ngon-triangulate

N-gon triangulate

Synthesizes one hexagon by dissolving a cube edge, then asserts the hygiene / tangent contracts on that defective mesh. Inverse of mesh-hygiene-audit (clean mesh, no ngons) and neighbor of triangulate-tangents (calc_tangents aborts on any n-gon).

Why this pathology: the audit's n-gon gate is a snapshot on clean geometry. AI-generated meshes often leave dissolved n-gons; glTF silently triangulates, so an export-only check is vacuous (12 tris either way).

Pre-assertion (pathology exists): exactly 1 face with 6 loops, 5 faces total. --no-dissolve exits 3.

Handling (second axis): Mesh.calc_tangents aborts with tris/quads until bmesh.ops.triangulate on that face; afterward 4 tris + 4 quads, 28 loops, tangents succeed. Count-only glTF tris = 12 for a cube or this mesh. --skip-triangulate exits 4.

No gallery still. A hexagon on a cube does not read at thumbnail without fake annotation.

Verified behaviour

Mesh.calc_tangents raises RuntimeError on any face with more than four loops. The message is byte-identical on 4.5.11 LTS, 5.1.2, and 5.2.1 LTS:

Error: Tangent space can only be computed for tris/quads, aborting

TANGENT_ABORT matches on the tris/quads substring, so the check survives a reword of the surrounding sentence but still fails if the abort stops happening at all.

Affected versions: all three targeted series. Nothing here is version-gated: the abort, the counts and both falsifier exits are the same on 4.5.11 LTS, 5.1.2 and 5.2.1 LTS, so neither falsifier is expected to exit 0 anywhere.

The UV map comes first. calc_tangents checks for a UV map before it looks at face sizes. On a mesh with no UV layer it fails with a different message, and the n-gon abort is never reached:

Error: Tangent space computation needs a UV Map, "(null)" not found, aborting   # 4.5.11 LTS
Error: Tangent space computation needs a UV Map, "" not found, aborting         # 5.1.2, 5.2.1 LTS

So a mesh that has both problems shows only the UV error; add a UV map and the tris/quads abort appears next. build() adds UVMap so the check witnesses the n-gon abort, and because the match is on tris/quads, a missing UV map would fail the check (exit 4) rather than pass it.

Re-verified

Run on 4.5.11 LTS, 5.1.2 and 5.2.1 LTS with a scratch probe that repeats the construction:

Claim	4.5.11	5.1.2	5.2.1
n-gon + UV map: tris/quads abort, byte-identical text	yes	yes	yes
n-gon, no UV map: UV-map error instead	"(null)"	""	""
glTF index count / 3, dissolved mesh	12	12	12
glTF index count / 3, plain cube	12	12	12
default / --no-dissolve / --skip-triangulate exit	0 / 3 / 4	0 / 3 / 4	0 / 3 / 4

Falsifiers

Each flag breaks one stage and lands on the assertion that stage feeds. Neither announces a failure; both let a real check catch the mesh.

Flag	What it breaks	Exit
--no-dissolve	never makes the n-gon, so the pre-assertion finds no pathology (ngon count 0 != 1)	3
--skip-triangulate	skips bmesh.ops.triangulate, so the handling assertion sees the n-gon survive (ngons=1 tris=0 quads=4)	4

API reference

Name	4.5 LTS	5.2 LTS
Mesh.calc_tangents	4.5	5.2
bmesh.ops.dissolve_edges	4.5	5.2
bmesh.ops.triangulate	4.5	5.2

Run

blender --background --python ngon_triangulate.py --
blender --background --python ngon_triangulate.py -- --no-dissolve
blender --background --python ngon_triangulate.py -- --skip-triangulate

Exit codes

Per-script sequential checks. 9 is a valid check code; there is no rule against it.

Code	Meaning
0	Success
1	Uncaught exception (the __main__ wrapper prints ERROR: <type>: <message>)
2	argparse / usage
3	Pathology missing: n-gon count, loops, or face count (--no-dissolve lands here)
4	calc_tangents / triangulate handling (--skip-triangulate lands here, via the real handling assertion)

The blender-smoke workflow runs the check on Blender 5.2 LTS and 4.5 LTS (5.1 on the weekly cron, the needs-5.1 PR label, or manual dispatch). Smoke does not pass --no-dissolve or --skip-triangulate.