attribute-domain-shear

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: dual pinwheel from the same closed-form palette the check asserts. CORNER (left) holds eight crisp petals to the hub; naive POINT (right) smears — petal colors bleed across the shared hub and ring verts into a swirl. The broken state is in-frame by design: the right fan is the falsification variant. Fully matte petal materials (Specular IOR Level = 0) so the flat color data carries no specular line, per docs/VISUAL-STYLE.md.

Run

blender --background --python attribute_domain_shear.py --
blender --background --python attribute_domain_shear.py -- --output shear.png
blender --background --python attribute_domain_shear.py -- --output shear.png --engine cycles

Exits non-zero on failure. The blender-smoke workflow runs the check on Blender 4.5 LTS and 5.1. The --output render path additionally measures framing against the Layer 1 band via examples/gallery_framing.py (exit 10 on violation) before writing the still.