Parent Inverse Orrery

A runnable example that assembles a brass orrery — sun, three planets on pivot arms, one moon — entirely through the data API, witnessing the parenting contract that generated Blender code gets wrong most often. child.parent = pivot alone re-interprets the child's local matrix in the pivot's space and the child visibly teleports; keeping the world transform takes the two-line idiom:

child.parent = pivot
child.matrix_parent_inverse = pivot.matrix_world.inverted()

What it witnesses: the check first proves the trap is real (a bare-parented probe jumps by more than half a unit), then that the idiom restores the probe's world position to within 1e-5. It also asserts the second half of the contract — matrix_world is the last-evaluated matrix, stale after any transform edit until bpy.context.view_layer.update() — and finally that every planet and the two-level moon land exactly on their closed-form orbit positions after the pivots spin.

Run#

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

# Falsifier: parent without MPI. Must exit non-zero (orbit closed form).
blender --background --python parent_inverse_orrery.py -- --skip-mpi

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

Staging#

Render path only; the rig the checks measure is untouched. The still uses the Standard view transform: under the default AgX the orange sun clipped to a peach-white bulb and read as a lamp. Lighting follows the house recipe (shaped key, faint fill, rim, warm wedge on the back wall) on the dark matte studio floor; the old rig (key 1500 W, fill 500 W, a glossy grey floor) lit the frame to mean luma 0.33, above the gallery's calibration band. The render path gates framing through gallery_framing.check_framing (exit 10), which it never did before.

Exit codes#

Per-script sequential checks. 9 is a valid check code; there is no rule against it. 10 is the shared framing 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 --skip-mpi (expects exit 6).