Pre-reg — TRUE arm surface texture via the mjSpec recompile path

2026-08-14, drafted 09:2xZ, posted in-channel before the gate read. Queue item sim-arm-surface-texture-mjspec; the escalation registered by the micro-texture refutation (05:4xZ 08-14): statistically-matched screen-space grain read MORE fake — the encoder wants coherent surface-tracking structure, not matched marginals. This read prices that structure.

Plain words. The real robot arm is 3D-printed, and printing leaves fine horizontal ridges — layer lines — on every surface. Our simulated arm is perfectly smooth. Last time we tried to fake the ridges by sprinkling matched noise over the arm’s pixels on screen, and the policy’s vision encoder called the bluff: the sprinkle looked less real, because the encoder cares about structure that sticks to the surface, not about pixel statistics. So this time we did it properly: we baked a real striped texture into the arm’s materials inside the physics model itself, so the stripes live on the surface and move with it. The physics is provably untouched — the recompiled model is bit-identical in every physical field. Now we render 100 identical scene pairs — smooth arm vs striped arm — and ask the encoder which looks more like real robot footage.

What landed before this read (instrument facts, commit e408f9e)

• arm_texture='v2': a quasi-periodic layer-line texture asset (256², pinned private RNG seed 20260814) built at compile time and cube-mapped onto the 18 PLA link materials via an mjSpec recompile. Cube shrink-wrap anchors the bands in OBJECT space — they track the surface through every pose, the property the refutation demanded and screen-space fields cannot have. Servo casings out of scope (their residual is a specular glint tail; no specular-map path in the classic renderer).
• Physics hard bar: 11/11 oracles green — every physics field of the recompiled model bit-equal to the from_xml_path baseline, ids unrenumbered, qpos trajectories bit-equal over settled resets AND a scripted 60-tick excursion, shared RNG streams untouched.
• Zero-clip generator: tanh soft-bound keeps the texture strictly inside center·(1±amplitude) — zero clipped texels (clipping would silently break the grade-time mean compensation; the renderer MODULATES lit color by the texture, so PLA albedo is divided by the realized texture mean and the fitted photometric grade is preserved exactly).
• Reflection rider (registered): the tabletop reflects the arm (mat_reflectance 0.02), and a TRUE surface texture rides that physical light path — mechanism confirmed by zeroing the reflectance (0 out-of-halo diff px). The locality oracles therefore allow out-of-arm-halo diffs ONLY on the dilated reflective-geom mask, |Δ| ≤ 24, < 1% of frame; anything else is a leak and aborts.
• Fit honesty (reports/analysis__arm_surface_texture_fit.json): period chosen by lc-response probe over {6, 10, 16, 24, 32} texture rows/line — the response is monotonic in period (fine bands die in the composite blur chain); 32 is frozen at the registered plausibility bound (≥ 8 bands per link face; coarser stops being print-layer-like). Amplitude quadrature-solved on the real PLA local-contrast median 8.36 and capped at the 0.42 no-clip headroom: realized lc 6.43 vs 4.66 graded — the instrument closes ~41% of the quadrature lc gap and cannot close the rest without clipping (same finding class as the v1 servo contrast cap). This read prices a partial-fidelity coherent texture, stated as such.

Question

Does TRUE surface-tracking layer-line structure on the printed links — at the maximum no-clip amplitude — move the top view toward real on the pinned encoder probe, where the statistically-matched screen-space grain moved it AWAY?

Design

fontaine/scripts/sim_arm_surface_texture_read.py: TWO production v3 instances (numpy post) over the same 20 seeds × 5 appearance draws — arm_photometrics='v1' alone vs grade + arm_texture='v2' — hooked at _composite exactly like the micro-texture read (arms: v3_photo / plate_only / only_links_photo vs v3_surf / only_links_surf). Encoder probe: er_60k trunk, knn5 vs held-out real-B, top camera. In-run oracles: hooked frames bit-exact vs production observations; qpos bit-equal across instances per slot; strict outside-arm-halo bit-equality off the reflective rider region; rider bounds as registered above.

Registered anchors / aborts

ABORT (no claims) unless in-run v3_photo knn5 AUROC in 0.698 ± 0.005 (the banked photometric-read anchor; same gate the micro-texture read used and passed at 0.6977). Anchors: banked v3 0.713, only_links_photo 0.652, real_fg 0.328, clean anchor; the REFUTED micro-texture primary +9.33e-07 CI95 [+8.27, +10.42]e-07 (AUROC 0.698 → 0.751) — the delta this escalation must NOT reproduce.

Decision rule (frozen before the read)

• PRIMARY: paired top Δknn5 CI95 (10k resamples, rng 0) of v3_surf vs v3_photo entirely below 0 → coherent surface-tracking structure reads more real where matched marginals read more fake — the refutation’s mechanism hypothesis is CONFIRMED; arm_texture='v2' becomes a promotion candidate at its declared partial fidelity, and closing the residual lc gap (a clip-free channel: normal-map-free band contrast is exhausted) becomes a priced follow-up.
• CI95 entirely ABOVE 0 → SECOND refutation, now WITH coherent structure: the failure was never about coherence, and the arm-texture direction goes cold at this abstraction level — v1-graded stays the arm frontier; magnitude compared against the micro-texture’s +9.33e-07.
• CI95 straddles 0 → no measurable effect at partial fidelity: the texture stays opt-in and unclaimed; the residual-fidelity follow-up is priced but NOT auto-queued (the channel is capped — a straddle at 41% of the gap does not license extrapolation).
• MECHANISM (record + interpretation aid): same rule for only_links_surf vs only_links_photo — the links-only composite is the sharper version of the same question (the micro-texture’s mechanism read was +1.30e-06, unambiguous).
• Record-only riders: only_links_surf vs plate_only; clean anchor; reflection-rider per-slot stats (max |Δ|, changed fraction).

Cost

CPU renders (100 paired slots × 2 instances + per-slot seg renders) + ~0.02 GPU-h embeds (500 sim + 400 real frames) on the er_60k trunk. GPU is idle by design (R1-A boundary pends the owner call) — the embed job does not conflict.

RESULTS (09:2xZ 08-14, executed same session — SECOND REFUTATION, adjudicated by the frozen rule)

All gates green: in-run v3_photo knn5 AUROC 0.698 dead-center in the abort band; hooked frames bit-exact vs production ×100; qpos bit-equal across instances ×100; strict outside-halo equality held (zero leak pixels); reflection rider silent in the composited output (max |Δ| 0 — the ≤14-count raw reflection is fully absorbed by the PSF blur + uint8 quantization; the rider bound was needed for the raw-frame oracles only).

• PRIMARY — CI entirely ABOVE zero → the registered second refutation. v3_surf vs v3_photo paired Δknn5 +3.07e-07, CI95 [+2.42, +3.71]e-07 (14/100 slots closer); AUROC 0.698 → 0.718. TRUE surface-tracking structure at the maximum no-clip amplitude reads MORE fake, exactly like the screen-space grain did — the coherence hypothesis is disconfirmed as the missing ingredient. Per the frozen rule: the arm-texture direction goes cold at this abstraction level; arm_photometrics='v1' (0.698) stays the arm frontier. Magnitude context: about a third of the micro-texture’s harm (+9.33e-07, 0.751) — coherent structure hurts less, but it confidently hurts.
• MECHANISM — consistent. only_links_surf vs only_links_photo +1.98e-07, CI95 [+1.36, +2.59]e-07 (27/100 closer), AUROC 0.652 → 0.671 (micro-texture: +1.30e-06, 0.740).
• Mechanism observations (diagnostic, not new claims):
  1. The cube shrink-wrap does NOT render clean horizontal layer lines — on several link faces the bands come out as radial “sunburst” fans (visible in the strip’s Δ panel). The realized structure is coherent and surface-tracking but not print-layer-like everywhere.
  2. Our bands are pure albedo modulation. Real print layers are RELIEF — their contrast is shading/specular structure that moves with the light, which the classic renderer cannot express without a normal-map path (the same limitation that put servo glints out of scope). Two refutations now say the encoder rejects added arm content at this rendering abstraction; the surviving hypothesis is that the residual lives in light-transport structure, not albedo statistics or albedo geometry.
• What this buys the ledger: the escalation branch registered by the micro-texture refutation is now CLOSED with a measured answer, the v1-graded arm stays the production frontier, and no further texture rung is auto-queued (the clip-capped albedo channel is exhausted; a normal-map/renderer-upgrade rung would be a new design decision, priced only if the owner wants it).

Artifacts: analysis · fit record · chart · strip