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3D preview of NX parts from their lightweight representation

2026-11-06

Question

Can TeamCraft show a native Siemens NX part (.prt) in the 3D viewer without NX, a Parasolid licence or a translation service?

Findings

  • An NX part is a stream container (SPLMSSTR, see src/nxpart.rs). Next to the NX data (/Root/UG_PART/UG_PART, which embeds the exact geometry as a zlib-compressed Parasolid transmit block) every saved part carries /Root/FastLoad/JT: a short index followed by a complete JT 9.4 file (Version 9.4 JT DM 10.8.0.1) with the tessellation NX uses for fast loading.
  • Assemblies and drawings have no such stream (checked with the NIST assembly and with a real project folder: 23 of 27 .prt files had one, the rest were assemblies, a drawing backup and one part saved without it).
  • The JT file holds three levels of detail: one segment of type 7 (LOD 0, the finest) and two of type 6. Each is a Tri-Strip Set Shape LOD element with topologically compressed data, the most involved encoding of the format: 24 integer packets (codecs bitlength, arithmetic, chopper) drive a dual-mesh coder that rebuilds the triangles; coordinates are quantised per axis, the per-corner normals use Deering’s normal code. Coordinates are metres.
  • JT 9.5 is published by Siemens as the JT File Format Reference and was standardised as ISO 14306 (2012); the encoding is documented there. Open implementations to compare against: oce-jt (C++, LGPL-2.1, an Open CASCADE fork), jt-java, JTSharp. TeamCraft (MPL-2.0) implements the algorithms from the format reference and copies no code.
  • The Parasolid block cannot be tessellated without a Parasolid kernel (commercial); no open reader exists.
  • LOD 0 is coarse: 16–30 facets around a cylinder, up to 0.45 mm away from the true surface on a 60 mm part. But every corner carries the exact surface normal of the B-rep. With curved point-normal triangles (Vlachos et al. 2001) and creases handled through the cross product of the two face normals, a 3 × 3 refinement brings the deviation on the cylinders of a real part from 448 µm to 10 µm (mean 35 µm → 0.6 µm), and the mesh stays closed.
  • A prototype decoded all NX parts of the NIST fixtures and of a real project folder (23 parts, 128 to 9 760 triangles): every mesh is closed (each edge shared by two triangles), consistently oriented, with a positive volume; the normals all agree with the triangle orientation.

Decision

Implement: src/mesh/jt.rs reads LOD 0 of the JT stream of NX parts (JT 9.x only), src/mesh/pn.rs refines it along the normals up to 3 × 3 (about 500 000 triangles at most). NX parts get the format id nx-part in the 3D preview; assemblies and drawings keep their preview image.

Not now: stand-alone .jt files (the NIST assembly’s is JT 10.5 with a different element layout and a scene graph with transforms), the Parasolid block.

Addendum 2026-11-07: assemblies and thumbnails

The JPEG NX stores in a file is small (about 240 × 180 px) and poor for assemblies. NX parts therefore get a thumbnail rendered from their 3D preview, and assemblies a 3D preview put together from their parts:

  • An NX assembly has no /Root/FastLoad/JT of its own. Its /Root/FastLoad/Structure stream is an NX object serialisation (“OM”: header, object table of offsets and tags). The largest object holds the placement records (two rotation rows and the translation in mm, numbers as a tag byte plus a shortened big-endian double, all divided by 4), the component names (the file names without .prt, upper case) and the name index of every instance. Another object is a hash map from instance to record; instances without a record sit at the origin.
  • Verified against the JT 10.5 export of the NIST assembly (scene graph, property table and transform attributes decoded for this): all 36 instances match in part, rotation and translation (differences below 0.00001 mm). A real assembly with two levels of sub-assemblies (a pintle injector, 59 placed parts, 0.63 million triangles) renders correctly.
  • Sub-assemblies keep their placements in their own file (same format); placements compose as row vectors (p · R + t).
  • The component files are found by file name like file references; their saved folders are in /Root/FastLoad/ExternalReferences, but inside TeamCraft the name match is what counts.
  • Two of three exploration agents worked out the mapping independently (one from the JT ground truth, one from the .prt alone); a third ruled out the other streams (RMFastLoad: attributes and mass properties, UG_PART: the same transforms keyed by internal object maps).

Decision: src/nxasm.rs reads the components, services::assemblies composes the preview from the parts’ previews (cache key = content hashes of everything used), the thumbnail of parts and assemblies is drawn from the 3D preview (mesh/render.rs).

Hidden components (layers)

NX leaves components out of its own picture when they sit on a layer the assembly hides:

  • The Structure stream stores the layer of every component that is not on the default layer, as an int map (instance → layer 1..=256). A scan of a large SVN (5 760 .prt files) found such a map in exactly three assemblies.
  • The layer settings are in /Root/UG_PART/UG_PART, an OM section like the structure stream: one UGS::LAYER_parameter object with four masks of 17 32-bit words (selectable, visible, two copies with an extra bit), the work layer and the constants 1, 1. Integers there are signed (one byte = value
    • 1; 13, 21, 29 or 32 bits). The same layout decoded in all 5 760 files.
  • Checked against NX’s pictures: an assembly that hides layers 2–255 leaves out exactly its components on layers 9 and 19 (TeamCraft does the same); an assembly with every layer visible shows its components on layers 17 and 19.

Known gap: a component can also be invisible because the bodies inside its own part sit on hidden layers (a chamber whose own NX picture is empty). The lightweight data does not tell which layer a body is on, so such a component is still shown. Units of assemblies modelled in inches are not handled (all files seen are in mm).

Sources

  • Siemens, JT File Format Reference, version 9.5 (2010), and ISO 14306:2012 Industrial automation systems and integration – JT file format specification for 3D visualization.
  • A. Vlachos, J. Peters, C. Boyd, J. L. Mitchell, Curved PN Triangles, ACM Symposium on Interactive 3D Graphics, 2001.
  • oce-jt, jt-java, JTSharp – compared against only.
  • NIST CAD test files in tests/fixtures/cad/nist (see Openly licensed CAD files).