gn-modifier-inputs

GN Modifier Inputs

A runnable example that attaches one Geometry Nodes tree to three carrier objects and writes a per-modifier Float Height input through the version-appropriate API. The tree is a parametric spiral staircase: from Height it derives the step count (round((Height - 0.3) / 0.1)), instances the carrier mesh — one bevelled oak tread with its brass baluster — up a helix at 22.5° per step, and adds a teal newel post and a brass helical handrail. Written heights 1 / 2 / 3 m give 7 / 17 / 27 treads, so the three staircases read left to right as one design at three settings of one input.

The closed form: the newel post runs from the floor at z=0 to exactly z=Height and every other part stays inside that span, so the evaluated Z-extent must equal the written height. If a write did not land, that staircase falls back to the socket default (1 m, 7 treads) and the extents are no longer distinct — in the render, the tall staircases collapse to copies of the short one.

What it witnesses: Blender 5.2 removed ID-property assignment on NodesModifier. mod["Socket_1"] = 2.0 raises TypeError rather than silently no-opping. 4.5 LTS and 5.1 still require that dict form; mod.properties does not exist there (AttributeError). 5.2+ writes mod.properties.inputs.Socket_1.value. After the write, the depsgraph must be updated or evaluated_get still sees the previous height.

Follows geometry-nodes-python.

Staging: the default dark studio. The three staircases stand on 1.62 m centres (XS); the check reads only each object's evaluated Z extent and zmin, never X, so the layout is free. All three share one tree and one set of materials — the only thing that differs between them is the modifier input. The key carries a 36° spread so its pool stays on the stairs, and the rim sits high behind them so it does not wash the foreground floor.

Run

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

# Portable falsifier: write 1.0 to every modifier. Must exit non-zero.
# (--same-scale is kept as an alias for the pre-staircase flag name.)
blender --background --python gn_modifier_inputs.py -- --same-height

# Force one side of the split (must fail on the other series, not all three):
blender --background --python gn_modifier_inputs.py -- --api dict
blender --background --python gn_modifier_inputs.py -- --api rna

# Also render a still (EEVEE on a GPU host; use --engine cycles on GPU-less hosts):
blender --background --python gn_modifier_inputs.py -- --output stairs.png
blender --background --python gn_modifier_inputs.py -- --output stairs.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	Height input identifier missing on the tree interface
4	Modifiers do not share one node_group
5	Version-path write raised (--api dict on 5.2, --api rna on 4.5)
6	Version-path read raised
7	Readback ≠ intended height (--same-height lands here)
8	Evaluated Z-extent ≠ intended height
9	Evaluated mesh not sitting on z=0
10	Gallery framing violation
11	Evaluated extents not distinct
12	--output produced no file

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, --same-height, or --api dict/rna.

Falsification

Probe	Binary	Result
--api dict	5.2.1 LTS	exit 5, TypeError: id properties not supported for this type
--api rna	4.5.11 LTS	exit 5, AttributeError: 'NodesModifier' object has no attribute 'properties'
--api dict	5.1.2	exit 0 (5.1 still takes the dict form)
--api rna	5.1.2	exit 5, AttributeError: 'NodesModifier' object has no attribute 'properties'
--same-height	5.2.1, 5.1.2, 4.5.11	exit 7, readback 1.0 != written 2.0 on SpiralStair.H2
RAIL_H = -0.5 (treads climb past the post)	5.2.1, 4.5.11	exit 8, evaluated Z-extent 1.934334 != height 1.0
--api auto	5.2.1, 5.1.2, 4.5.11	exit 0, extents 1.000 / 2.000 / 3.000