diff --git a/ROADMAP.md b/ROADMAP.md
index 8f4b307..2c0061e 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -7,8 +7,8 @@ LS-PrePost, with hexahedral meshing as the centre of gravity, plus its own linea
 |---|---|---|
 | M0 Foundation | Workspace, quality gates (`cargo xtask ci`), FE model, command engine with rollback and panic guard, CLI, geometry boundary on SolidCraft's kernel and tessellator | done |
 | M1 Surface preparation | Robust adaptive-precision predicates; weld, closed/manifold/orientation checks and feature edges on the tessellated boundary; mesh quality metrics | next |
-| M2 Tetrahedral meshing | 3D Delaunay (Bowyer–Watson), boundary recovery, Delaunay refinement to a size field, Voronoi-based optimization (CVT/ODT), sliver removal; linear and quadratic tets | planned |
-| M3 Hexahedral meshing (core) | Method from the literature survey: a hex mesher whose output is always valid, then quality-driven methods; hex-dominant fallback | planned |
+| M2 Tetrahedral meshing | Bowyer–Watson Delaunay, robust constrained Delaunay boundary recovery (Diazzi et al. 2023), Delaunay refinement to a size field, ODT smoothing, Klingner–Shewchuk sliver removal; linear and quadratic tets | planned |
+| M3 Hexahedral meshing (core) | Octree/grid dual all-hex (valid on any input), then hex-dominant by tet recombination, then polycube for blocky parts; see docs/meshing/literature.md | planned |
 | M4 Model definition | Materials, sections, sets and surfaces, boundary conditions, loads, steps, output requests | planned |
 | M5 Solver decks | Abaqus input, Nastran bulk data, LS-DYNA keyword: writers first, then readers | planned |
 | M6 Linear solver | Element library, sparse assembly, linear static and modal analysis, verified against analytic solutions | planned |
diff --git a/docs/adr/0001-libraries.md b/docs/adr/0001-libraries.md
index d2d2ad4..7a4fda7 100644
--- a/docs/adr/0001-libraries.md
+++ b/docs/adr/0001-libraries.md
@@ -39,6 +39,26 @@ CC0 and public domain.
 | Elmer | GPL-2.0 / LGPL-2.1 | multiphysics solver |
 | deal.II, FreeFEM, Salome | LGPL | FE libraries, pre-processing |
 | OpenCASCADE | LGPL-2.1 | B-rep kernel |
+| Attene's CDT, Indirect_Predicates, VolumeMesher | GPL / LGPL | robust CDT, indirect predicates (implement from the papers) |
+| TetWild (original), Evocube, robustPolycube, ultimaille, invertible-maps | GPL-3.0 / AGPL-3.0 | tet meshing, polycube, injective maps |
+| libHexEx, libHexHex, QGP3D, MC3D, CoMISo, Graphite | GPL | integer-grid-map hex extraction, quantization |
+| Shewchuk's Triangle | non-commercial only | 2D meshing |
+| geogram `third_party/tetgen`, `third_party/triangle` and their wrappers | AGPL / non-commercial | (the rest of geogram is BSD-3 and readable) |
+| libigl `copyleft/` modules | GPL | (libigl core is MPL-2.0 and readable) |
+| OpenVolumeMesh, CGoGN | believed LGPL, unverified | treat as forbidden until checked |
+
+**Repositories without a licence are all rights reserved: never read them** (for example Gao
+2019's octree hex code, LoopyCuts). Readable references for the meshing work (re-implement from
+the paper, note anything adapted in ATTRIBUTION.md): Shewchuk's `predicates.c` (public domain),
+`georust/robust` (Apache-2.0), geogram (BSD-3, exclusions above), fTetWild (MPL-2.0), CinoLib,
+Gen-Adapt-Ref, HexaLab, robust_hex_dominant_meshing (MIT), quartet (ISC). The full table with
+sources is in `docs/meshing/literature.md`.
+
+**Test geometry** follows the same rule as test data: nothing produced with a commercial CAD or
+CAE product. That excludes ABC (Onshape), MAMBO (files from commercial translators) and the HexMe
+models that come from ABC, GrabCAD or Siemens NX; HexMe's Gmsh-made models are allowed (CC-BY-4.0).
+Thingi10K (per-model Creative Commons licences, authoring tool unrecorded) is pending an owner
+decision. The default corpus is generated: SolidCraft-built parts and analytic shapes.
 
 **Commercial products** (Abaqus, Nastran, LS-DYNA, LS-PrePost, HyperMesh, ANSA, Femap, Patran,
 ANSYS) are never run, scripted or used as a source of data. Deck formats come from the vendors'
diff --git a/docs/architecture.md b/docs/architecture.md
index 579cd5d..2095559 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -43,12 +43,18 @@ Rules beyond the layer order:
    so feature edges and face-based loads and BCs survive meshing.
 2. **Surface preparation.** Weld coincident vertices, check the surface is closed, manifold and
    consistently oriented, find feature edges (face boundaries, sharp dihedral angles).
-3. **Tetrahedral meshing.** 3D Delaunay tetrahedralization of the boundary points with exact
-   predicates, boundary recovery, Delaunay refinement to a size field, then Voronoi-based
-   optimization (centroidal Voronoi / optimal Delaunay) and sliver removal.
-4. **Hexahedral meshing (the core).** All-hex first where it can be guaranteed valid, with
-   hex-dominant as the fallback; the method is chosen from the literature survey
-   (`docs/meshing/literature.md`).
+3. **Tetrahedral meshing.** Bowyer–Watson Delaunay on Shewchuk's adaptive predicates (symbolic
+   perturbation for degeneracies), boundary recovery by the robust constrained Delaunay method of
+   Diazzi et al. 2023 (implicit Steiner points on indirect predicates, gift-wrapping fallback), so
+   every boundary triangle and its B-rep face tag survives. Then constrained Delaunay refinement to a
+   size field, ODT smoothing interleaved as in Tournois et al. 2009, and the Klingner–Shewchuk 2007
+   improvement schedule against slivers.
+4. **Hexahedral meshing (the core).** Octree/grid dual all-hex first, the one family that meshes
+   any input validly (Livesu et al. 2021 dual schemes, Pitzalis et al. 2021 refinement, Gao et al.
+   2019 feature projection and padding, Garanzha et al. 2021 untangling). Then hex-dominant by tet
+   recombination (Pellerin et al. 2018), then polycube (DualCube labelling) for blocky parts;
+   frame-field/integer-grid-map methods stay research. Validity uses a certified trilinear Jacobian
+   check (Marschner et al. 2020), not corner Jacobians alone.
 5. **Quality.** Scaled Jacobian, aspect ratio, dihedral angles; every generated mesh is checked
    for positive Jacobians and conserved volume before it reaches the model.
 
diff --git a/docs/meshing/literature.md b/docs/meshing/literature.md
new file mode 100644
index 0000000..ffbb5f4
--- /dev/null
+++ b/docs/meshing/literature.md
@@ -0,0 +1,906 @@
+# Volume meshing literature survey for ModelCraft (tet → hex)
+
+Compiled 2026-10-13. Scope: watertight triangulated B-rep tessellation in, then (1) tetrahedral mesh by
+Delaunay methods with Voronoi/ODT improvement, then (2) all-hex and hex-dominant meshes.
+
+## How this was verified
+
+- **DOIs.** Every DOI below resolved through the DOI handle API (`doi.org/api/handles/<doi>` → responseCode 1).
+  Each title, first author, year and venue was also checked against Crossref metadata. One wrong DOI was
+  caught and fixed: `10.1145/276884.276889` is Amenta's *Voronoi filtering*, and Shewchuk's SoCG'98 paper is
+  `.276894`.
+- **Other URLs** (author PDFs, arXiv, project pages, repos) returned HTTP 200 on 2026-10-13. Two exceptions:
+  `cs.nyu.edu/~yixinhu/tetwild.pdf` (403) and `zenodo.org/record/6642020` (403 to a scripted client, though the
+  Zenodo REST API returned the record). Neither is used as a primary link; I cite the DOI instead.
+- **Section numbers** were read from the PDFs for: Shewchuk 1997, 1998, 2002; Klingner–Shewchuk; Labelle–Shewchuk;
+  Diazzi 2021 and 2023; fTetWild; Attene 2020; Livesu 2021; Garanzha 2021; Pietroni survey; HexMe; DualCube. For any
+  other paper I give no section number rather than guess one.
+- **Licences** come from GitHub/GitLab licence metadata, LICENSE files or README licence statements. I read no
+  source code. "none" means the repo has no licence, which makes it all-rights-reserved: treat it as unreadable
+  for a clean-room project, same as GPL.
+
+### Licence legend for associated code
+
+| Status | Meaning |
+|---|---|
+| **READ OK** | Permissive (MIT, BSD, Apache-2.0, ISC, MPL-2.0 file-level, public domain). May be read for reference. Still re-implement from the paper. |
+| **FORBIDDEN** | GPL, LGPL or AGPL. Do not open. |
+| **NO LICENCE** | All rights reserved. Do not open. |
+
+---
+
+## Part A: Tetrahedral meshing
+
+### A0. Robust predicates and exactness (foundation for everything)
+
+**A0.1 Shewchuk 1997, adaptive-precision predicates.** **Verdict: build on.**
+- **Citation.** J. R. Shewchuk. *Adaptive Precision Floating-Point Arithmetic and Fast Robust Geometric Predicates.* Discrete & Computational Geometry 18(3):305–363, 1997.
+- **Links.** DOI [10.1007/PL00009321](https://doi.org/10.1007/PL00009321). PDF: https://people.eecs.berkeley.edu/~jrs/papers/robustr.pdf
+- **Method.** Floating-point *expansions* (non-overlapping sums of doubles) with error-free transforms:
+  - Two-Sum and Fast-Two-Sum (§2.3).
+  - Expansion-Sum / Fast-Expansion-Sum (§2.4).
+  - Two-Product with Dekker splitting (§2.5).
+  - Scale-Expansion (§2.6).
+  - Compression (§2.7).
+  - Adaptive evaluation (§3): compute the determinant in plain doubles with a certified error bound. Escalate through
+    increasingly exact stages only when the sign is uncertain.
+- **Predicates.** `orient2d`, `orient3d`, `incircle` and `insphere`, each with stage-wise error bounds (§4.2).
+  §5 lists caveats: overflow, underflow, and the assumption of round-to-even IEEE arithmetic.
+- **Problem solved.** Exact sign of the core Delaunay predicates at near-double speed.
+- **Robustness.** Exact for inputs in double precision. It does not handle constructed points such as
+  intersections or Steiner points; that is what A0.3 is for. It does not resolve true degeneracies (cospherical
+  points); that is what A0.2 is for.
+- **Complexity.** Low to moderate: about 1–2k lines. The error-bound constants must be derived exactly as in the paper.
+- **Code.**
+  - `predicates.c` at https://www.cs.cmu.edu/~quake/robust.html is **public domain** (the page says so explicitly):
+    READ OK. The same page notes that *Triangle* is copyrighted and not public domain; don't read it.
+  - Rust port `georust/robust` (Apache-2.0, https://github.com/georust/robust): READ OK. Useful as a differential
+    test oracle.
+- **Why.** Every downstream algorithm (Bowyer–Watson, CDT, refinement, octree/surface classification) assumes
+  exact `orient3d` and `insphere`.
+
+**A0.2 Symbolic perturbation (Simulation of Simplicity).** **Verdict: build on.**
+- **Citations.**
+  - H. Edelsbrunner, E. P. Mücke. *Simulation of Simplicity: a technique to cope with degenerate cases in geometric
+    algorithms.* ACM TOG 9(1):66–104, 1990. DOI [10.1145/77635.77639](https://doi.org/10.1145/77635.77639).
+  - O. Devillers, M. Teillaud. *Perturbations for Delaunay and weighted Delaunay 3D triangulations.* CGTA 44(3):160–168,
+    2011. DOI [10.1016/j.comgeo.2010.09.010](https://doi.org/10.1016/j.comgeo.2010.09.010).
+- **Method.** When `insphere` is exactly zero, break the tie with a consistent symbolic perturbation. Devillers–Teillaud
+  give a perturbation for `insphere` that needs only `orient3d` evaluations on sorted point indices. That is much
+  simpler than general SoS.
+- **Problem solved.** Cospherical and coplanar inputs, which are ubiquitous in CAD tessellations (grids, cylinders,
+  regular polygons). The Delaunay triangulation stays unique and the Bowyer–Watson cavity stays a star-shaped ball.
+- **Complexity.** Low, given A0.1.
+- **Code.** No code was read.
+
+**A0.3 Attene 2020, indirect predicates.** **Verdict: build on (from the paper only).**
+- **Citation.** M. Attene. *Indirect Predicates for Geometric Constructions.* Computer-Aided Design 126:102856, 2020.
+- **Links.** DOI [10.1016/j.cad.2020.102856](https://doi.org/10.1016/j.cad.2020.102856). arXiv: https://arxiv.org/abs/2105.09772
+- **Method.** Represent constructed points implicitly, never rounding them:
+  - LPI: line–plane intersection.
+  - TPI: three-plane intersection.
+  - LNC: point on a segment.
+- **Evaluation.** Predicates on implicit points are expanded symbolically into homogeneous polynomials. They are then
+  evaluated with a three-level filter (§5):
+  - static/semi-static floating-point filter (§5.1);
+  - interval arithmetic (§5.2);
+  - exact expansions (§5.3).
+- The paper also covers caching (§5.4) and generating predicate code automatically from a description (§6).
+- **Problem solved.** Exact predicates on Steiner and intersection points without rational or bignum arithmetic.
+  This is the enabling trick of A2.3 and A2.4.
+- **Robustness.** Exact. The cost grows with the "depth" of the construction, so the method is designed for one
+  level of implicit points defined by explicit inputs.
+- **Complexity.** Moderate: about 3–5k lines, mostly generated. Write a small Rust code generator, as the paper does.
+- **Code.** `MarcoAttene/Indirect_Predicates` is **LGPL-2.1**: FORBIDDEN.
+- **Reference alternative.** Lévy's PCK (next entry) is BSD.
+
+**A0.4 Lévy 2016, Predicate Construction Kit (PCK).** **Verdict: reference.**
+- **Citation.** B. Lévy. *Robustness and efficiency of geometric programs: The Predicate Construction Kit (PCK).*
+  Computer-Aided Design 72:3–12, 2016. DOI [10.1016/j.cad.2015.10.004](https://doi.org/10.1016/j.cad.2015.10.004).
+- **Method.** A code generator from a predicate formula to:
+  - a semi-static filter;
+  - an exact expansion fallback;
+  - SoS for degenerate cases.
+- It is used in geogram's Delaunay and restricted-Voronoi code.
+- **Code.** `BrunoLevy/geogram` is **BSD-3-Clause** (LICENSE file): READ OK, with one caveat. geogram vendors
+  `third_party/tetgen` (AGPL) and `third_party/triangle` (non-free), and has wrappers `delaunay/delaunay_tetgen.*` and
+  `delaunay/delaunay_triangle.*`. **Never open those.** Geogram's own `delaunay_3d.cpp`, `parallel_delaunay_3d.cpp`,
+  `voronoi/RVD*.cpp`, `CVT.cpp` and its numerics/PCK files are BSD.
+- **Verdict reason.** It is the best permissively licensed reference for a production Delaunay + RVD + predicate stack.
+
+---
+
+### A1. Delaunay construction
+
+**A1.1 Bowyer 1981 / Watson 1981, incremental cavity insertion.** **Verdict: build on.**
+- **Citations.**
+  - A. Bowyer. *Computing Dirichlet tessellations.* The Computer Journal 24(2):162–166, 1981.
+    DOI [10.1093/comjnl/24.2.162](https://doi.org/10.1093/comjnl/24.2.162).
+  - D. F. Watson. *Computing the n-dimensional Delaunay tessellation with application to Voronoi polytopes.*
+    The Computer Journal 24(2):167–172, 1981. DOI [10.1093/comjnl/24.2.167](https://doi.org/10.1093/comjnl/24.2.167).
+- **Method.** Insert a point, find all tetrahedra whose circumsphere contains it (the cavity), delete them, and star the
+  cavity boundary to the new point.
+- **Robustness.** With exact `insphere`/`orient3d` (A0.1) and SoS (A0.2) the cavity is always a star-shaped ball and
+  the method never fails. With floating-point predicates it corrupts the mesh on degenerate CAD input.
+- **Complexity.** Low for the core. Practical speed needs:
+  - BRIO insertion order: N. Amenta, S. Choi, G. Rote, *Incremental constructions con BRIO*, SoCG 2003, 211–219,
+    DOI [10.1145/777792.777824](https://doi.org/10.1145/777792.777824). Use it together with Hilbert/Morton sorting.
+  - Visibility or straight walking for point location: O. Devillers, S. Pion, M. Teillaud, *Walking in a
+    triangulation*, IJFCS 13(2):181–199, 2002, DOI [10.1142/S0129054102001047](https://doi.org/10.1142/S0129054102001047).
+  - A compact tet-adjacency array.
+- **Code.** For a permissive reference, see geogram `delaunay_3d.cpp` (BSD).
+
+**A1.2 Marot, Pellerin, Remacle 2019, "One machine, one minute, three billion tetrahedra".** **Verdict: reference (later).**
+- **Citation.** IJNME 117(9):967–990, 2019. DOI [10.1002/nme.5987](https://doi.org/10.1002/nme.5987).
+- **Method.** Parallel Delaunay kernel: domain partitioning on a space-filling curve, lock-free cavity work,
+  cache-friendly data layout.
+- **Use.** Only once single-threaded correctness is nailed down.
+- **Code.** HXT lives inside Gmsh (GPL): FORBIDDEN.
+
+---
+
+### A2. Boundary recovery: constrained or conforming Delaunay
+
+**A2.1 Shewchuk 2002, CDTs and provably good boundary recovery.** **Verdict: build on (theory).**
+- **Citation.** J. R. Shewchuk. *Constrained Delaunay Tetrahedralizations and Provably Good Boundary Recovery.*
+  Proc. 11th International Meshing Roundtable, 2002.
+- **Links.** No DOI exists. PDF: https://people.eecs.berkeley.edu/~jrs/papers/cdtbasic.pdf
+- **Method.** A PLC does not always have a CDT. If every segment is *strongly Delaunay*, a CDT exists. The paper
+  protects edges by splitting them (§3 Edge Protection, §5 Provably Good Edge Protection) until this holds. Then it
+  builds the CDT (§6), for example by incremental facet insertion (§7).
+- **Problem solved.** The existence theory behind every practical CDT code.
+- **Robustness.** Proven termination and existence. The provable protection inserts more Steiner points than needed.
+- **Complexity.** Medium.
+- **Companion papers.**
+  - Shewchuk, *General-dimensional constrained Delaunay and constrained regular triangulations I*, DCG 39:580–637,
+    2008. DOI [10.1007/s00454-008-9060-3](https://doi.org/10.1007/s00454-008-9060-3).
+    PDF: https://people.eecs.berkeley.edu/~jrs/papers/cdtj1.pdf
+  - Shewchuk, *Updating and constructing CDTs by flips* (SoCG 2003). PDF: https://people.eecs.berkeley.edu/~jrs/papers/cdtflip.pdf
+
+**A2.2 Si and Gärtner 2005; Si 2010; Si 2015; Shewchuk and Si 2014.** **Verdict: reference (papers only; TetGen is AGPL).**
+- **Citations.**
+  - H. Si, K. Gärtner. *Meshing Piecewise Linear Complexes by Constrained Delaunay Tetrahedralizations.* IMR 14,
+    147–163, 2005. DOI [10.1007/3-540-29090-7_9](https://doi.org/10.1007/3-540-29090-7_9).
+  - H. Si. *Constrained Delaunay tetrahedral mesh generation and refinement.* Finite Elements in Analysis and Design
+    46(1–2):33–46, 2010. DOI [10.1016/j.finel.2009.06.017](https://doi.org/10.1016/j.finel.2009.06.017).
+  - H. Si. *TetGen, a Delaunay-based quality tetrahedral mesh generator.* ACM TOMS 41(2):11, 2015.
+    DOI [10.1145/2629697](https://doi.org/10.1145/2629697).
+  - J. R. Shewchuk, H. Si. *Incrementally constructing and updating constrained Delaunay tetrahedralizations with
+    finite-precision coordinates.* Engineering with Computers 30:253–269, 2014.
+    DOI [10.1007/s00366-013-0331-0](https://doi.org/10.1007/s00366-013-0331-0).
+    PDF (IMR'12 version): https://people.eecs.berkeley.edu/~jrs/papers/cdtupdate.pdf
+- **Method.**
+  1. Delaunay tetrahedralization of the vertices.
+  2. Segment recovery, with Steiner points placed using a protecting-ball rule that needs far fewer points than
+     Shewchuk 2002.
+  3. Face recovery by cavity retriangulation.
+  4. Optional quality refinement (radius–edge bound) under the CDT.
+- **Limitations.**
+  - Steiner points are rounded to doubles, which needs tolerances and can fail.
+  - Diazzi et al. 2023 §3.5 ("Possible failures in tetgen") also show a **theoretical** gap: cavity expansion can fail
+    even in exact arithmetic (their Fig. 4).
+- **Code.** TetGen is **AGPL**: FORBIDDEN. Read the papers only.
+
+**A2.3 Diazzi, Panozzo, Vaxman, Attene 2023, robust CDT.** **Verdict: BUILD ON. This is the recommended boundary-recovery algorithm.**
+- **Citation.** L. Diazzi, D. Panozzo, A. Vaxman, M. Attene. *Constrained Delaunay Tetrahedrization: A Robust and
+  Practical Approach.* ACM TOG 42(6):181 (SIGGRAPH Asia), 2023.
+- **Links.** DOI [10.1145/3618352](https://doi.org/10.1145/3618352). arXiv: https://arxiv.org/abs/2309.09805
+- **Method.** Si–Gärtner's pipeline with every Steiner point represented **implicitly** with indirect predicates (A0.3),
+  so no geometric tolerance is ever needed.
+  - **Background, §3:**
+    - §3.1 problem statement;
+    - §3.2 PLC characteristics;
+    - §3.3 segment recovery;
+    - §3.4 face recovery;
+    - §3.5 TetGen failure modes.
+  - **Algorithm, §4:**
+    - §4.1 why exact rational numbers are not enough;
+    - §4.2 rational Steiner points;
+    - §4.3 implicit Steiner points (§4.3.1 indirect `orient3d`/`inSphere`);
+    - §4.4 a modified gift-wrapping algorithm that replaces Si's cavity expansion where that can fail;
+    - §4.5 interior/exterior labelling;
+    - §4.6 the full algorithm.
+  - **Rounding, §5.1:** output can be rounded to floating-point coordinates afterwards.
+- **Results.** **100% success on the 4,408 valid Thingi10K models.** The output conforms exactly to the input. It is
+  parameter-free.
+- **Problem solved.** Exact boundary conformity, which CAE needs for face groups, contact pairs, shared interfaces
+  between parts and boundary-condition tagging, together with a termination guarantee.
+- **Limitations.**
+  - The input must be a valid PLC (no self-intersections). Fine for a B-rep tessellator that guarantees
+    watertight, non-self-intersecting output. Otherwise run A2.4 or A2.5 first.
+  - Rounding to doubles "almost always" stays valid but is not guaranteed. Keep implicit points through the quality
+    phase, or round with a validity check.
+  - Steiner counts can be large on nasty inputs. The paper notes about 9% of models needing over 50k.
+- **Complexity.** High: about 8–12k lines of Rust including the predicate generator. It is the single most valuable
+  investment in the tet pipeline.
+- **Code.** `MarcoAttene/CDT` is **GPL, or LGPL by build option** (README): FORBIDDEN.
+
+**A2.4 Diazzi and Attene 2021, convex polyhedral meshing.** **Verdict: reference.**
+- **Citation.** L. Diazzi, M. Attene. *Convex polyhedral meshing for robust solid modeling.* ACM TOG 40(6):259, 2021.
+- **Links.** DOI [10.1145/3478513.3480564](https://doi.org/10.1145/3478513.3480564). arXiv: https://arxiv.org/abs/2109.14434
+- **Method.**
+  1. Delaunay of the vertices.
+  2. Map tets to the constraints they intersect (§3.3).
+  3. Binary space partition of the tets by constraint planes, with implicit points (§3.4).
+  4. Virtual constraints (§3.5).
+  5. Identify constrained facets (§3.6).
+  6. Partition cells into inside/outside (§3.7).
+- **Output.** A convex polyhedral mesh that conforms exactly even to **self-intersecting** input. It can be
+  tetrahedralized afterwards.
+- **Use for us.** A repair path for dirty B-reps: overlapping faces, booleans, imported STEP/STL with intersections.
+- **Code.** `MarcoAttene/VolumeMesher` is **GPL-3**: FORBIDDEN.
+
+**A2.5 Hu et al. 2018, TetWild; Hu et al. 2020, fTetWild (envelope approach).** **Verdict: reference; use as a fallback design for dirty input.**
+- **Citations.**
+  - Y. Hu, Q. Zhou, X. Gao, A. Jacobson, D. Zorin, D. Panozzo. *Tetrahedral Meshing in the Wild.* ACM TOG 37(4):60,
+    2018. DOI [10.1145/3197517.3201353](https://doi.org/10.1145/3197517.3201353).
+  - Y. Hu, T. Schneider, B. Wang, D. Zorin, D. Panozzo. *Fast Tetrahedral Meshing in the Wild.* ACM TOG 39(4):117,
+    2020. DOI [10.1145/3386569.3392385](https://doi.org/10.1145/3386569.3392385). arXiv: https://arxiv.org/abs/1908.03581
+- **Method.** The input surface may move within an ε-envelope:
+  - §3.2 envelope test;
+  - §3.3 preprocessing;
+  - §3.4 incremental triangle insertion into a floating-point tet mesh. Each insertion is rejected if it would
+    create an inverted tet;
+  - §3.5 AMIPS-driven split/collapse/swap/smooth improvement that never leaves the envelope.
+- **Results.** fTetWild guarantees a **valid floating-point** output (TetWild produced rational coordinates that may
+  invert after rounding). It meshes 98.7% of Thingi10K in under 2 minutes.
+- **Limitations.**
+  - fTetWild does **not** guarantee every input triangle is in the output.
+  - The output surface only approximates the input. Sharp features and face IDs must be re-projected.
+  - That is acceptable for "meshes in the wild" but awkward for CAE, where face groups must match B-rep faces exactly.
+- **Complexity.** High.
+- **Code.**
+  - `wildmeshing/fTetWild` is **MPL-2.0**: READ OK (its own `src/`; it pulls in geogram and libigl).
+  - Original `Yixin-Hu/TetWild` is **GPL-3.0** per GitHub metadata: FORBIDDEN.
+  - `wildmeshing/wildmeshing-toolkit` is MIT: READ OK.
+
+**A2.6 Cherchi, Livesu, Scateni, Attene 2020, mesh arrangements.** **Verdict: reference (needed only for self-intersecting input).**
+- **Citation.** *Fast and robust mesh arrangements using floating-point arithmetic.* ACM TOG 39(6):250, 2020.
+  DOI [10.1145/3414685.3417818](https://doi.org/10.1145/3414685.3417818).
+- **Method.** Resolves all self-intersections of a triangle soup with indirect predicates.
+- **Code.** `mlivesu/InteractiveAndRobustMeshBooleans` is MIT per GitHub, a related later work. **Caution:** check
+  whether it bundles Attene's LGPL predicate headers before opening any file.
+
+---
+
+### A3. Quality: Delaunay refinement, ODT/CVT, sliver removal
+
+**A3.1 Shewchuk 1998, tetrahedral Delaunay refinement; thesis 1997.** **Verdict: build on.**
+- **Citation.** J. R. Shewchuk. *Tetrahedral Mesh Generation by Delaunay Refinement.* SoCG 1998, 86–95.
+- **Links.**
+  - DOI [10.1145/276884.276894](https://doi.org/10.1145/276884.276894).
+  - PDF: https://people.eecs.berkeley.edu/~jrs/papers/delref3d.pdf
+  - Thesis: https://www.cs.cmu.edu/~quake-papers/delaunay-refinement.pdf
+- **Method.** Insert the circumcentres of tets whose radius–edge ratio exceeds B. Encroached subsegments are split
+  first, then encroached subfacets.
+- **Guarantee.** Termination with B = 2 when input angles are at least 90°.
+- **Limitations.** It bounds radius–edge ratio, not dihedral angles. **Slivers survive**, so pair it with A3.3–A3.6.
+  Small input angles, common in CAD, need the CDT-based variants: Si 2010 (A2.2), and Shewchuk & Si, SoCG 2014,
+  DOI [10.1145/2582112.2582138](https://doi.org/10.1145/2582112.2582138).
+- **Complexity.** Medium, on top of A2.3.
+- **Code.** None read.
+
+**A3.2 Cheng, Dey, Shewchuk, *Delaunay Mesh Generation* (CRC, 2012).** **Verdict: reference (the textbook).**
+- **Links.** DOI [10.1201/b12987](https://doi.org/10.1201/b12987).
+- **Use.** The consolidated treatment of 3D refinement, small-angle handling and sliver theory. Buy it.
+
+**A3.3 Alliez, Cohen-Steiner, Yvinec, Desbrun 2005, Variational Tetrahedral Meshing.** **Verdict: build on (ODT smoothing).**
+- **Citation.** ACM TOG 24(3):617–625, 2005. DOI [10.1145/1073204.1073238](https://doi.org/10.1145/1073204.1073238).
+- **Method.** Minimize the ODT energy: the L1 error between the paraboloid and its piecewise-linear interpolant.
+  Alternate between:
+  - moving each interior vertex to the volume-weighted average of its incident tets' circumcentres (closed form);
+  - re-Delaunaying the connectivity.
+- **Findings.** ODT outperforms CVT for tet quality. Results improve with a sizing field.
+- **Limitations.**
+  - Boundary vertices need special handling, and the paper's boundary treatment is the weak point.
+  - Slivers remain at the end. The paper uses a final perturbation/exudation pass.
+- **Complexity.** Low to moderate, given a Delaunay kernel.
+
+**A3.4 Chen and Xu 2004; Chen and Holst 2011, Optimal Delaunay Triangulations.** **Verdict: build on (theory and update formulas).**
+- **Citations.**
+  - L. Chen, J. Xu. *Optimal Delaunay triangulations.* J. Computational Mathematics 22(2):299–308, 2004. No DOI.
+    Publisher page: https://ojs.global-sci.org/index.php/JCM/article/view/11631. Author page with PDF link:
+    https://www.math.uci.edu/~chenlong/CX2004.html
+  - L. Chen, M. Holst. *Efficient mesh optimization schemes based on Optimal Delaunay Triangulations.* CMAME
+    200(9–12):967–984, 2011. DOI [10.1016/j.cma.2010.11.007](https://doi.org/10.1016/j.cma.2010.11.007).
+- **Method.** The Delaunay triangulation minimizes the interpolation error of ‖x‖² over all triangulations of a point
+  set. Chen–Holst give the exact and approximate vertex-update rules (ODT smoothing), including a boundary-aware variant.
+- **Complexity.** Low.
+
+**A3.5 Tournois, Wormser, Alliez, Desbrun 2009, Delaunay refinement interleaved with optimization.** **Verdict: build on (the overall schedule).**
+- **Citation.** *Interleaving Delaunay Refinement and Optimization for Practical Isotropic Tetrahedron Mesh
+  Generation.* ACM TOG 28(3):75, 2009. DOI [10.1145/1531326.1531381](https://doi.org/10.1145/1531326.1531381).
+- **Method.** Alternate Delaunay refinement and ODT/NODT smoothing. The refinement uses the radius–edge and boundary
+  criteria. NODT is a Natural ODT variant for boundary vertices. Finish with sliver perturbation and exudation.
+- **Results.** Minimum dihedral angles are substantially better than refinement alone.
+- **Problem solved.** The practical recipe used by CGAL Mesh_3-style generators.
+- **Complexity.** Moderate.
+- **Code.** CGAL is GPL: FORBIDDEN. This paper is the readable source of the scheme.
+
+**A3.6 Klingner and Shewchuk 2007, Aggressive Tetrahedral Mesh Improvement.** **Verdict: BUILD ON (final sliver-removal stage).**
+- **Citation.** B. M. Klingner, J. R. Shewchuk. IMR 16, 3–23, 2007.
+- **Links.** DOI [10.1007/978-3-540-75103-8_1](https://doi.org/10.1007/978-3-540-75103-8_1).
+  PDF: https://people.eecs.berkeley.edu/~jrs/papers/aggress.pdf
+- **Method.**
+  - §2: quality measures. Use volume–length, or the minimum sine of dihedral angles.
+  - §3, operations:
+    - §3.1 optimization-based smoothing with an active-set method on the worst quality;
+    - §3.2 edge removal;
+    - §3.3 multi-face removal;
+    - §3.4 vertex insertion;
+    - §3.5 composites.
+  - §4.2: hill-climbing schedule that accepts only operations improving the local minimum quality.
+- **Results.** Dihedral angles are usually between **31° and 149°**, or 23°–136° with another objective.
+- **Problem solved.** Kills slivers that ODT and refinement leave behind. Works on any valid tet mesh.
+- **Robustness.** Monotone in worst quality, so it never makes the mesh worse.
+- **Limitations.** Slow. Boundary vertices are constrained.
+- **Complexity.** Moderate: edge removal requires Shewchuk's dynamic programming over the edge ring.
+  PDF: https://people.eecs.berkeley.edu/~jrs/papers/edge.pdf, not separately verified.
+- **Code.** The Stellar code's licence was not verified, so it is not read.
+
+**A3.7 Cheng, Dey, Edelsbrunner, Facello, Teng 2000, Sliver exudation.** **Verdict: reference.**
+- **Citation.** J. ACM 47(5):883–904, 2000. DOI [10.1145/355483.355487](https://doi.org/10.1145/355483.355487).
+- **Method.** Assign weights to vertices so the weighted Delaunay triangulation contains no slivers. This gives a
+  provable but tiny dihedral bound.
+- **Use.** Klingner–Shewchuk dominates it in practice. Useful as a cheap pass inside the A3.5 schedule.
+
+**A3.8 Labelle and Shewchuk 2007, Isosurface stuffing.** **Verdict: reference (useful only for organic, smooth domains).**
+- **Citation.** ACM TOG 26(3):57, 2007.
+- **Links.** DOI [10.1145/1276377.1276448](https://doi.org/10.1145/1276377.1276448).
+  PDF: https://people.eecs.berkeley.edu/~jrs/papers/stuffing.pdf
+- **Method.** BCC lattice plus snapping and warping with stencils (§3). It has **guaranteed** dihedral bounds (§4).
+- **Limitations.** It does **not preserve sharp features**, so it is wrong for CAD.
+- **Code.** `crawforddoran/quartet` is ISC: READ OK. Do not read `alecjacobson/isostuffer` (LGPL-3).
+
+---
+
+### A4. Voronoi-based: CVT, restricted Voronoi diagrams, Lp-CVT
+
+**A4.1 Du, Faber, Gunzburger 1999; Du and Wang 2003.** **Verdict: reference.**
+- **Citations.**
+  - *Centroidal Voronoi Tessellations: Applications and Algorithms.* SIAM Review 41(4):637–676, 1999.
+    DOI [10.1137/S0036144599352836](https://doi.org/10.1137/S0036144599352836).
+  - Q. Du, D. Wang. *Tetrahedral mesh generation and optimization based on centroidal Voronoi tessellations.*
+    IJNME 56(9):1355–1373, 2003. DOI [10.1002/nme.616](https://doi.org/10.1002/nme.616).
+- **Method.** Lloyd relaxation toward a CVT. The dual Delaunay triangulation gives well-shaped tets apart from slivers.
+- **Assessment.** Alliez 2005 shows ODT beats CVT for tet quality, so CVT is not our tet smoother.
+
+**A4.2 Liu, Wang, Lévy, Sun, Yan, Lu, Yang 2009, CVT energy smoothness and quasi-Newton.** **Verdict: build on (the CVT solver).**
+- **Citation.** ACM TOG 28(4):101, 2009. DOI [10.1145/1559755.1559758](https://doi.org/10.1145/1559755.1559758).
+- **Method.** The CVT energy is C². Minimize it with L-BFGS instead of Lloyd for an order-of-magnitude speedup.
+
+**A4.3 Yan, Lévy, Liu, Sun, Wang 2009 (restricted Voronoi diagram); Yan et al. 2013 (clipped Voronoi diagram).** **Verdict: build on (the RVD and clipped-VD kernel).**
+- **Citations.**
+  - Yan et al. *Isotropic Remeshing with Fast and Exact Computation of Restricted Voronoi Diagram.* CGF 28(5):1445–1454,
+    2009. DOI [10.1111/j.1467-8659.2009.01521.x](https://doi.org/10.1111/j.1467-8659.2009.01521.x).
+  - D.-M. Yan, W. Wang, B. Lévy, Y. Liu. *Efficient computation of clipped Voronoi diagram for mesh generation.*
+    CAD 45(4):843–852, 2013. DOI [10.1016/j.cad.2011.09.004](https://doi.org/10.1016/j.cad.2011.09.004).
+- **Method.** Compute the Voronoi diagram restricted to a surface (or clipped by a volume). Propagate over triangles
+  and clip each against the bisector half-spaces of the nearest seeds (Sutherland–Hodgman).
+- **Problem solved.** The integration domain for surface remeshing and for CVT/Lp-CVT inside a closed volume.
+- **Robustness.** Exact with PCK-style predicates (geogram).
+- **Complexity.** Moderate.
+- **Code.** geogram `voronoi/RVD*.cpp` (BSD): READ OK.
+
+**A4.4 Lévy and Liu 2010, Lp Centroidal Voronoi Tessellation.** **Verdict: reference (hex-dominant point placement, superseded for that purpose by Sokolov 2016).**
+- **Citation.** B. Lévy, Y. Liu. *Lp Centroidal Voronoi Tessellation and its Applications.* ACM TOG 29(4):119, 2010.
+  DOI [10.1145/1778765.1778856](https://doi.org/10.1145/1778765.1778856).
+- **Method.** Replace the L2 CVT energy with an Lp norm in a local frame (anisotropy tensor). As p grows, Voronoi cells
+  become cubes aligned to a field. Feature constraints come from integrating over the RVD of the surface and its
+  sharp edges.
+- **Use.** Produces cube-like point sets for hex-dominant meshing.
+- **Limitations.** Non-convex energy. Needs a smooth frame field.
+
+---
+
+## Part B: Hexahedral meshing
+
+### B0. Survey and benchmark (read first)
+
+**B0.1 Pietroni et al. 2022, survey.** **Verdict: reference (required reading).**
+- **Citation.** N. Pietroni, M. Campen, A. Sheffer, G. Cherchi, D. Bommes, X. Gao, R. Scateni, F. Ledoux,
+  J.-F. Remacle, M. Livesu. *Hex-Mesh Generation and Processing: a Survey.* ACM TOG 42(2):16, 2022.
+- **Links.** DOI [10.1145/3554920](https://doi.org/10.1145/3554920). arXiv: https://arxiv.org/abs/2202.12670
+- **Structure.**
+  - §2.5 integer-grid maps;
+  - §4.6 grid-based;
+  - §4.7 polycube;
+  - §4.8 frame fields;
+  - §4.9 hex-dominant;
+  - §5 topological operators;
+  - §6 geometric optimization;
+  - §8 open problems;
+  - §9 resources.
+- **Key quote (§4.6).** Grid-based methods "really stand out in terms of robustness. To date, they are the only fully
+  automatic methods capable of successfully hex-meshing any input shape... they are the only automatic methods
+  currently implemented in professional software." It also notes their cost: orientation dependence, intricate
+  high-valence singular structure, and no coarse block structure.
+- **§8.1.** Frame-field methods are "limited to rather simple shapes" fully automatically. No characterization of
+  hex-meshable fields exists. Polycube graph characterization is limited to 3-connected graphs and, in the Eppstein–Mumford
+  form, genus 0.
+
+**B0.2 Beaufort, Reberol, Kalmykov, Liu, Ledoux, Bommes 2022, Hex Me If You Can (HexMe).** **Verdict: reference (benchmark, with a provenance caveat in C).**
+- **Citation.** CGF 41(5):125–134 (SGP), 2022.
+- **Links.**
+  - DOI [10.1111/cgf.14608](https://doi.org/10.1111/cgf.14608).
+  - arXiv: https://arxiv.org/abs/2111.10295
+  - Dataset DOI [10.5281/zenodo.6642020](https://doi.org/10.5281/zenodo.6642020), **CC-BY-4.0**.
+  - Catalog: https://hexme.algohex.eu
+- **Contents.** 189 tet meshes from 63 CAD models in three categories: simple, nasty and industrial. Each comes with
+  feature tags.
+- **Baseline result.** A state-of-the-art frame-field → IGM → HexEx pipeline meshes all features correctly on only
+  **19 of 189**. On nasty and industrial shapes about half the meshes preserve under 1–8% of surfaces.
+- **Why it matters.** This is the evidence that frame-field methods are not production-ready for CAD.
+
+---
+
+### B1. Grid / octree-based (robust family)
+
+**B1.1 Schneiders 1996.** **Verdict: reference (historical).**
+- **Citation.** R. Schneiders. *A grid-based algorithm for the generation of hexahedral element meshes.* Engineering
+  with Computers 12:168–177, 1996. DOI [10.1007/BF01198732](https://doi.org/10.1007/BF01198732).
+- **Method.** An octree with templates resolves hanging nodes (3-refinement). The boundary layer is projected onto the
+  surface.
+
+**B1.2 Zhang and Bajaj 2006.** **Verdict: reference.**
+- **Citation.** *Adaptive and quality quadrilateral/hexahedral meshing from volumetric data.* CMAME 195:942–960, 2006.
+  DOI [10.1016/j.cma.2005.02.016](https://doi.org/10.1016/j.cma.2005.02.016).
+- **Method.** Dual contouring on an octree with templates, followed by quality improvement.
+
+**B1.3 Maréchal 2009, octree-based all-hex with sharp features.** **Verdict: reference (feature handling ideas).**
+- **Citation.** L. Maréchal. *Advances in Octree-Based All-Hexahedral Mesh Generation: Handling Sharp Features.*
+  IMR 18, 65–84, 2009. DOI [10.1007/978-3-642-04319-2_5](https://doi.org/10.1007/978-3-642-04319-2_5).
+- **Method.** A 2-refinement octree. Hanging nodes are removed by **dualization** with flat transition schemes. Sharp
+  features are captured by snapping dual vertices and by padding.
+- **Significance.** The basis of commercial octree hex meshers.
+- **Limitation.** The scheme set is incomplete: Livesu 2021 shows it misses some transitions.
+
+**B1.4 Gao, Shen, Panozzo 2019, Feature Preserving Octree-Based Hexahedral Meshing.** **Verdict: reference (pipeline design).**
+- **Citation.** CGF 38(5):135–149 (SGP), 2019. DOI [10.1111/cgf.13795](https://doi.org/10.1111/cgf.13795).
+- **Pipeline.**
+  1. Octree refinement driven by surface approximation.
+  2. Dual-based hex extraction.
+  3. Feature-preserving surface projection.
+  4. Padding.
+  5. Global optimization that maximizes the minimum scaled Jacobian while fitting features.
+- **Results.** The authors report high robustness on a large dataset.
+- **Code.** `gaoxifeng/Feature-Preserving-Octree-Hex-Meshing` has **NO LICENCE**: do not read.
+
+**B1.5 Livesu, Pitzalis, Cherchi 2021, Optimal Dual Schemes for Adaptive Grid Based Hexmeshing.** **Verdict: BUILD ON (the core of the first hex mesher).**
+- **Citation.** ACM TOG 41(2):15, 2022 (online 2021).
+- **Links.** DOI [10.1145/3494456](https://doi.org/10.1145/3494456). arXiv: https://arxiv.org/abs/2103.07745
+- **Method, part 1.**
+  - Enumerates **all** transitions dual methods must handle (§4: §4.1 convex, §4.2 concave, §4.3 around corners).
+  - Shows prior schemes miss some of them.
+  - Gives the minimal scheme set with the simplest singular structure (§5).
+- **Method, part 2.**
+  - Extends support from strongly to **weakly balanced** grids (§6; discussion in §7.1). Coarser meshes for the same accuracy.
+  - Compares with Gao 2019 and Maréchal 2009 (§7.2).
+  - Spells out the implementation details that earlier papers left implicit.
+- **Robustness.** Combinatorially complete for balanced grids, so the dual is always all-hex. Geometry
+  (projection/padding/untangling) is outside the paper's scope.
+- **Limitations.**
+  - Most grid methods do not guarantee the output keeps the input's genus or component count (survey §4.6).
+  - Sharp features need a separate projection and optimization stage.
+- **Complexity.** Moderate: the schemes are tables plus a balancing pass.
+- **Code.** Integrated into `mlivesu/cinolib`, **MIT** (paper p. 2 and §8): READ OK. Do not open cinolib's optional
+  TetGen/Triangle wrappers if present.
+
+**B1.6 Pitzalis, Livesu, Cherchi, Gobbetti, Scateni 2021, Generalized Adaptive Refinement.** **Verdict: build on (refinement and balancing stage).**
+- **Citation.** ACM TOG 40(6):257, 2021. DOI [10.1145/3478513.3480508](https://doi.org/10.1145/3478513.3480508).
+- **Method.** Generalizes balanced adaptive refinement so it works on any input hex mesh, not just a regular grid,
+  with conforming templates and no dualization.
+- **Use.** Lets us refine a coarse aligned hex mesh, for example polycube output, as well as an octree.
+- **Code.** `cg3hci/Gen-Adapt-Ref-for-Hexmeshing` is **MIT**: READ OK.
+
+---
+
+### B2. Polycube maps
+
+**B2.1 Tarini, Hormann, Cignoni, Montani 2004, PolyCube-Maps.** **Verdict: reference (the concept).**
+- **Citation.** ACM TOG 23(3):853–860, 2004. DOI [10.1145/1015706.1015810](https://doi.org/10.1145/1015706.1015810).
+
+**B2.2 Gregson, Sheffer, Zhang 2011, All-hex mesh generation via volumetric polycube deformation.** **Verdict: reference.**
+- **Citation.** CGF 30(5):1407–1416, 2011.
+  DOI [10.1111/j.1467-8659.2011.02015.x](https://doi.org/10.1111/j.1467-8659.2011.02015.x).
+- **Method.** Rotation-driven volumetric deformation toward axis-aligned normals, then labelling, then hex sampling.
+- **Limitation.** Heuristic cleanup of invalid labellings.
+
+**B2.3 Livesu, Vining, Sheffer, Gregson, Scateni 2013, PolyCut.** **Verdict: reference.**
+- **Citation.** ACM TOG 32(6):171, 2013. DOI [10.1145/2508363.2508388](https://doi.org/10.1145/2508363.2508388).
+- **Links.** Project: https://www.cs.ubc.ca/labs/imager/tr/2013/polycut/
+- **Method.** Graph-cut labelling with monotonicity. Applies the Eppstein–Mumford validity conditions.
+- **Limitation.** Not guaranteed to converge to a valid polycube (survey §4.7).
+
+**B2.4 Huang et al. 2014 (ℓ1 polycube); Fang et al. 2016 (closed-form induced polycube); Fu et al. 2016.** **Verdict: reference.**
+- **Citations.**
+  - Huang, Jiang, Shi, Tong, Bao, Desbrun. *ℓ1-Based Construction of Polycube Maps from Complex Shapes.* ACM TOG
+    33(3):25, 2014. DOI [10.1145/2602141](https://doi.org/10.1145/2602141).
+  - Fang, Xu, Bao, Huang. *All-hex meshing using closed-form induced polycube.* ACM TOG 35(4):124, 2016.
+    DOI [10.1145/2897824.2925957](https://doi.org/10.1145/2897824.2925957).
+  - Fu, Bai, Liu. *Efficient volumetric PolyCube-map construction.* CGF 35(7):97–106, 2016.
+    DOI [10.1111/cgf.13007](https://doi.org/10.1111/cgf.13007).
+- **Method.** Deformation-based polycube construction with different energies.
+- **Limitation.** They "seldom converge to a topologically consistent polyhedron" without a valid guiding labelling
+  (survey §4.7).
+
+**B2.5 Guo, Liu, Yan, Liu 2020, Cut-enhanced PolyCube-maps.** **Verdict: reference.**
+- **Citation.** *Cut-enhanced PolyCube-maps for feature-aware all-hex meshing.* ACM TOG 39(4):106, 2020.
+  DOI [10.1145/3386569.3392378](https://doi.org/10.1145/3386569.3392378).
+- **Method.** Interior cuts let polycubes align to features that a plain polycube cannot.
+
+**B2.6 Dumery, Protais, Mestrallet, Bourcier, Ledoux 2022, Evocube.** **Verdict: reference.**
+- **Citation.** *Evocube: a Genetic Labelling Framework for Polycube-Maps.* CGF 41(6):467–479, 2022.
+- **Links.** DOI [10.1111/cgf.14649](https://doi.org/10.1111/cgf.14649). arXiv: https://arxiv.org/abs/2205.00738
+- **Method.** Genetic search over labellings, scored by validity and distortion. Slow, and still with no guarantee.
+- **Code.** `LIHPC-Computational-Geometry/evocube` is **GPL-3**: FORBIDDEN.
+
+**B2.7 Protais, Reberol, Ray, Corman, Ledoux, Sokolov 2022, Robust Quantization for Polycube Maps.** **Verdict: build on (when polycube is built).**
+- **Citation.** CAD 150:103321, 2022. DOI [10.1016/j.cad.2022.103321](https://doi.org/10.1016/j.cad.2022.103321).
+- **Method.** Robust integer quantization of a polycube map's chart sizes. Guarantees a valid hex layout from a valid
+  labelling.
+- **Code.** `fprotais/robustPolycube` is **AGPL-3**: FORBIDDEN.
+
+**B2.8 Mandad, Chen, Bommes, Campen 2022, Intrinsic Mixed-Integer Polycubes.** **Verdict: reference.**
+- **Citation.** CAGD 94:102078, 2022. DOI [10.1016/j.cagd.2022.102078](https://doi.org/10.1016/j.cagd.2022.102078).
+
+**B2.9 Snoep, Speckmann, Verbeek 2025, DualCube.** **Verdict: BUILD ON (when polycube is built). The first labelling method with a validity guarantee.**
+- **Citation.** *Robust Construction of Polycube Segmentations via Dual Loops.* CGF 44(5):e70195 (SGP), 2025.
+- **Links.** DOI [10.1111/cgf.70195](https://doi.org/10.1111/cgf.70195). arXiv: https://arxiv.org/abs/2402.00652
+- **Method.**
+  - Work in the **dual** loop structure of a polycube. §3 gives the dual structure, its characterization and the
+    valid modifications.
+  - §4: start from a trivial polycube of the right genus, then iteratively add loops (§4.1) while keeping validity.
+    Primalize into a segmentation (§4.2) and optimize the loop structure (§4.3).
+  - §5.2 covers hex meshing.
+- **Guarantee.** "Guaranteed to return a valid polycube segmentation for 3D models of any genus." It is valid at every
+  iteration, so it can be stopped at any time.
+- **Limitations.** Quality and feature alignment trade against complexity. Feature preservation is weaker than with
+  feature-driven labellings.
+- **Companion.** *Polycubes via Dual Loops*, arXiv https://arxiv.org/abs/2410.16865 (2024): the dual-loop
+  characterization for arbitrary genus.
+- **Code.** The repo named in search results (`maximsnoep/polycube-layouts-via-iterative-dual-loops`) returned 404.
+  Licence unknown.
+
+---
+
+### B3. Frame fields and integer-grid maps (IGM)
+
+**B3.1 Nieser, Reitebuch, Polthier 2011, CubeCover.** **Verdict: reference (defines the IGM pipeline).**
+- **Citation.** *CubeCover – Parameterization of 3D Volumes.* CGF 30(5):1397–1406, 2011.
+  DOI [10.1111/j.1467-8659.2011.02014.x](https://doi.org/10.1111/j.1467-8659.2011.02014.x).
+- **Method.** Frame field → seamless volumetric parametrization whose integer iso-surfaces form hexes. It is the 3D
+  analogue of Bommes, Zimmer, Kobbelt, *Mixed-integer quadrangulation*, ACM TOG 28(3):77, 2009,
+  DOI [10.1145/1531326.1531383](https://doi.org/10.1145/1531326.1531383).
+
+**B3.2 Huang, Tong, Wei, Bao 2011, Boundary aligned smooth 3D cross-frame field.** **Verdict: reference.**
+- **Citation.** ACM TOG 30(6):143, 2011. DOI [10.1145/2070781.2024177](https://doi.org/10.1145/2070781.2024177).
+- **Method.** Spherical-harmonic (SH) frame representation, which removes the octahedral-symmetry ambiguity.
+
+**B3.3 Li, Liu, Xu, Wang, Guo 2012, All-hex meshing using singularity-restricted field.** **Verdict: reference.**
+- **Citation.** ACM TOG 31(6):177, 2012. DOI [10.1145/2366145.2366196](https://doi.org/10.1145/2366145.2366196).
+- **Method.** Local operations repair singularity graphs to be hex-meshable.
+
+**B3.4 Ray, Sokolov, Lévy 2016, Practical 3D frame field generation.** **Verdict: build on (only if the frame-field family is pursued).**
+- **Citation.** ACM TOG 35(6):233, 2016. DOI [10.1145/2980179.2982408](https://doi.org/10.1145/2980179.2982408).
+- **Method.** SH-based smoothing with local Euler-angle optimization and boundary alignment. Fast and simple.
+
+**B3.5 Solomon, Vaxman, Bommes 2017, Boundary element octahedral fields.** **Verdict: reference.**
+- **Citation.** ACM TOG 36(3):28, 2017. DOI [10.1145/3065254](https://doi.org/10.1145/3065254).
+
+**B3.6 Liu, Zhang, Chien, Solomon, Bommes 2018, Singularity-constrained octahedral fields.** **Verdict: reference.**
+- **Citation.** ACM TOG 37(4):93, 2018. DOI [10.1145/3197517.3201344](https://doi.org/10.1145/3197517.3201344).
+- **Method.** Enumerates local meshability conditions for valence-3 and valence-5 singular edges. Computes a field
+  conforming to a given singularity graph.
+- **Limitation.** The singular graph is user- or heuristic-supplied.
+
+**B3.7 Corman and Crane 2019, Symmetric Moving Frames.** **Verdict: reference.**
+- **Citation.** ACM TOG 38(4):87, 2019. DOI [10.1145/3306346.3323029](https://doi.org/10.1145/3306346.3323029).
+- **Links.** PDF: https://www.cs.cmu.edu/~kmcrane/Projects/SymmetricMovingFrames/paper.pdf
+- **Method.** Cartan's moving frames. Frame fields with *prescribed* singularity graph and non-orthogonal frames,
+  obtained as connection 1-forms with integrability constraints.
+
+**B3.8 Palmer, Bommes, Solomon 2020, Algebraic representations for volumetric frame fields.** **Verdict: reference.**
+- **Citation.** ACM TOG 39(2):16, 2020.
+- **Links.** DOI [10.1145/3366786](https://doi.org/10.1145/3366786). arXiv: https://arxiv.org/abs/1908.05411
+- **Method.** Octahedral and odeco frames as varieties. Manifold-optimization (MBO-style) field computation. Supports
+  non-orthogonal frames.
+
+**B3.9 Lyon, Bommes, Kobbelt 2016, HexEx; Kohler, Heistermann, Bommes 2025, HexHex.** **Verdict: reference (needed only for the frame-field family).**
+- **Citations.**
+  - *HexEx: Robust Hexahedral Mesh Extraction.* ACM TOG 35(4):123, 2016.
+    DOI [10.1145/2897824.2925976](https://doi.org/10.1145/2897824.2925976).
+  - *HexHex: Highspeed Extraction of Hexahedral Meshes.* ACM TOG 44(4), 2025.
+    DOI [10.1145/3730940](https://doi.org/10.1145/3730940). Project: https://cgg.unibe.ch/publications/hexhex/
+- **Method.** Extract the hex mesh from an IGM, tolerating some defects (HexEx). HexHex assumes a *locally injective*
+  map and is unconditionally robust, with about 30× speedup through a propeller data structure and conservative
+  rasterization.
+- **Code.**
+  - libHexEx is **GPLv3** (RWTH page): FORBIDDEN.
+  - `cgg-bern/libHexHex` LICENSE is **GPL** (header plus `GPL.txt`): FORBIDDEN.
+
+**B3.10 Brückler, Bommes, Campen: 3D motorcycle complex (CGF 2022), QGP3D (TOG 2022), cell-complex collapsing (TOG 2023), Integer-Sheet-Pump (CGF 2024).** **Verdict: reference.**
+- **Citations.**
+  - DOI [10.1111/cgf.14470](https://doi.org/10.1111/cgf.14470): *The 3D Motorcycle Complex for Structured Volume
+    Decomposition*, CGF 41(2):221–235.
+  - DOI [10.1145/3528223.3530123](https://doi.org/10.1145/3528223.3530123): *Volume parametrization quantization for
+    hexahedral meshing*, TOG 41(4):60.
+  - DOI [10.1145/3618384](https://doi.org/10.1145/3618384): *Collapsing Embedded Cell Complexes for Safer Hexahedral
+    Meshing*, TOG 42(6):205.
+  - DOI [10.1111/cgf.15131](https://doi.org/10.1111/cgf.15131): *Integer-Sheet-Pump Quantization*, CGF 43(5).
+- **Method.** Robust quantization of seamless volume maps into valid IGMs through the motorcycle complex.
+- **Code.** QGP3D, MC3D, TrulySeamless3D and C4HexMeshing are all **GPL-3**: FORBIDDEN.
+
+**B3.11 Liu and Bommes 2023, Locally Meshable Frame Fields.** **Verdict: reference (best current evidence of the family's state).**
+- **Citation.** ACM TOG 42(4), 2023. DOI [10.1145/3592457](https://doi.org/10.1145/3592457).
+- **Result.** Raises the valid-IGM success rate on HexMe "**from 2% to 58%**" (project page abstract). Local
+  meshability is necessary but not sufficient.
+- **Code.** No code link on the project page.
+
+**B3.12 Nigolian, Campen, Bommes 2023, Expansion Cones.** **Verdict: reference (injective volumetric maps).**
+- **Citation.** ACM TOG 42(4), 2023. DOI [10.1145/3592421](https://doi.org/10.1145/3592421).
+- **Use.** Relevant for polycube maps too.
+
+---
+
+### B4. Loop- and sweep-based block decomposition
+
+**B4.1 Livesu, Pietroni, Puppo, Sheffer, Cignoni 2020, LoopyCuts.** **Verdict: reference.**
+- **Citation.** *LoopyCuts: Practical Feature-Preserving Block Decomposition for Strongly Hex-Dominant Meshing.*
+  ACM TOG 39(4):121, 2020. DOI [10.1145/3386569.3392472](https://doi.org/10.1145/3386569.3392472).
+- **Method.** Trace field-aligned surface loops, cut the volume along them into simple blocks, then mesh the blocks.
+- **Guarantee.** Strongly hex-dominant output, *not* all-hex.
+- **Fit.** Good for CAD with features.
+- **Complexity.** High.
+- **Code.** `mlivesu/LoopyCuts` has **NO LICENCE**: do not read.
+
+**B4.2 Reberol, Verhetsel, Henrotte, Bommes, Remacle 2023, all-hex boundary layers.** **Verdict: reference (CFD boundary layers later).**
+- **Citation.** *Robust Topological Construction of All-hexahedral Boundary Layer Meshes.* ACM TOMS 49(1), 2023.
+  DOI [10.1145/3577196](https://doi.org/10.1145/3577196).
+
+---
+
+### B5. Hex-dominant via tet recombination or aligned points
+
+**B5.1 Meshkat and Talmor 2000.** **Verdict: reference.**
+- **Citation.** *Generating a mixed mesh of hexahedra, pentahedra and tetrahedra from an underlying tetrahedral mesh.*
+  IJNME 49(1–2):17–30, 2000.
+  DOI [10.1002/1097-0207(20000910/20)49:1/2<17::AID-NME920>3.0.CO;2-U](https://doi.org/10.1002/1097-0207(20000910/20)49:1/2%3C17::AID-NME920%3E3.0.CO;2-U).
+- **Method.** Graph search for tet groups forming hexes or prisms.
+
+**B5.2 Yamakawa and Shimada 2003, rectangular-solid cell packing.** **Verdict: reference.**
+- **Citation.** IJNME 57(15):2099–2129, 2003. DOI [10.1002/nme.754](https://doi.org/10.1002/nme.754).
+- **Method.** Pack cells to place hex-friendly points, then recombine.
+
+**B5.3 Baudouin, Remacle, Marchandise, Henrotte, Geuzaine 2014, frontal hex-dominant.** **Verdict: reference.**
+- **Citation.** Adv. Model. Simul. Eng. Sci. 1:8, 2014. DOI [10.1186/2213-7467-1-8](https://doi.org/10.1186/2213-7467-1-8).
+- **Method.** Frontal point insertion along a cross field, Delaunay of the points, then recombination.
+
+**B5.4 Botella, Lévy, Caumon 2016, tet recombination.** **Verdict: reference.**
+- **Citation.** *Indirect unstructured hex-dominant mesh generation using tetrahedra recombination.* Comput. Geosci.
+  20(3):437–451, 2016. DOI [10.1007/s10596-015-9484-9](https://doi.org/10.1007/s10596-015-9484-9).
+
+**B5.5 Sokolov, Ray, Untereiner, Lévy 2016, Hexahedral-Dominant Meshing.** **Verdict: reference (strong candidate for the hex-dominant product).**
+- **Citation.** ACM TOG 35(5):157, 2016. DOI [10.1145/2930662](https://doi.org/10.1145/2930662).
+- **Method.**
+  1. Frame field.
+  2. Global parametrization, tolerating defects.
+  3. Point set from the integer iso-values.
+  4. Delaunay of the points.
+  5. Recombine tets into hexes.
+  6. Fill the rest with tets, prisms and pyramids.
+- **Result.** Robust hex-dominant output even where the parametrization is defective.
+- **Code.** Graphite is GPL-2: FORBIDDEN.
+
+**B5.6 Gao, Jakob, Tarini, Panozzo 2017, field-guided polyhedral agglomeration.** **Verdict: reference.**
+- **Citation.** ACM TOG 36(4):114, 2017. DOI [10.1145/3072959.3073676](https://doi.org/10.1145/3072959.3073676).
+- **Method.** Instant-Meshes-style position field in 3D, giving a polyhedral, mostly-hex mesh.
+- **Code.** `gaoxifeng/robust_hex_dominant_meshing` is **MIT**: READ OK.
+
+**B5.7 Pellerin, Johnen, Verhetsel, Remacle 2018, vertex-based tet-to-hex identification.** **Verdict: build on (the recombination step).**
+- **Citation.** *Identifying combinations of tetrahedra into hexahedra: a vertex based strategy.* CAD 105:1–10, 2018.
+  DOI [10.1016/j.cad.2018.05.004](https://doi.org/10.1016/j.cad.2018.05.004).
+- **Method.** Enumerate **all** potential hexes in a tet mesh exhaustively from vertex neighbourhoods. This is faster
+  and complete compared with pattern-matching on tet groups. A selection step (maximum compatible set) then follows.
+- **Fit.** Reuses our tet pipeline directly.
+- **Code.** Implemented in Gmsh (GPL): FORBIDDEN.
+
+**B5.8 Verhetsel, Pellerin, Remacle 2019, hexahedrizations of small quadrangulations.** **Verdict: skip for now (research).**
+- **Citation.** *Finding hexahedrizations for small quadrangulations of the sphere.* ACM TOG 38(4):53, 2019.
+  DOI [10.1145/3306346.3323017](https://doi.org/10.1145/3306346.3323017).
+
+---
+
+### B6. Hex quality improvement and untangling
+
+**B6.1 Livesu, Sheffer, Vining, Tarini 2015, edge-cone rectification.** **Verdict: reference.**
+- **Citation.** *Practical hex-mesh optimization via edge-cone rectification.* ACM TOG 34(4):141, 2015.
+  DOI [10.1145/2766905](https://doi.org/10.1145/2766905).
+- **Method.** Local-global optimization that makes each corner's edge cone well-shaped. It fixes inverted elements,
+  with surface constraints.
+
+**B6.2 Xu, Gao, Chen 2018, edge-angle optimization.** **Verdict: reference.**
+- **Citation.** *Hexahedral mesh quality improvement via edge-angle optimization.* Computers & Graphics 70:17–27, 2018.
+  DOI [10.1016/j.cag.2017.07.002](https://doi.org/10.1016/j.cag.2017.07.002).
+- **Note.** This is the "Xu et al. 2018" hex improvement and untangling paper.
+
+**B6.3 Marschner, Palmer, Zhang, Solomon 2020, sum-of-squares hex repair.** **Verdict: reference.**
+- **Citation.** *Hexahedral Mesh Repair via Sum-of-Squares Relaxation.* CGF 39(5):133–147, 2020.
+  DOI [10.1111/cgf.14074](https://doi.org/10.1111/cgf.14074).
+- **Method.** Polynomial (SOS) relaxation of the trilinear-hex Jacobian positivity. It certifies validity of the
+  *trilinear* element, not just its corners.
+- **Assessment.** Heavy machinery, since it needs an SDP solver. Reference for the **validity check**: corner Jacobians
+  are insufficient for trilinear hexes.
+
+**B6.4 Garanzha, Kaporin, Kudryavtseva, Protais, Ray, Sokolov 2021, Foldover-free maps in 50 lines of code.** **Verdict: BUILD ON (the untangler for every hex family and for polycube/IGM maps).**
+- **Citation.** ACM TOG 40(4):102, 2021.
+- **Links.** DOI [10.1145/3450626.3459847](https://doi.org/10.1145/3450626.3459847). arXiv: https://arxiv.org/abs/2102.03069
+- **Method.**
+  - §2.1: a variational barrier energy, with χ-regularization of det J.
+  - §2.2: penalty method. ε_k is driven to zero (§4.2).
+  - §2.3: L-BFGS resolution scheme.
+- **Result.** Untangles meshes reliably and is simple to implement. The paper reports it on a large benchmark (§3.1).
+- **Code.** `ssloy/invertible-maps` is **AGPL-3**: FORBIDDEN. The paper itself is enough, by design.
+
+**B6.5 Visualization and metrics.** **Verdict: reference.**
+- **HexaLab** (MIT, https://github.com/mlivesu/HexaLab, https://www.hexalab.net) is READ OK. It is a hex quality
+  viewer whose metrics match the literature's.
+
+---
+
+## Part C: Datasets and benchmarks
+
+| Dataset | Content | Licence | Usable for ModelCraft? |
+|---|---|---|---|
+| **Thingi10K**: Zhou & Jacobson 2016, arXiv https://arxiv.org/abs/1605.04797; site https://ten-thousand-models.appspot.com; tooling https://github.com/Thingi10K/Thingi10K | 10,000 STL meshes. 45% self-intersecting, 50% non-solid. The 4,408 "valid" subset is the standard tet-meshing robustness benchmark (fTetWild, Diazzi 2023). | Tooling Apache-2.0. **Each model has its own licence** (10 licences, including CC-BY-NC and GPL). Filter by the `license` field. | **Conditionally.** Use CC-BY / CC0 / public-domain models for robustness testing. **Provenance risk:** Thingiverse records no authoring tool, and many uploads were modelled in commercial CAD. Under the "no test data produced with commercial CAD products" rule this is an **owner decision**. |
+| **HexMe**: DOI 10.5281/zenodo.6642020, https://hexme.algohex.eu | 189 tet meshes from 63 CAD models, with feature tags. | Dataset **CC-BY-4.0**. The source models come from the **ABC dataset (Onshape terms), GrabCAD (GrabCAD terms)**, MAMBO (Apache-2.0), and models made with **Gmsh and Siemens NX** (the paper's own statement). | **Only a filtered subset.** The ABC/Onshape, GrabCAD and Siemens NX models are produced with commercial CAD: exclude them. Gmsh-authored ones are acceptable as data. |
+| **MAMBO**: https://gitlab.com/franck.ledoux/mambo | STEP models: basic, simple, medium. Designed for hex block-meshing evaluation. | **Apache-2.0** | **Risky.** The STEP headers I sampled say `'Spatial InterOp 3D'` as originating system and authors `'Cubit 15.2'` / `'MGX'`. These are commercial or closed tools. Treat as commercial-CAD-produced unless the owner rules otherwise. |
+| **ABC**: Koch et al., CVPR 2019, DOI [10.1109/CVPR.2019.00983](https://doi.org/10.1109/CVPR.2019.00983), arXiv https://arxiv.org/abs/1812.06216, https://deep-geometry.github.io/abc-dataset/ | 1M CAD models (B-rep and meshes). | "Copyright of the CAD models is owned by their creators", under **Onshape Terms of Use**. The models come from Onshape. | **No.** Produced with a commercial CAD product, and the licence is not open. Skip. |
+
+**Recommendation for test data.** Build ModelCraft's own corpus:
+- **Generated cases.** Parametric primitives plus booleans from our own kernel:
+  - cubes and cylinders with holes;
+  - fillets;
+  - thin walls;
+  - multi-material interfaces;
+  - genus-n plates;
+  - a "nasty" series modelled on HexMe's categories by description, not by copying its models.
+- **Analytic cases.** Sphere and torus volumes, with exact volumes for checks.
+- **Robustness stress.** The CC-licensed Thingi10K subset, if the owner approves.
+- **Hex-meshing claims.** Report results on our own corpus. Report the HexMe Gmsh-authored subset separately.
+
+---
+
+## Part D: Recommended pipeline (opinionated)
+
+### Step 1: Robust predicates (weeks 1–3)
+1. Shewchuk adaptive `orient2d`, `orient3d`, `incircle` and `insphere` (A0.1, §2–§4).
+   - Port the expansion primitives from the paper.
+   - Use `predicates.c` (public domain) and `georust/robust` (Apache-2.0) only as differential oracles in tests.
+2. Devillers–Teillaud symbolic perturbation for `insphere` (A0.2).
+3. Indirect predicates for LPI/TPI implicit points (A0.3, §4–§5). Use a small build-time code generator that emits a
+   floating-point filter, an interval filter and an expansion fallback. Model it on PCK (A0.4, BSD-readable). Do not
+   use Attene's LGPL code.
+4. **Tests.** Exhaustive near-degenerate fuzzing against an exact rational reference (`num-rational` / BigInt in tests only).
+
+**If wrong.** Every later stage inherits silent topological corruption. This is the one stage where "almost right" is fatal.
+
+### Step 2: Delaunay plus boundary recovery (weeks 3–12)
+1. Bowyer–Watson with BRIO and Hilbert order, straight walking, and a compact adjacency array (A1.1).
+2. **Boundary recovery: implement Diazzi et al. 2023 (A2.3).**
+   - Si–Gärtner segment recovery (§3.3) and face recovery (§3.4).
+   - Implicit Steiner points (§4.3).
+   - The gift-wrapping fallback (§4.4), which is mandatory: it closes the theoretical gap in TetGen.
+   - Inside/outside labelling (§4.5).
+   - **Why CDT over the envelope approach.** The B-rep tessellator gives a watertight, non-self-intersecting PLC. CAE
+     needs exact face conformity: B-rep face IDs on boundary triangles, shared interfaces in assemblies, contact
+     surfaces. CDT preserves this by construction. fTetWild (A2.5) does not preserve all input triangles.
+3. Keep **fTetWild's design (MPL-2.0, readable) as the documented fallback** for imported dirty geometry. Do not build
+   it now. Convex polyhedral meshing (A2.4) is the alternative if exact conformity on self-intersecting input is ever
+   required.
+
+**If wrong.** If the tessellator emits self-intersections (common at tolerant B-rep edges), the CDT rejects the input.
+- **Mitigation.** Run an exact self-intersection check before CDT.
+- **Then route to repair.** Use the arrangement method (A2.6), or the fTetWild path once it exists.
+
+### Step 3: Quality (weeks 12–18)
+1. Constrained Delaunay refinement with a radius–edge bound of about 2 and a sizing field (A3.1, A2.2/Si 2010 for small
+   input angles).
+2. Interleave with ODT vertex smoothing following Tournois 2009 (A3.5). Use the Chen–Holst update formulas (A3.4) and
+   keep boundary vertices on their B-rep faces.
+3. **Final pass: the Klingner–Shewchuk schedule (A3.6, §3–§4.2).** Smoothing, edge removal, multi-face removal and
+   vertex insertion, monotone in worst quality.
+4. **Targets.** Report min/max dihedral angles and an FE quality metric. Use Shewchuk's *What is a good linear
+   element?*, PDF https://people.eecs.berkeley.edu/~jrs/papers/elemj.pdf, for the metric choice.
+5. **CVT/RVD (A4.2–A4.3).** Build only for **surface remeshing** and for Lp-CVT if needed later. Not for tet
+   smoothing, where ODT is better.
+
+**If wrong.** ODT alone leaves boundary slivers, which can make FE results wrong. Step 3.3 is the guard. Do not ship without it.
+
+### Step 4: Hex meshing. Build the grid/octree dual family first.
+
+**Order.**
+1. **(a) Octree dual all-hex.** Livesu 2021 schemes (B1.5) with Pitzalis 2021 refinement (B1.6). Then the Gao 2019
+   stage list (B1.4): feature projection, padding layer, and global min-scaled-Jacobian optimization. Use the Garanzha
+   2021 untangler (B6.4) for any inverted elements. Validate with the Marschner 2020 insight (B6.3) that corner
+   Jacobians do not certify trilinear hexes; use a subdivided or Bernstein Jacobian check.
+2. **(b) Hex-dominant by recombination.** Frame-field-aligned point placement:
+   - Ray 2016 field (B3.4);
+   - frontal or Sokolov-style points (B5.3/B5.5);
+   - our own Delaunay;
+   - Pellerin 2018 vertex-based hex identification (B5.7) and selection;
+   - pyramids, prisms and tets for the rest.
+   This reuses all of steps 1–3.
+3. **(c) Polycube for blocky CAD parts.**
+   - DualCube labelling (B2.9): guaranteed valid.
+   - Volumetric map with an injective solve (B6.4).
+   - Robust quantization (B2.7, from the paper only).
+   - Pitzalis refinement (B1.6) for grading.
+4. **(d) Frame-field/IGM all-hex last, as R&D.** All mature implementations are GPL. HexMe shows 19/189 fully correct
+   outputs, and the best 2023 method reaches 58% valid IGMs.
+
+**Why octree first.**
+- It is the **only family with a robustness guarantee on arbitrary input** (survey §4.6, quoted in B0.1).
+- It is the only family shipped in professional automatic hex meshers.
+- Every component has a permissive reference: CinoLib MIT for the dual schemes, Gen-Adapt-Ref MIT for refinement,
+  geogram BSD.
+- A CAE customer gets *a* valid all-hex mesh for any part on day one, which polycube and frame-field methods cannot promise.
+
+**Risks per family.**
+
+| Family | Main risk | Mitigation |
+|---|---|---|
+| Octree dual | Poor element quality at the boundary (low scaled Jacobian near concave features); orientation dependence; high element count; edges where more than 3 faces meet; genus not guaranteed (survey §4.6). | Padding layer. Feature-aware projection plus global optimization (B1.4). Align the octree to the part's principal axes. Weakly balanced grids (B1.5 §6) reduce element count. A topology check rejects genus changes. |
+| Hex-dominant recombination | Solvers must support pyramids and prisms; hex ratio is often only 60–90%; pyramids hurt accuracy. | Check the solver targets (Abaqus/Nastran element libraries support mixed meshes). Report the hex ratio honestly. |
+| Polycube | High distortion on organic or highly non-axis-aligned parts; features at oblique angles are lost; the injective volumetric map can fail. | Use only when the labelling distortion score is acceptable; otherwise fall back to octree. The Garanzha untangler handles the map. |
+| Frame-field IGM | Low success rate on CAD (HexMe, Liu & Bommes 2023); mixed-integer solves; all reference code is GPL. | Treat as R&D. Keep it out of the critical path. |
+
+**If the octree-first call is wrong.** Customers wanting high-quality structured hex for thin-walled or extruded parts
+would find octree meshes too unstructured. The answer then is sweeping and block decomposition (LoopyCuts-style, B4.1),
+not frame fields. That family is not covered in depth here and would need a follow-up survey on sweeping and
+multi-sweeping.
+
+---
+
+## Licence summary of associated code (metadata only; no source read)
+
+| Repo / code | Licence | Status |
+|---|---|---|
+| Shewchuk `predicates.c` | Public domain | READ OK |
+| Shewchuk Triangle | Proprietary (non-commercial) | FORBIDDEN |
+| `georust/robust` | Apache-2.0 (GitHub metadata) | READ OK |
+| `BrunoLevy/geogram` | BSD-3, **except** `third_party/tetgen` (AGPL), `third_party/triangle` and the `delaunay_tetgen/triangle` wrappers | READ OK, with exclusions |
+| `wildmeshing/fTetWild` | MPL-2.0 | READ OK |
+| `wildmeshing/wildmeshing-toolkit` | MIT | READ OK |
+| `Yixin-Hu/TetWild` | GPL-3.0 | FORBIDDEN |
+| TetGen | AGPL | FORBIDDEN |
+| CGAL, Gmsh (incl. HXT, recombination), Netgen | GPL / LGPL | FORBIDDEN |
+| `MarcoAttene/CDT`, `Delaunay3D` | GPL, or LGPL by option | FORBIDDEN |
+| `MarcoAttene/Indirect_Predicates` | LGPL-2.1 | FORBIDDEN |
+| `MarcoAttene/VolumeMesher` | GPL-3 | FORBIDDEN |
+| `libigl/libigl` | MPL-2.0 core; GPL `copyleft/` modules (GitHub reports GPL-3 because both licence files exist) | Core READ OK; `copyleft/` FORBIDDEN |
+| `mlivesu/cinolib` | MIT | READ OK |
+| `cg3hci/Gen-Adapt-Ref-for-Hexmeshing` | MIT | READ OK |
+| `mlivesu/HexaLab` | MIT | READ OK |
+| `crawforddoran/quartet` (isosurface stuffing) | ISC | READ OK |
+| `alecjacobson/isostuffer` | LGPL-3 | FORBIDDEN |
+| `gaoxifeng/robust_hex_dominant_meshing` | MIT | READ OK |
+| `gaoxifeng/Feature-Preserving-Octree-Hex-Meshing` | none | NO LICENCE |
+| `mlivesu/LoopyCuts` | none | NO LICENCE |
+| `LIHPC-Computational-Geometry/evocube` | GPL-3 | FORBIDDEN |
+| `LIHPC-Computational-Geometry/dds-hexmeshing` (HexMe-related workflow) | MIT | READ OK |
+| `fprotais/robustPolycube`, `ultimaille`, `marchinghex` | AGPL-3 | FORBIDDEN |
+| `ssloy/invertible-maps` (foldover-free) | AGPL-3 | FORBIDDEN |
+| libHexEx (RWTH) | GPLv3 | FORBIDDEN |
+| `cgg-bern/libHexHex` | GPL | FORBIDDEN |
+| `HendrikBrueckler/QGP3D`, `MC3D`, `TrulySeamless3D`, `C4HexMeshing` | GPL-3 | FORBIDDEN |
+| `mlivesu/CoMISo` (mixed-integer solver fork) | GPL-3 | FORBIDDEN |
+| Graphite (`BrunoLevy/GraphiteThree`) | GPL-2 | FORBIDDEN |
+| OpenVolumeMesh, CGoGN | Not checked in this pass. Believed LGPL. | Treat as FORBIDDEN until verified |
+
+## Unverified items
+- Stellar (Klingner's mesh improvement code): licence not checked.
+- `https://people.eecs.berkeley.edu/~jrs/papers/edge.pdf`: listed on Shewchuk's page but not fetched.
+- The DualCube code repo: 404 at the name given in search results.
+- The HexMe workflow repo URL: not stated in the paper. `dds-hexmeshing` is a related LIHPC repo, not confirmed as the
+  HexMe workflow.