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3D preview of Inventor parts and assemblies

2026-11-08

Question

Can TeamCraft show native Autodesk Inventor parts (.ipt) and assemblies (.iam) in the 3D viewer without Inventor, Inventor View or a translation service, and with the same quality as the NX and SOLIDWORKS previews (3D preview of NX parts, 3D preview of SOLIDWORKS files)?

Findings

Inventor files are OLE compound files. Below the storage RSeStorage, a document is split into segments. RSeSegInfo lists them, and each segment has two streams with the same id:

  • M<id>, the meta stream:
    • the text RSe Meta Stream Version 8, the segment name (UTF-16) and its class UUID, then two dates;
    • then a zlib body: the length of every section in the segment, and the table of section classes (UUIDs).
  • B<id>, the data stream: an 18-byte header, then zlib. Inflated, it is a flat list of sections: u32 class word | payload | u32 length again | u8 0. The low byte of the class word indexes the class table, and the repeated length is a check.

Sections are named after the first 32 bits of their class UUID (for example D79AD3F3). Inside the sections, arrays share one form: u32 0x30000002, u32 count, u32 count, u32 element type, then the elements. The element type names a class of the table: a point (3 × f32), a unit vector or a 2D point.

Parts: the display tessellation

  • Every part has one PmGraphicsSegment. It is the scene graph Inventor draws the part with.

  • One section of class D79AD3F3 per B-rep face holds:

    • the owning face (a key);
    • the vertices (f32 xyz, centimetres, part coordinates);
    • a plain u32 triangle list, counter-clockwise around the outward normal;
    • a unit surface normal per vertex;
    • the uv parameters and edge flags, which the preview does not need.

  • A simpler class A79EACD2 has the same arrays without the extras. It is used:

    • for the quads of work planes (owned by no face, and left out);
    • for a second triangulation of a face;
    • in a few files, as the only mesh of a face.

  • Bodies: the faces come in body groups. A record before each group tells solid bodies (1) from surface bodies and construction copies (0). The preview takes the solid bodies; a part without any shows all its surfaces.

  • No transforms: every body is stored in part coordinates.

  • Watertight without tolerance: neighbouring faces share their boundary vertices bit for bit. Welding on the exact f32 values closes the mesh, whereas welding with a 1 µm tolerance breaks tiny features.

  • Tested files:

    • all 873 parts of the Autodesk Inventor 2022 sample set (saved with Inventor 2019 and 2022);
    • the two NIST parts in tests/fixtures/cad (Inventor 2021).

  • Results:

    ResultParts
    Decode failures0
    Closed and consistently oriented790
    Watertight, but touching themselves (springs, propeller blades, multi-body mould plates)18
    Open, because they are true surface models9
    Empty: sketches, route and skeleton parts without a body56

    NIST ctc-01 has the same bounding box as its NX and STEP versions: 800 × 450 × 150 mm.

  • Fineness and PN refinement: the resolution is Inventor’s display quality, similar to SOLIDWORKS. Every vertex carries the exact surface normal, so the same PN-triangle refinement as for NX and SOLIDWORKS applies.

  • Stored preview image: the summary information (\x05Zrxrt4arFafyu34gYa3l3ohgHg, property 17) holds a PNG of 180 or 512 pixels, which TeamCraft already shows.

Assemblies: occurrences and placements

  • An assembly stores no mesh of its own. Its AmGraphicsSegment holds proxies, so the preview is put together from the part files, as for NX and SOLIDWORKS.

  • Which files: the stream UFRxDoc holds:

    • the table of referenced files: the author’s absolute path, a file id, and flags for iPart/iAssembly factories and members;
    • one record per occurrence: occurrence id, file id, title, and 8 flag bytes. Bit 0x40 of the last flag byte means not visible.

  • Where: the AmRxSegment holds one section 232792BC per occurrence:

    • a packed 4 × 4 matrix: one u32 mask says which elements are +1, 0 or −1, and the others follow as f64;
    • the occurrence id, to join with UFRxDoc.

    The matrix uses column vectors (p' = R·p + t) and centimetres.

  • Sub-assemblies are files of their own and are placed relative to their parent.

  • Checks across all 249 sample assemblies:

    • all of them parse;
    • all 7 700 occurrences resolve to a file;
    • 233 of the 243 assemblies that store a bounding box reproduce it exactly with the decoded placements. With the transposed rotation only 40 do, all symmetric cases.
    • The misses are six cable and harness assemblies, whose wires are virtual components, and one at 0.9 %.
    • The visibility bit agrees with Inventor’s own DWFx exports.

  • Flexible sub-assemblies (addendum 2026-11-11). A parent re-poses children deeper down with override sets (class F17A4562): a path of occurrence ids from the parent to the sub-assembly, and refs to override sections (class 2238DCC5). Each override holds the child’s persistent key (offset 120 of its placement tail) and a packed matrix that replaces the child’s local placement. The overrides of the outermost assembly win. Applying them makes the Test Station bounding box exact (1e-8; 7.2 % off without), and the Blower pulleys turn. Kind 1 overrides (positional representations, with an empty path) are not applied.

  • Suppressed components (addendum 2026-11-11). Each occurrence record’s property block holds a name record 01 05 01 00 00 00 05 and, 31 bytes after its end, the record 01 07 01 00 00 00 07 whose byte is 1 for a suppressed occurrence. That is the layout Instance counts of Inventor assemblies verified with real suppressed components; in the sample set it sits at that place for 2 564 of 2 565 occurrences (all with the byte 0). Suppressed components are left out.

  • Not decoded:

    • design views other than the one active when the file was saved (DesignViewSegment): in the sample set only Buffer Prep Skid shows a difference, a room enclosure hidden in its Default view;
    • the weld beads of weldments, which are stored in the assembly itself.

The exact geometry

  • PmBRepSegment holds an Autodesk ShapeManager body (ASM BinaryFile, a fork of the ACIS SAB format). Its records can be read, but tessellating trimmed NURBS, blends and offset surfaces ourselves is weeks of work.
  • The display tessellation with PN refinement is good enough for a preview, so the exact geometry is not needed now.

Prior art, licences and law

  • InventorLoader (FreeCAD add-on, GPL-2.0) reads parts through their ShapeManager body and does not support assemblies. It confirmed the section framing and the UUID naming. No code was taken: TeamCraft is MPL-2.0.
  • The Open Design Alliance SDK is paid. Autodesk’s own readers (Inventor View / Apprentice) are Windows-only and not redistributable.
  • No Autodesk software or SDK was used, and nothing was decompiled. The layout was found by analysing files, which EU law permits for interoperability (Directive 2009/24/EC Art. 6, UrhG §69e).
  • “Inventor” and “ShapeManager” are trademarks of Autodesk; they are named only to say which files are read.
  • The Autodesk sample set carries no licence that allows redistribution, so it is not part of the repository. The tests use the two openly licensed NIST parts.
  • There is no openly licensed Inventor assembly yet. The assembly reader is tested with assemblies built in the test code.

Decision

  • inventor-part: read the face meshes of PmGraphicsSegment (solid bodies, or all faces of a surface-only part), weld them on their exact coordinates and refine them with PN triangles like NX and SOLIDWORKS.
  • inventor-assembly:
    • compose the preview from the part and sub-assembly files, found by file name;
    • use the placements of AmRxSegment, with the poses a parent gives children of flexible sub-assemblies, and leave out the occurrences that UFRxDoc marks as not visible or suppressed;
    • this is the same pipeline as NX and SOLIDWORKS assemblies (services/assemblies.rs).
  • The thumbnails of Inventor parts and assemblies are rendered from the 3D preview instead of the stored PNG. An assembly none of whose parts are in TeamCraft keeps the stored image.
  • Later, if needed: design views, weld beads and the ShapeManager geometry.

Addendum 2026-11-12: Zstandard segments (#271)

A user’s 44 Inventor parts all failed with damaged compressed data. The files were intact, but every M<id> and B<id> stream below RSeStorage held a Zstandard frame (magic 28 B5 2F FD, RFC 8878) instead of zlib (78 5E). Which Inventor version started writing Zstandard is not known: the version could not be read from those files, and every file we have is zlib – the Autodesk samples (Inventor 2019 and 2022), NIST 2021 and the NIST STC set saved with Inventor 2024. No openly licensed Inventor file with Zstandard segments was found.

What changed in src/invpart.rs:

  • the body of a meta or data stream is Zstandard when the magic stands where the zlib body would start (after the u8 of the meta stream, after the 18-byte header of the data stream), zlib when a valid zlib header stands there (RFC 1950 check bits), and otherwise the first Zstandard magic within 64 bytes before or after that offset;
  • the search is there because the header length in front of a Zstandard frame could not be checked against real bytes: if newer files changed the u8 flag or the 18-byte header, the frame is still found;
  • frames that follow each other directly are read one after the other;
  • the output is capped like the zlib output, and the window a frame asks for (which the decoder allocates up front) is capped at 32 MiB;
  • the decoder is the pure-Rust ruzstd (MIT), the same crate the Fusion archives use.

Tests repack a NIST part with Zstandard (every segment, the frame also a few bytes off) and check that every segment unpacks to the same bytes as from the zlib original, and that a Zstandard part gives the same preview mesh as with zlib; damaged Zstandard streams are errors, never panics. The fuzz target invpart gets a small synthetic seed with Zstandard segments (fuzz/seeds/invpart/zstd-segments.ipt).

Failed conversions are cached, so the preview cache (3), the thumbnail cache (7) and the shape signatures (3) got new versions: the parts that failed before are converted again.

Known gap: no real Zstandard file is in the tests. If one turns up under an open licence, it belongs in tests/fixtures/cad with a fixture test, and the header bytes in front of the frame should be checked against it.