3D preview of SketchUp models

2026-11-08

Question

Can TeamCraft show a SketchUp model (.skp) in the 3D viewer without SketchUp, the SketchUp SDK or a conversion service?

Findings

• SketchUp is a polygon modeller: a model is made of planar faces bounded by loops of edges (the first loop is the outer boundary, further loops are holes), plus component definitions and instances of them (groups are definitions with a single instance). There is no separate display tessellation as in NX or SOLIDWORKS – the faces are the geometry and only need to be triangulated. Units are inches, Z is up.
• Two unrelated containers exist, both opening with the same UTF-16 header (FF FE FF 0E "SketchUp Model", then the version such as {25.0.575}):
  • SketchUp 2021 and later (“VFF”): a ZIP archive behind the header. model.dat is a tag–length–value tree (u16 tag, u32 length; about forty tags are containers). Vertices (C409: id, 3 × f64), edges (B80B: start and end vertex), faces (AC0D: normal, loops of coedges referencing an edge and a direction), instances (6419: definition id and a 13-double transform), definitions (7C15) and layers (8C3C: name, hidden byte). Hide flags and layers sit in a common D007 record. The archive also carries meta/model_thumbnail.png.
  • SketchUp 3 to 2020 (“classic”): one uncompressed MFC CArchive object stream with a single store map. Objects cannot be skipped without knowing their class layout, about 30 classes occur (vertices, edges, loops, faces, curves, layers, materials, textures, attributes, definitions, instances, groups, images, guides, section planes, text, dimensions, line styles), and several field sizes and quirks depend on the version.
• OpenSKP (github.com/iamahsanmehmood/openskp, MIT) is an open reader and writer for both containers with ports in C++, Python, Rust, TypeScript, C# and Dart, and a format description (docs/BINARY_FORMAT.md, the docstring of legacy.py). Its Rust port (packages/rust, crate openskp 1.5.0) has no dependencies but is not on crates.io. The crate published there under the same name (0.5.0, hew3d/openskp) is a different project under GPL-3.0 and was not used.
• The OpenSKP repository ships 40 .skp test files (MIT) from SketchUp 3 to 2026: 31 classic, 9 VFF, with curves, nested components, scaled instances, layers and hidden geometry; several models are saved in more than one version.
• A prototype (Python, outside the repository) with its own VFF decoder and OpenSKP’s classic walker loaded all 40 files; 35 contain faces (47 495 faces, 247 972 triangles), the other 5 are empty templates or hold only edges. On the 9 VFF files the own decoder produced exactly the faces, instances, definitions and – after triangulation and placement – the same world mesh triangle for triangle as OpenSKP. The same model saved in several versions (nesting, arcs, instance_scale_term, legacy_vN) gives identical meshes across both containers.
• Instance transforms: p' = s · (R p + t) with R’s rows (m0 m1 m2) (m3 m4 m5) (m6 m7 m8), t = (m9 m10 m11) and s = m12 (SketchUp’s homogeneous scale term). Verified on the models with skewed rotations: with the transposed matrix the window frames of one model fall apart, and another model matches its stored thumbnail from the same view. A mirrored instance (negative determinant) reverses the triangle order.
• Triangulation: a simple ear clipper with a hand-made hole bridge failed on about 400 real faces (holes, loops touching themselves). The earcut algorithm (mapbox/earcut, ISC) handles them; the triangle area matches the loop area within 1e-3 on all but a handful of faces whose loops overlap themselves in the source data. Inner loops that overlap each other do occur; then the outer loop is triangulated alone and triangles whose centre lies in an inner loop are dropped (as OpenSKP does).
• Visibility: hidden faces and instances (flag bit 0 of the display flags) and everything on a hidden layer are left out, an instance with its whole subtree; image definitions (textured rectangles) as well. OpenSKP itself reports these flags but does not filter.
• Most SketchUp models are not solids (single faces, open shells, faces turned inside out): only 4 of the 35 meshes are closed. The viewer and the thumbnail renderer have to draw both sides; closedness and volume are no test criteria here.

Decision

Implement src/mesh/sketchup.rs: the VFF container (SketchUp 2021+) with an own TLV reader on the existing zip crate, an earcut triangulation, placement of nested components, visibility filtering, units converted to millimetres. .skp gets the format id sketchup in the 3D preview and a thumbnail rendered from the mesh. The OpenSKP test files go into tests/fixtures/cad with their MIT licence (all but one 5.5 MB file).

The classic container (SketchUp 3 to 2020) is decided separately: taking over OpenSKP’s MIT Rust walker with attribution, or leaving it out for now. A git dependency is ruled out by deny.toml, and writing an own reader for the classic stream is not worth it.

Not now: colours, materials and textures, edges without faces, scenes.

Addendum 2026-11-08: SketchUp 3 to 2020

Decision: take over OpenSKP’s Rust port. Its packages/rust folder (crate openskp 1.5.0, MIT, no dependencies) is kept unchanged in vendor/openskp (origin, commit and licence in vendor/README.md, listed in the licence table of the README) and used as a path dependency with a fixed version – no git dependency, no crates.io package. src/mesh/sketchup.rs calls openskp::parse_skp only for classic files, catches a panic as a damaged file, and places and triangulates OpenSKP’s definitions, faces and instances exactly like those of its own 2021+ reader (layers are matched by name there). The tests expect the triangle counts and boxes of the prototype for all 26 classic test files with faces, and the same triangles for a model saved by SketchUp 2013, 2017, 2020 and 2021.

The crates.io package of the same name (openskp 0.5.0, hew3d/openskp) is a different project under GPL-3.0 and is not used.

Sources

• OpenSKP, MIT licence, © 2024–2026 Ahsan Mehmood, commit e7b4674: docs/BINARY_FORMAT.md, packages/python/src/openskp/{_core,legacy,scene}.py, packages/rust/src/ (vendored unchanged in vendor/openskp for the classic container; Python modules run unchanged in the prototype for comparison).
• earcut, mapbox, ISC licence (algorithm ported).
• No SketchUp installation, no SketchUp SDK; nothing decompiled.