shape-key-blend

Shape-Key Blend

A runnable example that authors a relative shape key entirely through the data API — shape_key_add, per-vertex key_blocks["Tall"].data[i].co, and .value — then reads the blend back from the depsgraph-evaluated mesh. The Tall key both lifts and flares the top face, so the silhouette is a truncated pyramid.

What it witnesses: shape keys do not rewrite mesh.vertices. The undeformed mesh stays at Basis; every evaluated vertex matches the closed-form blend co = basis + value × (key − basis). The check also asserts the flared top half-extent (0.5 + value × flare) so a uniform-scale mistake cannot pass.

Run

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

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

It exits non-zero on failure (missing keys, wrong value, per-vert blend mismatch, or flare miss). The blender-smoke workflow runs the check on Blender 4.5 LTS and 5.1.