Attribute Domain Shear

A runnable example that witnesses what POINT versus CORNER means on Mesh.color_attributes once the mesh has shared vertices — the domain is not a storage detail, it decides where colors can live. Companion to color-attribute-wheel (which covers color_attributes.new() versus the deprecated alias, CORNER sizing == len(loops), and active_color); this example covers the trap one step later, when AI code knows the API exists but authors per-face colors into a POINT-domain attribute.

Pipeline arc neighbors: attribute authoring in color-attribute-wheel, mesh topology gates in mesh-hygiene-audit, tangent-space UV contracts in triangulate-tangents.

What it witnesses: a pinwheel of K=8 triangles around one raised hub vertex shared by every wedge (plus a shared outer ring). The contract, all closed form:

• Storage sizes. CORNER attr data == len(loops) == 3K; POINT attr == len(vertices) == K+1.
• CORNER authoring is exact. The hub corner of wedge i reads palette[i] within 1e-6 — K faces at one vertex may disagree there.
• POINT naive authoring shears by construction. A per-wedge authoring loop ("paint each wedge its color") rewrites every shared vertex once per neighbor, and the last write wins: the hub reads palette[K-1], ring vert i reads palette[i] — except ring vert 0, which the wrap-around last wedge rewrites to palette[K-1]. The measured mean deviation from intended equals the palette closed form (0.751031) exactly.

What each check catches on failure: wedge 3 miscolored in the CORNER pass (exit 4); naive writes reversed, so the hub reads palette[0] (exit 5); a ring vert corrupted, breaking the overwrite-ordering witness (exit 6); a constant palette, collapsing the shear so the probe cannot distinguish naive from correct (exit 7). Sizes wrong for the declared domain (exit 3).

Version witness: output is byte-identical on Blender 4.5.11 LTS and 5.1.2 — the color_attributes domain API is stable across both.

Render as proof: two striped patio parasols, painted by the same two authoring functions the check asserts. The canopy is the check's fan grown into fabric: eight gores around one shared apex vertex, each gore a strip of faces whose vertices all sit on the two seams it shares with its neighbours, so the naive loop touches exactly the kind of shared vertex the check measures. CORNER (left) keeps crisp crimson and cream stripes. Naive POINT (right) rewrites every seam and the apex with the later gore's color: the stripes smear pink along the seams, and gore 0, crimson by intent, renders cream because its seam and the apex were last written by gore 7. That gore is turned to face the camera. The broken state is in frame by design: the right parasol is the falsification variant. The canopy fabric is fully matte (Specular IOR Level = 0) so the flat color data carries no specular line, per docs/VISUAL-STYLE.md.

The render uses a two-tone palette (crimson and cream, alternating). It is not the check's palette. The check keeps its eight distinct hues because an off-by-two ordering bug would pass unnoticed under a period-2 palette. A parasol reads as broken only when stripes everyone expects to be crisp go soft. Both palettes go through the same assign_corner and assign_point_naive. Those functions take a faces_per_wedge stride (gore-major face order). It is 1 on the check's fan, so the check's writes are unchanged, and its output is identical before and after.

The staging is render-only and does not touch the checked mesh. Each parasol is eight named parts with five materials: canopy, aluminium ribs and stretchers, brass runner, finial and tilt knuckle, teak upper and lower pole, and a cast-iron base. The canopy tilts 24 degrees toward the camera about the knuckle so the whole top reads. The pair stands on a terracotta paver patio (a render-only 10 x 5 deck of beveled pavers with grout joints, two tones alternated by a fixed index hash, passed to the framing gate as stage like the floor), and each base has a slate placard on a brass post in front of it, the domain name inlaid in cream enamel. The render path runs the framing gate (exit 10) and the asset-quality floors on one parasol (exit 11).

Run#

blender --background --python attribute_domain_shear.py --
blender --background --python attribute_domain_shear.py -- --no-overwrite
blender --background --python attribute_domain_shear.py -- --output shear.png
blender --background --python attribute_domain_shear.py -- --output shear.png --engine cycles

Exit codes#

Per-script sequential checks. 9 is a valid check code; there is no rule against it. 10 is the shared framing helper and 11 the shared asset-quality helper.

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. Its catalog falsifier is --no-overwrite (expects exit 5).