ConformalLabpp/doc/reviewer/usability-audit-2026-05-31.md

# Usability & Documentation Audit — ConformalLabpp v0.10.0

**Date:** 2026-05-31
**Auditor:** External reviewer (Claude Sonnet 4.6)
**Branch:** `review/usability-audit-2026-05-31`
**Base:** `main` (post external-audit-2026-05-30 merge)
**Focus:** Documentation quality, API usability, new-user experience

This document is self-contained. A new session can pick up any finding
and act on it without prior context. Each finding includes exact file paths,
the concrete problem, and a precise fix with acceptance criteria.

Status legend: 🔴 Usability bug · 🟡 Doc error/stale · 🟠 Missing content

---

## How to read this document in a new session

```bash
git checkout review/usability-audit-2026-05-31

# No build needed for most fixes — documentation and example changes only.
# For changes that touch compilable code (examples), verify with:
cmake -S code -B build-cgal -DWITH_CGAL_TESTS=ON -DCONFORMALLAB_LOW_MEMORY_BUILD=ON
cmake --build build-cgal --target conformallab_cgal_tests -j1
ctest --test-dir build-cgal -R '^cgal\.' --output-on-failure
```

---

## FINDING-U1 — 🔴 CRITICAL: All examples show the trivial identity map, not real conformal flattening

### Location
- `README.md` lines 100–121 ("Minimal usage")
- `code/examples/example_euclidean.cpp` lines 72–83 (Step 4)
- `code/examples/example_layout.cpp` lines 40–49 (`set_natural_theta`)
- `code/examples/example_hyper_ideal.cpp` lines 89–98 (Step 3)

### Problem

Every example uses the "natural theta" trick:
```cpp
auto G0 = euclidean_gradient(mesh, x0, maps);
for (auto v : mesh.vertices())
    if (maps.v_idx[v] >= 0)
        maps.theta_v[v] -= G0[maps.v_idx[v]];
```

This sets Θ_v to the *actual angle sums at x=0*, making x*=0 the equilibrium.
The result: `u_v ≈ 0` everywhere — **no deformation**. The map is identity.

Natural theta is a valid test pattern (it proves Newton converges in 0 or 1
steps and guards against solver regressions). But as the *only* pattern shown to
a new user, it gives the false impression that the library produces trivial
output. The real use case — **conformally flatten a surface to the plane** —
is never demonstrated.

### What is missing

A "real-world" example that shows the actual use case:

```cpp
// Conformally flatten a mesh to the plane.
// Set Θ_v = 2π (regular interior vertex, zero cone angle defect) for all
// free vertices; enforce_gauss_bonnet makes the assignment topologically
// consistent.  The result is a flat conformal map with minimal angle distortion.
for (auto v : mesh.vertices())
    maps.theta_v[v] = conformallab::TWO_PI;
conformallab::enforce_gauss_bonnet(mesh, maps);
auto res = conformallab::newton_euclidean(mesh, x0, maps);
auto layout = conformallab::euclidean_layout(mesh, res.x, maps);
// layout.uv[v.idx()] now holds 2D coordinates of the flattening
```

### Fix

1. **Add a new example** `code/examples/example_flatten.cpp`:
   - Takes a real mesh from `code/data/` (e.g. `cathead.obj` or `torus_8x8.off`)
   - Sets Θ_v = 2π, calls `enforce_gauss_bonnet`, solves Newton, computes layout
   - Saves a UV-mapped OFF file
   - Shows non-trivial u_v values at output

2. **Update README "Minimal usage"** to show the flatten use case instead of
   (or in addition to) natural theta.  Add a comment explaining what natural
   theta is and when to use it.

3. **Add a comment** to all existing examples explaining that natural theta
   is a *testing convenience*, not a typical use case.

### Acceptance criteria
- [ ] `example_flatten.cpp` compiles and runs with a real mesh input
- [ ] Output shows non-trivial `u_v ≠ 0` and a visible UV parameterisation
- [ ] README "Minimal usage" demonstrates real flattening (not identity)
- [ ] Existing examples have a comment: "Note: natural theta → x* = 0 (testing
      pattern). For real flattening, set theta_v = TWO_PI and call enforce_gauss_bonnet."

---

## FINDING-U2 — 🔴 CRITICAL: No example for the CGAL public API

### Location
- `code/include/CGAL/Discrete_conformal_map.h` (the "user-facing entry")
- `code/examples/` (no example uses CGAL::discrete_conformal_map_euclidean)

### Problem

`Discrete_conformal_map.h` is documented as "User-facing entry point for the
Discrete_conformal_map package."  The Doxygen doc shows a minimal usage snippet:

```cpp
auto result = CGAL::discrete_conformal_map_euclidean(mesh);
```

But this snippet only exists in a Doxygen comment — no compilable example
demonstrates it. All four `code/examples/*.cpp` use the *internal* API
(`setup_euclidean_maps + newton_euclidean`). A CGAL user who reaches this
library via the CGAL ecosystem has no runnable starting point.

The CGAL API and the internal API also produce *different result types*:
- Internal: `NewtonResult.x` — DOF-index-indexed `std::vector<double>`
- CGAL: `Conformal_map_result.u_per_vertex` — vertex-index-indexed `std::vector<FT>`

This discrepancy is not explained anywhere for users.

### Fix

Add `code/examples/example_cgal_api.cpp`:
```cpp
// Demonstrates the CGAL public API (Discrete_conformal_map.h).
// Contrast with example_euclidean.cpp which uses the internal API directly.

#include <CGAL/Simple_cartesian.h>
#include <CGAL/Surface_mesh.h>
#include <CGAL/Discrete_conformal_map.h>
// ...
auto result = CGAL::discrete_conformal_map_euclidean(mesh);
// result.u_per_vertex[v.idx()] — per-vertex scale factor
// result.converged, result.iterations, result.gradient_norm
```

Also add to README: a short paragraph explaining when to use CGAL API
vs internal API (CGAL API: simpler, less control; internal API: full pipeline,
holonomy, period matrix).

### Acceptance criteria
- [ ] `example_cgal_api.cpp` compiles with `-DWITH_CGAL=ON`
- [ ] Example uses `CGAL::discrete_conformal_map_euclidean` (not internal API)
- [ ] README explains the two API levels and when to use each

---

## FINDING-U3 — 🟡 DOC ERROR: `contracts.md` incorrect after Finding-B

### Location
`doc/api/contracts.md` line 16

### Problem

```markdown
| `check_gauss_bonnet()` | `theta_v[v]` set for all vertices |
  Throws `std::runtime_error` if Σ(2π−Θᵥ) ≠ 2π·χ(M) |
```

After the external-audit-2026-05-30 Finding-B fix, calling
`check_gauss_bonnet(mesh, hyper_ideal_maps)` is a **compile error**
(the overload is `= delete`).  The contract table implies it works for
all map types — which is now false.

Also missing from the table: the HyperIdeal functional needs no
Gauss-Bonnet pre-check because its energy is strictly convex
(Springborn 2020 Theorem 1.3) — Newton converges for any target angles.

### Fix

```markdown
| `check_gauss_bonnet()` | `theta_v[v]` set · **Euclidean or Spherical maps only** |
  Throws if Σ(2π−Θᵥ) ≠ 2π·χ(M). **Not applicable to HyperIdealMaps** —
  deleted overload; HyperIdeal needs no pre-check (strictly convex energy). |
```

Also add a row for `enforce_gauss_bonnet`:
```markdown
| `enforce_gauss_bonnet()` | `theta_v[v]` set · **Euclidean or Spherical maps only** |
  Shifts all Θᵥ by δ = (lhs−rhs)/V so that GB holds exactly.
  **Not applicable to HyperIdealMaps** — deleted overload. |
```

### Acceptance criteria
- [ ] `check_gauss_bonnet` row notes "Euclidean/Spherical only, not HyperIdeal"
- [ ] `enforce_gauss_bonnet` row added with same note
- [ ] A brief explanation why HyperIdeal doesn't need GB: "strictly convex energy"

---

## FINDING-U4 — 🟡 DOC ERROR: Stale version in README (v0.9.0)

### Location
`README.md` line 17

### Problem

```markdown
**Status:** v0.9.0 — Phases 1–9a complete, Phase 8b-Lite CGAL API surface.
```

The current release is **v0.10.0** (tagged on `main`; CLAUDE.md confirms this).
CLAUDE.md was updated to v0.10.0 but the README was not.

### Fix

```markdown
**Status:** v0.10.0 — Phases 1–9b complete. Newton solvers for five DCE
models (Euclidean / Spherical / HyperIdeal / CP-Euclidean / Inversive-Distance),
priority-BFS layout in ℝ²/S²/Poincaré disk, Gauss–Bonnet, tree-cotree cut
graph, Möbius holonomy, period matrix (genus 1), fundamental domain,
halfedge_uv texture atlas, JSON/XML serialisation, CLI app.
277 tests passing, 0 skipped — see [`doc/api/tests.md`](doc/api/tests.md).
```

### Acceptance criteria
- [ ] README status line shows v0.10.0 and the correct feature list
- [ ] Test count updated to 277

---

## FINDING-U5 — 🟡 DOC ERROR: CGAL header comment describes superseded state

### Location
`code/include/CGAL/Discrete_conformal_map.h` lines 14–16

### Problem

```cpp
/// This header provides a single function — `discrete_conformal_map_euclidean`
/// — that computes a Euclidean discrete-conformal flattening … Spherical and
/// hyperbolic variants are scheduled for Phase 8b.2 once the Euclidean
/// pattern is validated
```

But the file already contains `discrete_conformal_map_spherical()` and
`discrete_conformal_map_hyper_ideal()` (and the other models).  The comment
describes Phase 8a state; the file is at Phase 8b-Lite.

### Fix

Update the `\file` Doxygen block at the top of `Discrete_conformal_map.h`:

```cpp
/*!
\file CGAL/Discrete_conformal_map.h
\ingroup PkgConformalMapRef

User-facing entry points for the Discrete_conformal_map package.

This header provides five functions covering all three DCE geometries:
  - `CGAL::discrete_conformal_map_euclidean`   (flat / ℝ²)
  - `CGAL::discrete_conformal_map_spherical`   (S², genus 0)
  - `CGAL::discrete_conformal_map_hyper_ideal` (H², genus ≥ 1)
  - `CGAL::discrete_circle_packing_euclidean`  → Discrete_circle_packing.h
  - `CGAL::discrete_inversive_distance_map`    → Discrete_inversive_distance.h
...
*/
```

### Acceptance criteria
- [ ] `\file` doc block lists all five entry functions
- [ ] No forward-reference to "planned Phase 8b.2" — it is already implemented

---

## FINDING-U6 — 🟡 STALE EXAMPLES: New gauge-vertex overload not reflected

### Location
- `README.md` lines 107–110
- `code/examples/example_euclidean.cpp` lines 62–68
- `code/examples/example_layout.cpp` lines 52–59 (`pin_first` helper)

### Problem

Finding-D (external-audit-2026-05-30) added a clean gauge-vertex overload:
```cpp
assign_euclidean_vertex_dof_indices(mesh, maps, gauge_vertex);
```

But all examples still use the old verbose loop:
```cpp
auto vit = mesh.vertices().begin();
maps.v_idx[*vit++] = -1;   // pinned
int idx = 0;
for (; vit != mesh.vertices().end(); ++vit)
    maps.v_idx[*vit] = idx++;
```

A new user copying from the examples will write the old error-prone pattern
instead of the new clean overload — defeating the purpose of the fix.

### Fix

Replace the old loop in all three locations with the new overload:
```cpp
// Pin the first vertex, assign sequential DOF indices to the rest.
int n = assign_euclidean_vertex_dof_indices(mesh, maps,
                                            *mesh.vertices().begin());
```

Same for `assign_vertex_dof_indices` (spherical) and
`assign_inversive_distance_vertex_dof_indices` wherever they appear in examples.

### Acceptance criteria
- [ ] README "Minimal usage" uses the overload
- [ ] `example_euclidean.cpp` uses the overload
- [ ] `example_layout.cpp` `pin_first` helper replaced with the overload
- [ ] Old manual loop removed from all user-facing code

---

## FINDING-U7 — 🟠 MISSING: `CONFORMALLAB_LOW_MEMORY_BUILD` absent from `getting-started.md`

### Location
`doc/getting-started.md` — "Build modes" section

### Problem

`getting-started.md` documents five compile-time modes but does not list
`CONFORMALLAB_LOW_MEMORY_BUILD=ON`.  A Raspberry Pi or low-RAM user who
reads `getting-started.md` (the natural first stop) cannot find the flag.
It only appears in CLAUDE.md and the README table.

### Fix

Add to `getting-started.md` after the `CONFORMALLAB_FAST_TEST_BUILD` entry:

```markdown
### Low-memory build (RAM-constrained CI / Raspberry Pi)

For machines with ≤ 4 GB RAM, the default CGAL build (PCH + -O3) OOMs
the compiler.  `LOW_MEMORY_BUILD` uses `-O0`, disables PCH, and sets
unity batch size to 1, keeping `cc1plus` peak at ~150-200 MB per TU:

```bash
cmake -S code -B build -DWITH_CGAL_TESTS=ON \
      -DCONFORMALLAB_LOW_MEMORY_BUILD=ON
cmake --build build --target conformallab_cgal_tests -j1
```

Tests run ~15× slower at -O0 but all pass.  Useful for CI runners with
limited memory.
```

### Acceptance criteria
- [ ] `getting-started.md` documents `CONFORMALLAB_LOW_MEMORY_BUILD`
- [ ] Example command shows `-j1` (required for memory safety)

---

## FINDING-U8 — 🟠 MISSING: `LOW_MEMORY_BUILD` absent from README compile-time table

### Location
`README.md` lines 53–82 ("Compile-time workflow modes")

### Problem

The README code block shows five compile-time patterns but omits
`CONFORMALLAB_LOW_MEMORY_BUILD`.  This is the flag that re-enabled CI
and is directly relevant to anyone building on resource-constrained hardware.

### Fix

Add to the README compile-time examples:
```bash
# Low-memory build: -O0, no PCH, unity batch 1 (~150 MB peak per cc1plus).
# Use on Raspberry Pi / CI runners with ≤ 4 GB RAM.  Tests run ~15× slower.
cmake -S code -B build -DWITH_CGAL_TESTS=ON -DCONFORMALLAB_LOW_MEMORY_BUILD=ON
cmake --build build --target conformallab_cgal_tests -j1
```

### Acceptance criteria
- [ ] README shows `LOW_MEMORY_BUILD` usage with a one-line explanation

---

## FINDING-U9 — 🟠 MISSING: `layout.uv` indexing semantics not documented

### Location
`code/examples/example_layout.cpp` line 127
`code/include/layout.hpp` — `Layout2D` struct definition

### Problem

```cpp
auto& p = layout.uv[v.idx()];  // example_layout.cpp:127
```

`Layout2D.uv` is a `std::vector` indexed by the raw integer `.idx()` of
`Vertex_index`.  This is undocumented: nowhere is it stated that the vector
length equals `num_vertices(mesh)` and that the index is contiguous.  In CGAL,
`vertex.idx()` is contiguous on a fresh `Surface_mesh` but may have gaps after
vertex removals.

The `halfedge_uv` field (also in `Layout2D`) has the same issue for halfedge
indices.

### Fix

In `layout.hpp`, add explicit documentation to the `Layout2D` struct:

```cpp
struct Layout2D {
    /// UV coordinate of vertex v in the layout plane.
    /// Indexed by `v.idx()` — length equals `mesh.number_of_vertices()`.
    /// \pre  No vertices have been removed from `mesh` since construction
    ///       (i.e., `mesh.is_valid(false)` and no compaction was performed).
    std::vector<Eigen::Vector2d> uv;

    /// UV of source(h) as seen from face(h), indexed by `h.idx()`.
    /// At seam halfedges the two opposite halfedges carry different UV values,
    /// enabling proper per-halfedge UV for GPU texture atlasing.
    std::vector<Eigen::Vector2d> halfedge_uv;
    ...
};
```

Also update `example_layout.cpp` to add a comment:
```cpp
// layout.uv is indexed by v.idx() — valid as long as no vertices were
// removed from the mesh after loading.
auto& p = layout.uv[v.idx()];
```

### Acceptance criteria
- [ ] `Layout2D.uv` and `Layout2D.halfedge_uv` have explicit Doxygen docs
      stating the index semantics and the no-compaction precondition
- [ ] `example_layout.cpp` has an inline comment at the access site

---

## Summary table

| ID | File | Type | Severity | Status |
|----|------|------|----------|--------|
| U1 | README + 4 examples | Usability | 🔴 Critical | ✅ Fixed 2026-05-31 |
| U2 | examples/ (missing) | Usability | 🔴 Critical | ✅ Fixed 2026-05-31 |
| U3 | `contracts.md:16` | Doc error | 🟡 Medium | 🟡 Open |
| U4 | `README.md:17` | Stale | 🟡 Medium | 🟡 Open |
| U5 | `Discrete_conformal_map.h:14` | Stale | 🟡 Medium | 🟡 Open |
| U6 | README + 2 examples | Stale | 🟡 Medium | 🟡 Open |
| U7 | `getting-started.md` | Missing | 🟠 Minor | 🟡 Open |
| U8 | `README.md` | Missing | 🟠 Minor | 🟡 Open |
| U9 | `layout.hpp` + example | Missing | 🟠 Minor | 🟡 Open |

**Priority order for fixing:**
1. U1 + U2 (new examples — most impactful for new users)
2. U3 (correctness — contracts.md wrong after Finding-B)
3. U4 + U5 + U6 (stale content — quick fixes)
4. U7 + U8 + U9 (missing docs — minor)