3D preview of Fusion designs from their display mesh

2026-11-08

Question

Can TeamCraft show an Autodesk Fusion design (.f3d) in the 3D viewer without Fusion, an Autodesk account or a translation service?

Findings

• A .f3d file is a ZIP archive. Most entries are compressed with Zstandard (ZIP method 93); a few small ones are stored. Common ZIP tools that know only Deflate refuse these entries. The archive holds one folder per document (FusionAssetName[Active]/…) with:

  • Breps.BlobParts/BREP.<uuid>.smb / .smbh: the exact geometry as binary Autodesk ShapeManager save files (ASM, the ACIS-derived kernel; .smbh with history). Tessellating them needs a B-rep kernel. truck cannot read them, and no open kernel does.
  • FusionDesignSegmentType1/…: the design (timeline, sketches, features).
  • Previews/small.png, Previews/big.png: thumbnails Fusion rendered when saving.
  • OGS.BlobFolder/OGS/DefaultScene/world and stream_mesh_000: the display scene of Fusion’s graphics system (OGS). Fusion stores it so that the design opens quickly. It holds the triangles Fusion draws for every face of every body.

• world is a binary serialisation of the scene graph. Class names are stored as u32 length + UTF-16LE: GroupNode, GeometryNode, OgsSimpleMesh, TransformAttribute, VisibilityAttribute, and the Fusion client types Component, Instance, Body, Faces, Face, Edges, Edge, ProfileFace, SketchCurve and so on. Shared objects are written once and then referenced by an id string (a pointer value printed as 16 hex digits).

• An OgsSimpleMesh record:

  "OgsSimpleMesh"  6 × f32 bounding box, u32 primitive type (4 = triangle list,
                   2/3 = lines, 5 = points), 3 bytes flags
  "VertexFormat"   id string; on first use: u32 count, count × 24 bytes
                   (u32 type: 1 = float2, 2 = float3; u32 semantic: 0 position,
                   1 normal, 2 uv, 3/4 tangent/binormal; u32 offset in floats; 3 × u32 0)
  vertex buffer    u32 1, u32 stream number, u32 offset, u32 byte length
  u8               1 = an index buffer follows
  "AIndexBuffer"   id string, u32 stream number, u8 0, u32 offset, u32 byte length
  

  The offsets point into stream_mesh_NNN, which holds the raw buffers: interleaved little-endian f32 vertices (position, normal, uv, …) and u32 triangle indices. The buffers follow each other without gaps.

• Coordinates are centimetres (Fusion’s internal unit). The meshes of a body’s faces share their boundary points bit for bit, so the faces weld into a closed surface. Every vertex carries the exact surface normal of its face.

• The sample (a mechanical key switch: 5 bodies with 179 faces in all) was compared with the STL and STEP files Fusion exported from the same design:

  	display mesh of the .f3d	STL export of Fusion
  triangles	1,108	4,490
  open edges after welding	0	0
  connected shells	4	4
  volume	0.98459 cm³	0.98485 cm³ (−0.03 %)
  bounding box	identical	identical

  The STEP export has the same box in millimetres (14.8 × 14.8 × 15.4 mm). The display mesh is coarser than an export, but it has exact normals, so the PN-triangle refinement of the NX preview (see 3D preview of NX parts) applies unchanged.

• Each body appears twice in the scene: once without meshes and once with the Face meshes. Edges, sketch profiles, sketch curves, dimensions, constraints, work planes and origin geometry are separate meshes under their own client types and are left out.

• Instances carry a TransformAttribute: a 4 × 4 f32 matrix, row-major, with the translation in the last row (p' = p · M). In the sample every instance and body transform is the identity, because the components are modelled in place.

• Prior art: cadmpeg-codec-f3d (Rust, Apache-2.0, clean-room) decodes the ZIP container, the ASM B-rep streams and the design records of .f3d files. It does not mention the OGS display scene. TeamCraft reads only the display scene and copies no code. The Zstandard decoder is ruzstd (pure Rust, MIT).

Decision

• Read the display mesh: open the ZIP, inflate world and the stream_mesh_* entries with ruzstd (sizes capped), and take the triangle meshes whose client type is Face. Weld them per body and refine them along their normals like NX parts. Convert centimetres to millimetres.
• Render the thumbnail from the 3D preview, as for NX. Fall back to Previews/big.png when the file has no display scene.
• The B-rep (.smb) stays unread: the display mesh already shows what Fusion shows.

Open points

• Instance transforms. The only sample places every component with the identity. An assembly whose components are moved, or that uses one component several times, is needed to confirm where the Instance transform applies. Until then the meshes are taken as stored.
• Hidden bodies and components. It is not yet known whether Fusion writes hidden bodies into the display scene, or how their VisibilityAttribute is set.
• .f3z (a design with its referenced designs) is a ZIP of .f3d files; it can be read once assemblies are confirmed.
• Files saved without a display scene (if Fusion writes such files) get the stored PNG.