prop-origin-transform

Prop Origin Transform

A runnable example that builds a street utility pedestal with a flanged conduit elbow and proves the origin / scale-apply / MPI contract a prop pipeline relies on before engine ingest — following operators and the data-API parenting idiom from parent-inverse-orrery.

Pipeline arc neighbors: MPI + stale matrix_world in parent-inverse-orrery, export origin sensitivity in gltf-export-roundtrip, collision bounds in collision-hull-proxy, and mesh hygiene in mesh-hygiene-audit. Origin at the base center with applied scale is what placement and physics ingest assume.

Scope: this witnesses the bpy-level contract a prop pipeline relies on (post-bake scale exactly (1,1,1), local bbox min.z == 0, world bbox unchanged, MPI so a parented accessory does not teleport). It is not an engine exporter — it proves the transform properties such an asset must have.

What it witnesses:

• Stale matrix_world. After a location edit, matrix_world is unchanged until view_layer.update() (same half of the orrery contract).
• Scale apply via data API. Non-uniform scale baked into verts; obj.scale == (1,1,1) exactly afterward.
• Origin at base center. Local bbox min.z == 0, XY centered; world AABB delta across the bake is 0 (gate 1e-5).
• Bare-parent trap. Parenting the accessory without MPI jumps it (0.298 m measured); setting matrix_parent_inverse restores world location (err 0).

What each check catches on failure: skipping scale apply leaves (1.15, 0.92, 1.08) (exit 5); skipping the origin bake leaves min.z = -0.58 (exit 6); skipping MPI leaves the accessory 0.298 m off (exit 8).

Version witness: identical numbers on Blender 5.2.1 LTS, 5.1.2 and 4.5.11 LTS.

The asset. The pedestal is one mesh with five materials: a precast concrete footing, a galvanised base plate with anchor nuts, a green cabinet on a black kick skirt, a hipped lid, a door with hinges, a T-handle and a hazard plate, louvred side vents, a conduit boss on the +X face and a sleeve in the pad. It is authored the way an import often arrives: origin at the geometric centre and coordinates divided by a non-uniform object scale. The bake lands it on exactly the designed shape. The accessory is a flanged PVC conduit elbow with bolt heads, a strap clamp and a coupling that drops into the sleeve.

The render. Two baked pedestals stand on a sidewalk slab. On the left the elbow is parented with MPI and stays bolted to its boss, seated in the sleeve. On the right, bare child.parent = parent applies the pedestal's transform a second time. The elbow is thrown 0.65 m, past the slab edge and into the air, and a glowing cyan outline (a Wireframe modifier on a copy) marks the seat it left. The slab raises both origins 0.20 m, so the doubled translation lifts the elbow instead of sliding it along the floor. A pivot ring on the floor is centred on each pedestal's origin, with red notches on its X axis and green notches on Y. The origin is under the base, so the ring frames it. There are no text labels: the ghost and the stranded elbow carry the proof. The check's closed forms are unchanged.

Run

blender --background --python prop_origin_transform.py --
blender --background --python prop_origin_transform.py -- --skip-mpi
blender --background --python prop_origin_transform.py -- --output origin.png
blender --background --python prop_origin_transform.py -- --output origin.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.

Code	Meaning
0	Success
1	Uncaught exception (FATAL wrapper)
2	argparse / usage
3	Stale matrix_world contract broken
4	World bbox moved across bake
5	Scale after bake is not (1,1,1)
6	Origin not at local base center
7	Bare parenting did not jump
8	MPI did not restore world location (--skip-mpi lands here)
9	--output produced no file
10	Gallery framing violation
11	Gallery asset-quality violation (render path only)

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 --skip-mpi.