JT File Documentation

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Technical details

Feature	Value
Full name	JT (Jupiter Tessellation)
File extension	.jt
MIME type	model/jt
Format type	Binary 3D CAD visualization (tessellated mesh + optional B-rep)
Developer	Siemens Digital Industries Software (origin: Engineering Animation / HP)
Introduced	Late 1990s; ISO 14306 (JT V1) published 2012
Standard	ISO 14306 — industrial automation, JT file format
Open standard	Yes — spec published free via the JT Open program
Byte order	Recorded in the header (little- or big-endian)
Geometry	Tessellated mesh with multiple LODs; optional precise / XT B-rep
Also stores	Assembly structure (LSG), PMI, metadata, materials
Header	80-byte version header, e.g. Version 10.5 JT (ASCII)
Magic number	No fixed hex; identify by the ASCII version string at offset 0
Compression	Per-segment (deflate / specialised geometry codecs)
Typical use	PLM visualization and data exchange (automotive, aerospace)
Related extensions	.step, .stp, .stl, .iges, .x_t, .prt
Free viewer	JT2Go (Siemens), FreeCAD, CAD Assistant
Specification	iso.org/standard/60572.html

What is a JT file?

JT stands for Jupiter Tessellation, a name left over from its development at Engineering Animation and HP in the late 1990s. It is a compact binary 3D CAD format now owned and developed by Siemens Digital Industries Software, and it became an international standard, ISO 14306 (JT V1), in 2012. A .jt file exists so that engineers, suppliers, and reviewers can open, rotate, measure, and mark up a 3D model without owning the heavyweight CAD system that authored it — NX, CATIA, Creo, or SolidWorks. It is the dominant visualization and data-exchange format in large automotive and aerospace supply chains, where it pairs with PLM systems such as Teamcenter.

The format has a dual nature that explains most of its design. It can store lightweight tessellated geometry (triangle meshes) at several levels of detail for fast display, and it can also carry precise B-rep (boundary-representation) solid geometry for true CAD interoperability. A mesh-only JT is small and quick to view; a JT with B-rep is larger but can be converted back into exact solids. This is the single most important thing to know about a JT file, because it determines what a conversion to STEP or Parasolid can actually recover.

The 80-byte version header

Every JT file opens with an 80-byte header. Unlike most binary formats, it does not start with a short fixed hex magic number. Instead the first bytes are a human-readable ASCII string identifying the JT version, for example Version 10.5 JT or Version 9.5 JT. That version string is how a parser (and a file-identification tool) recognises a JT file and decides which revision of the specification to apply, because the binary layout of later segments changed across JT 8, 9, and 10.

offset 0   : ASCII version string, e.g. "Version 10.5 JT"   (within 80 bytes)
           : byte-order flag (little- or big-endian)
           : reserved / TOC-offset field
offset 80  : binary payload begins
           : TOC (table of contents) -> data segments

The header also records the byte order used for the multi-byte integers and floats in the rest of the file, and a pointer to the table of contents. Because the TOC is located from the header rather than assumed to sit at a fixed place, a reader can jump straight to the segment index without scanning the whole file.

The table of contents and data segments

After the header, a JT file is organised as a set of independent data segments, each indexed by the table of contents. The TOC lists, for every segment, its unique identifier, its type, its absolute offset in the file, and its length. This is what makes JT efficient over a network or in a PLM repository: a viewer can read the TOC, then fetch only the segments it needs (say, the coarsest level of detail for a first display) and defer the rest.

Segments are typed. The common kinds are the logical scene graph, the shape LOD segments that hold the actual meshes, optional B-rep segments, PMI segments, and metadata segments. Each segment is independently compressed, so the geometry-heavy segments use codecs tuned for vertex and triangle data while metadata segments use general deflate. A single physical .jt file can also reference external JT files, which is how a large assembly is stored as a shallow structure file plus many per-part JT files.

The Logical Scene Graph and assembly structure

The Logical Scene Graph (LSG) is the backbone of a JT file. It is a directed acyclic graph of nodes that expresses the product structure: which parts exist, how they are instanced, and the transform that places each instance in space. Because it is a graph rather than a flat list, one part definition can be instanced many times (every identical bolt in an assembly points back to a single shape), which keeps big assemblies small.

Node types in the LSG include group and part nodes for structure, instance nodes that reference shared geometry, and attribute nodes that attach materials, colours, and transforms. The leaf shape nodes point into the shape LOD segments where the real triangle data lives. Walking the LSG from its root gives a viewer the full assembly tree, the visibility and transform of every instance, and the link from each instance to its mesh — everything needed to draw and to explode the assembly.

Tessellated geometry and levels of detail

The visualization side of JT is its tessellated geometry: triangle meshes approximating each part's surfaces. What sets JT apart from a plain mesh format like STL is that it stores several levels of detail (LODs) per shape. A coarse LOD has few triangles and loads instantly; finer LODs add triangles for close inspection. A viewer picks an LOD based on how large the part appears on screen, so a full car body can be spun smoothly and then examined in detail on demand.

The mesh data is compressed with geometry-specific encoders rather than generic byte compression. Vertex coordinates, normals, and topology are quantised and packed so that a JT mesh is markedly smaller than the equivalent STL or OBJ. This compact tessellation is exactly what makes JT usable for assemblies containing tens of thousands of parts.

Precise B-rep and PMI

A JT file may optionally include a B-rep segment holding exact boundary-representation geometry: the trimmed NURBS surfaces, edges, and vertices that define true solids, not an approximation. Siemens JT commonly uses XT B-rep, the same representation as the Parasolid kernel behind NX and Solid Edge, which is why a B-rep-bearing JT can transfer a genuine solid to those systems. When this segment is present, the JT is interoperable CAD data; when it is absent, the file is mesh-only and can only ever yield an approximate solid.

Product and manufacturing information (PMI) is carried in its own segments: dimensions, geometric tolerances, datums, surface finishes, and annotations, associated with the geometry they describe. PMI lets a downstream reviewer read the tolerances directly off the model rather than a separate 2D drawing, which is the point of a model-based-definition workflow.

Why a JT export can lose exact geometry

The mesh-versus-B-rep split has a direct practical consequence. Converting a JT to STEP (ISO 10303) gives you true solid geometry only if the JT contained precise B-rep; a mesh-only JT converts to a tessellated approximation that no CAD system will treat as a clean solid. The same caveat applies to Parasolid (.x_t) and IGES targets. Converting to STL or OBJ, by contrast, always works well, because those formats are meshes and JT already holds mesh data. If your goal is to edit the part in CAD, check first whether the JT carries B-rep; if your goal is to 3D print or render it, the tessellation is all you need.

Free tools cover both paths. JT2Go, Siemens' own viewer, opens any JT to view, measure, and read PMI. FreeCAD and Open Cascade CAD Assistant import JT and export STEP, STL, OBJ, or glTF, so they double as free converters. The professional Siemens tools (NX, Solid Edge, Teamcenter Visualization) author and read JT with full fidelity.

References

• ISO 14306 — Industrial automation systems and integration: JT file format specification
• Siemens — JT file format and JT Open program
• Open Cascade — CAD Assistant (opens and converts JT)

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