sky-texture-sun-elevation

Sky Texture Sun Elevation

A runnable example that builds a World whose Background Color is driven by ShaderNodeTexSky, then proves sun_elevation is load-bearing with two tiny Cycles EXR zenith probes — and stages a dual-elevation gallery diptych so the contract is visible at thumbnail scale.

What it witnesses: the Sky Texture contract AI-generated lighting code most often gets wrong across the 4.5 LTS → 5.1 window.

• Sky must drive Background Color. A near-black Background Strength alone is not a sky — the check asserts the Sky → Background → World Output links.
• sky_type renamed. 4.5 LTS uses NISHITA; 5.1 replaced it with MULTIPLE_SCATTERING (Nishita's successor). Assigning NISHITA on 5.1 fails; the enum no longer lists it.
• dust_density → aerosol_density. 4.5 exposes dust_density; 5.1 raises AttributeError and ships aerosol_density instead. AI code still emits dust_density.
• sun_elevation brightens zenith. Two CPU Cycles OPEN_EXR probes with a straight-up camera, Background Strength 0.05 (unclipped), compare zenith luminance at 8° vs 55°. Measured rise 2.25x on 5.1.2 and 1.50x on 4.5.11 LTS (gate ≥ 1.25).

What each check catches on failure: broken Sky→Background link (exit 6), wrong sky_type for the running version (exit 3), lost sun_elevation round-trip (exit 4), dust_density / NISHITA present on the wrong side of 5.0 (exit 5), a non-working sky (zenith floor, exit 7), and an elevation that does not brighten zenith (exit 8 — rise below 1.25).

Version witness: sky_type and the dust/aerosol rename are the divergence; sun_elevation itself is stable. Zenith rise differs by model (Nishita vs multiple scattering) but clears the same gate on both binaries.

Stage deviation

World carries a Nishita / multiple-scattering Sky Texture instead of the default near-black Background — the contract is the sky. Gallery still is a dual-elevation diptych (8° | 55°) so failure (identical panels) is visible at thumbnail scale. Layer 1 still holds: view_transform='Standard', designed terracotta materials, chosen camera, no helpers in frame.

Framing deviation

The hero is the sky itself — a world contract, not an object on the stage. The silhouette matte can only measure the cone reference props (0.369x / 0.489y), so the out-of-band number is a measurement artifact, not the composition; if ever wired to examples/gallery_framing.py, this example reports with deviation="world-contract subject; matte sees only the cone props" rather than enforcing.

Run

# Zenith-luminance correctness check (tiny Cycles CPU EXR probes) — the CI check:
blender --background --python sky_texture_sun_elevation.py --

# Falsifier: Sky→Background unlinked. Must exit non-zero (world links).
blender --background --python sky_texture_sun_elevation.py -- --unlink-sky

# Also render the gallery diptych (Cycles):
blender --background --python sky_texture_sun_elevation.py -- --output sky.png

Exit codes

Per-script sequential checks. 9 is a valid check code; there is no rule against it.

Code	Meaning
0	Success
1	Uncaught exception (FATAL wrapper)
2	argparse / usage
3	sky_type is not the identifier for this Blender
4	sun_elevation round-trip failed
5	dust_density / aerosol_density / NISHITA / MULTIPLE_SCATTERING trap
6	Sky → Background → World Output links broken (--unlink-sky lands here)
7	High-elevation zenith luma below floor
8	Zenith luma did not rise with elevation
9	--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 or --unlink-sky.