. Works in the browser, Node, and Bun.

Many CAD/3D formats save a rendered preview inside the file. cad-preview finds and returns it as ready-to-use image bytes (PNG/JPEG/BMP/GIF), so you can generate thumbnails without opening the CAD program.

import { extractPreview } from 'cad-preview'

const bytes = new Uint8Array(await file.arrayBuffer())
const preview = extractPreview(bytes, { filename: file.name })

if (preview) {
  // preview.data  -> Uint8Array of image bytes
  // preview.format -> 'png' | 'jpeg' | 'bmp' | 'gif'
  // preview.source -> which extractor produced it
  // preview.name   -> identifier when the file has several (see below)
} else {
  // No embedded preview — fall back to an icon or a 3D render.
}

Pass filename whenever you have it: a couple of formats (notably modern SolidWorks) have no reliable file magic and are recognized by extension.

Multiple previews

Some files hold several previews — a Bambu Studio 3MF has one render per build plate, a DWF has one per sheet, an OLE compound doc can carry several image streams, a SketchUp file can embed scene previews. extractPreviews returns them all, best-first; extractPreview returns just the first (the default pick).

import { extractPreviews } from 'cad-preview'

const all = extractPreviews(bytes, { filename: 'print.3mf' })
// -> [ { name: 'Metadata/plate_1.png', data, format, source },
//      { name: 'Metadata/plate_2.png', … }, … ]
// all[0] === extractPreview(...) ; use `name` to label a chooser.

Supported formats

Family	Extensions	How the preview is stored
SolidWorks (modern, ~2015+)	.sldprt .sldasm .slddrw	Proprietary binary container; a 640×480 PNG stored as a raw-DEFLATE stream. Reverse-engineered — see below.
SolidWorks (legacy, ≤2014)	.sldprt .sldasm .slddrw	OLE compound document with a PreviewPNG stream or a headerless DIB Preview stream.
Autodesk Inventor	.ipt .iam	OLE compound document with a PNG embedded inside a stream.
Solid Edge	.par .psm .asm .dft	OLE compound document with a DIB thumbnail embedded in a stream / property set.
Autodesk 3ds Max	.max	OLE compound document with a DIB thumbnail.
Autodesk Revit	.rvt .rfa	OLE compound document with a RevitPreview PNG stream.
Rhino 3DM	.3dm	openNURBS preview chunk — a Windows DIB whose pixels are zlib-compressed (or, rarely, uncompressed).
AutoCAD DWG	.dwg	Preview-image section (header pointer → sentinel → entry table); a headerless DIB (R13–R2010) or PNG (R2013+).
AutoCAD DXF	.dxf	Optional THUMBNAILIMAGE section; a Windows DIB hex-encoded across group-code 310 lines.
Blender	.blend	TEST file-block holding a bottom-up RGBA thumbnail. Uncompressed, gzip-, and zstd-compressed (Blender 3.0+) files.
SketchUp	.skp	Version header immediately followed by the thumbnail as an embedded PNG.
Autodesk DWF	.dwf	ZIP behind a (DWF V... prefix; each ePlot sheet stores a rasterized PNG.
3MF / FreeCAD / Fusion 360 / OPC ZIPs	.3mf .fcstd .f3d	ZIP package with a thumbnail/preview image part. Includes slicer 3MF projects (e.g. Bambu Studio), whose render is Metadata/plate_N.png.

The OLE compound-document family (legacy SolidWorks, Inventor, Solid Edge, 3ds Max, Revit, …) is handled generically: any embedded PNG or Windows DIB (BITMAPINFOHEADER / V4 / V5) in any stream is found and, if a DIB, transcoded to PNG. So many OLE-based CAD formats work without a dedicated extractor.

Previews stored as a Windows DIB are transcoded to PNG on the way out, so every result is a browser- and libvips/sharp-decodable image (never a raw BMP). Preview.format tells you which.

Not every file contains a preview — e.g. a SolidWorks file saved with "save preview graphics" off, or a raw geometry format (STL/STEP/OBJ/IGES) — in which case extractPreview returns null.

Roadmap

Raw geometry formats (STL, STEP, OBJ, IGES, Parasolid) embed no raster preview — generating a thumbnail for those needs an actual 3D render, which is out of scope for this (pure-parsing, no-GPU) library. New embedded-preview formats are welcome.

Why this exists

The pre-2015 OLE SolidWorks preview has long been extractable (libgsf's gsf cat file.SLDPRT PreviewPNG, the cfb package, or the Windows-only Document Manager API). But SolidWorks switched to a proprietary binary container in 2015, and there was no lightweight, cross-platform library to pull the preview out of those files. The high entropy of the compressed internal streams even leads people to assume the files are encrypted — they aren't. This package documents and implements the extraction so any project can generate SolidWorks thumbnails on Linux/macOS/Windows/in-browser.

Extending

Add your own format without forking:

import { registerExtractor } from 'cad-preview'

registerExtractor({
  name: 'my-format',
  canHandle: ({ data, lower }) => lower.endsWith('.myext'),
  extract: ({ data }) => {
    const image = /* locate the embedded image */ null
    return image ? { data: image, format: 'png', source: 'my-format' } : null
  },
})

The roadmap is to cover every 3D format that embeds a preview image; contributions of new extractors are welcome.

Notes

• Dependencies: cfb (OLE reader) and fflate (DEFLATE) — both pure-JS and browser-safe.
• This is clean-room interoperability with a file's own embedded preview image. "SolidWorks", "Inventor", "Fusion", and "3MF" are trademarks of their respective owners; this project is not affiliated with or endorsed by them.

License

MIT © Cameron Tacklind