bmesh-gear

Bmesh Gear

A runnable example that builds a 14-tooth gear entirely with bmesh — profile ring, face, extrude_face_region, translate — following the ownership contract from mesh-editing-and-bmesh and the always-free-bmesh rule: every bmesh.new() is paired with bm.free() in a try/finally.

What it witnesses: parametric bmesh construction has exactly predictable topology. The check asserts the closed-form counts — verts = 2 × (4 × teeth), faces = sides + 2 caps, edges = 3 × profile — and that the result is watertight (every edge borders exactly two faces). If an op leaks geometry or a face fails to close, the math catches it.

The still leans the gear back on a matte inclined display wedge. Both contacts are derived from the posed mesh: the lowest back-cap vertex sits 3 mm into the floor, and the wedge's slope lies in the back-cap plane, so the lean is carried rather than held in the air.

The finish follows how a gear blank is actually cut, and the render path changes only the material and lights — the mesh the check counts is untouched. The caps are lathe-faced: turning marks run concentric about the gear's own axis (object coordinates, so they centre on that axis rather than on a bounding-box corner), far finer than a pixel, so they read only as a narrow satin roughness band and a whisper of bump — the soft highlight that fans from the centre. The tooth flanks are hobbed, a rougher finish that spreads the light across every facet. The key highlight on the face is a softbox placed on the camera ray's reflection about the front cap, and a low bounce card gives the downward flanks something to mirror. There is no bore: the closed-form topology check counts exactly two rings and two caps, so the gear stays a solid blank rather than faking a hole in the shader.

Run

# Cheap correctness check (no render) — the CI check:
blender --background --python bmesh_gear.py --

# Falsifier: skip the extrude. Must exit non-zero (topology).
blender --background --python bmesh_gear.py -- --no-extrude

# Also render a still (EEVEE on a GPU host; use --engine cycles on GPU-less hosts):
blender --background --python bmesh_gear.py -- --output gear.png
blender --background --python bmesh_gear.py -- --output gear.png --engine cycles

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 (FATAL wrapper)
2	argparse / usage
3	Topology ≠ closed form (--no-extrude lands here)
4	Non-manifold edges
6	--output produced no file
10	--output framing violation (Layer 1 fill / margin gate, gallery_framing)

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 --output or --no-extrude.