Creo parts: a 3D preview from the stored B-rep

Research notes, 2026-11-08. Findings from reading the files only – no PTC software, no toolkit, nothing decompiled.

Summary

PTC Creo part files (name.prt.N) store no display tessellation of the body. Unlike NX (JT stream), SOLIDWORKS (display lists) or Inventor (graphics segments), the only geometry in the file is the exact B-rep: analytic surfaces with their contours, and every edge sampled as a short point list in the parameter spaces of both neighbouring faces. That sampling is what makes a preview possible without a geometry kernel: the face boundaries come ready-made, each face is ear-clipped in its parameter plane, mapped to 3D, welded with its neighbours and refined along the analytic normals (mesh/pn.rs, as for NX and SOLIDWORKS).

Status: src/mesh/creo/ reads planes, cylinders, cones, tori, bicubic splines, rolling-ball fillets and ruled surfaces. The NIST test part ctc-01 (Creo 10) gives a closed mesh whose bounding box is the STEP export’s to 0.05 mm; most other NIST parts are closed as well (see Verification and Gaps).

Container

Text header, then a table of contents in fixed-width text records:

#UGC:2 PART 2149 1860 800 1 1 15 4100 2023294 00000002cc \    last word: base offset (hex)
#- VERS … \  #- HOST … \  #- CMNM <name> \                    more header lines
#-END_OF_UGC_HEADER
#Creo  TM  10.0  (c) 2023 by PTC Inc. …
#UGC_TOC 2 32 81 17####…                                       32 entries of 81 bytes
<name> <off> <size> <usize> <ver> <kind> <x> <crc> <crc>      hex, '#'-padded
NEXT_TOC_ENTRY <off> …                                         chain to the next table

A section starts with the line #<name> at base + off and is size bytes long. The kind digit tells the storage: 9 plain tagged data, a/12 Unix compress (LZW, 1f 9d, 9–16 bit codes, no block mode in the files seen), 11 binary (JPEG previews, DIV-OL07BL graphics), b scrambled, c an LZW variant (only DispDataTable, not decoded). The inflated size is usize.

Sections that matter: VisibGeom (visible B-rep), NovisGeom (construction geometry, not shown), AllFeatur/FeatDefs (feature tree), MdlRefInfo (references – useful for assemblies later), THMB_IMG_MAIN (JPEG, not used as the thumbnail is rendered). SolidPrimdata, SolidPersistTable and CGFPersistTable contain tri-strips, but only for the PMI annotations (text, symbols, fonts), never for the body.

The tagged format

A section is a stream of values whose schema is written on first use. Field N is defined by e0 TT name 00 (type TT; in order of appearance), its value follows. The writer remembers, for every field, the transition that followed it last time – how many struct levels close and which field comes next – and writes only what differs:

Types: 00 struct, 01 05 07 08 09 0b integers, 02 double, 03/0a C string, 04 byte, 06 float32-precision number. Integers: 00–7f, 80–bf + 1 byte, c0–df + 2 bytes, e8–eb + 1–4 bytes, f5 <int> negated, f6 = −1.

Doubles are IEEE-754 big-endian values with a compressed top:

• one-byte constants: 0f 1, 10 −1, 11 π, 12 π/2, 13 2π, 14 3π/2, 15 −π, 16 −π/2, 18 0, 0d 0.25, 0e 0.5, 0c 0.1, 0b 0.06, 04 1−ε, 03 0.125 (05–0a unknown);
• near-constants 00/01/02 + constant token + 1/2/3 bytes that replace the lowest bytes of that constant (e.g. 00 14 d4 = 3π/2 + 2 ulp);
• group tokens 19–4a: ten groups of five, the group fixes the top byte (3c 3d 3e 3f 40 bc bd be bf c0), the sub-type the payload (7 bytes; 2 bytes + the last byte repeated; 2 bytes + zeros; 3 bytes; 6 bytes);
• class tokens 4b–df: the top 16 bits from a fixed table (tagged::double::class_top), then 6 mantissa bytes;
• escapes e8–ef with explicit bytes (ed = all eight).

With this, 99.9 % of the edge samples of the 17 NIST parts land on the same 3D point from both neighbouring faces (chord check, tolerance 1e-5).

The B-rep in VisibGeom

Sld_VisGeom / active_geom
  srf_array[]   geom_id, geom_type (34 plane, 36 cylinder, 37 cone, 38 torus, 40 spline,
                41 fillet, 42 ruled), orient (±1), envlp (uv box), outline (3D box),
                srf_prim_ptr(<kind>) { local_sys[4][3], radius | half_angle | radius1, radius2 | … },
                frst_cntr_ptr / next_cntr_ptr { frst_cntr_crv_hdr_ptr, trv }, topol_ref_data { list_id, list_type }
  crv_array[]   crv_id, type, crv_pnt_dir[2], crv_pnt_arr[n][4] = (u0 v0 u1 v1),
                crv_hdr_geom_ptr[0/1], next_crv_hdr_ptr[0/1]
  body_array[]  body_id, outline, first_geom_ptr, mat_name_ptr …

• local_sys: rows 0–2 are the rotation by columns (X, Y, Z), row 3 the origin. All 2 492 frames of the test parts are right-handed and orthonormal.
• Plane: O + u·X + v·Y. Cylinder: O + |r|(cos u·X + sin u·Y) + v·Z. Cone: radius v·tan(half_angle). Torus: O + (R1 + r cos v)(cos u·X + sin u·Y) + r sin v·Z. Which sign each parameter has, whether an angle starts at 0 or π, and which torus angle is u is not stored; it is resolved by walking from planes with the identity frame across shared edges and keeping the variant whose points coincide with the neighbour’s (brep::resolve).
• Edges: crv_pnt_arr holds the same points in both faces’ parameter spaces (row i of both sides is one 3D point); crv_pnt_dir gives the direction per side, next_crv_hdr_ptr the next edge of the contour on that side. The sampling is the display resolution (about 18° on circles).
• Contours: trv = 1 is an outer contour, trv = −1 a hole. One surface record may carry several regions (ctc-01: the four feet of the plate share one plane record, each with its own outer contour). A hole belongs to the region that contains its first point.
• Faces of the solid body have topol_ref_data.list_type = 98 (list_id = body id). Surfaces with list_id = −1 are loose or cosmetic and are not meshed.
• Twins: a periodic face is split in two halves of which only one has a record; the other is referenced by edges only (ctc-01: 212 beside 210). It shares the primitive of the known curved face behind a straight seam edge (type 1 or 8, constant u) and is rebuilt from the edges that reference it.
• Normals: triangles counter-clockwise in the parameter plane have the normal ∂u×∂v; orient = −1 turns it. The signed volume of the whole mesh decides the global sense.

Freeform faces

• splsrf (40): an interpolating bicubic C² spline (de Boor) through the grid i_points[iu·nv + iv] with knots u_params/v_params, end tangents end_u_tangts[e·nv + iv], end_v_tangts[e·nu + iu] and the corner twists end_uv_deriv[eu·2 + ev]; derivatives are with respect to the real parameters. Edge samples are hit to 8·10⁻⁴ of the face size; the rest is the spline’s own approximation of circular neighbours.
• fillet_srf (41): a rolling-ball blend. With s = par_v_0 + v (par_v_1 − par_v_0), centre C = ctr(s), radius vector A = pnt(s) − C and circle axis T = tan(s), the face is C + cos u · A + sin u · (T × A); the three curves are clamped C² splines (i_pnts, params, end_tangts). Exact to 10⁻⁷ on ctc-02.
• ruled_srf (42): C1(u) + v (C2(u) − C1(u)) with u normalised along both curves; a curve(conic) is the ellipse O + c1 cos t X + c2 sin t Y, t = t0 + u (t1 − t0), a curve(spline) a clamped spline.
• Freeform faces carry absolute 3D data and seed the convention resolution of the analytic faces (ftc-07 has no plane with the identity frame).
• Every edge is evaluated once on its most exact neighbour (plane before cylinder … before spline) and both faces use those vertices; edges whose sides disagree by more than 10⁻³ of the model size keep separate vertices, so real errors stay visible as open edges. Edge ends that meet in a contour are unified across the contours.

Verification

• 28 NIST parts (22 FTC/CTC in Creo 4 and Creo 10, 5 STC, 1 HTC); the FTC and CTC definitions come with STEP exports.
• ctc-01: 145 body faces (8 twins), 151 regions, closed (0 open edges), bounding box equal to the STEP export, volume 14.7·10⁶ mm³; the PN refinement keeps the mesh closed.
• Closed as well (prototype): ctc-04, ctc-05, ftc-06, ftc-08, ftc-10, ftc-11; a handful of open edges on ctc-03 (4), ftc-07 (8), ftc-09 (4); ctc-02 keeps about 1 200 open edges on planes with twin halves. Bounding boxes equal to STEP for ctc-01–ctc-04, ftc-06, ftc-09; ctc-05, ftc-07 and ftc-08 match Creo’s own body outline rather than the STEP definition.
• Prototype and notes: cad-decoding-notes/creo.md and creo-prototypes/ next to the repository (not in it).

Gaps

• The mesh has vertices only on the face boundaries: freeform faces show as facets between their edge samples until interior sample points are added. The twist layout of end_uv_deriv is plausible but only affects the interior and could not be checked against edges.
• Model units: read from UnitSystemDef since 2026-11-12 and converted to millimetres (3D preview of Creo assemblies); the NIST part ctc-05 is stored in inches, ctc-01 in mm.
• Open edges: the prototype leaves a few open edges on some parts (4–8 on ctc-03, ftc-07, ftc-09) and about 1 200 on ctc-02, where planes are split into twin halves; faces whose stored record is wrong or missing are not rebuilt (the prototype has a least-squares primitive fit for them). Only ctc-01 is a repository fixture and checked by the Rust tests.
• Assemblies (.asm.N): no freely licensed test file was found (NIST has no Creo assembly; the GitHub finds carry no licence). Component names are in MdlRefInfo; placements have not been looked at.
• Drawings and other Creo files keep the stored JPEG.

Licences and prior art

The layout was worked out from the files alone, which the EU software directive (2009/24/EC, art. 6) and § 69e UrhG allow for interoperability. No open implementation of the Creo binary format was found, nothing was copied. “Creo” is PTC’s trade mark and is only used to name the format. Only the openly licensed NIST files are committed as fixtures.