Pre-registration: stage-2 sign-convention probe — optical-flow cross-check

2026-08-05 ~23:3xZ. Posted BEFORE any probe code runs on the candidate cells. Follow-up to the stage-1 screen (owner hypothesis 14:55Z: flipped sign conventions, esp. wrist_roll on mirrored wrist-cam mounts; the two-stage plan was agreed then — this freezes stage 2’s scope). CPU-only, runs beside the live box batch and draws chain on spare cores, ionice’d. Zero GPU.

Question

Stage 1 found model-vs-truth mirror signatures but is structurally blind in both directions: it cannot see an internally-consistent mirror-world repo the model partially fit, and a genuine-looking anti-correlation can still be a model failure on a hard repo rather than a data fault. Stage 2 asks the pixels: does the recorded joint velocity move the world in the same direction as it does in healthy repos? A flipped-convention repo shows optical flow opposite to what its recorded velocities predict, relative to the population.

Candidate cells (primary reads)

The stage-1 mirror-signature shortlist, verbatim:

Specificity controls (pre-declared expected outcomes): Dongkkka/koch_arm_gripper_pick_red_pen · shoulder_pan (stage-1 “tracked-but-offset”, med frame corr +0.76) must read NORMAL; kevin510/lerobot-cat-toy-placement · wrist_roll (the ±180° wraparound pathology, wrap census) must read NORMAL — wraps are not mirrors. Either control reading MIRRORED ⇒ instrument-fault presumption: no candidate verdicts ship, a debug post does. The remaining 4 stage-1 cells run record-only.

All three candidates verified present in the local corpus copy (~/datasets/mcobzarenco/community_curated_v0), robot_type so100, 30 fps, two 480×640 cams (image, image2 — no wrist label, see ego-cam rule), AV1 videos decode via torchcodec, state+action parquet streams intact (checked this session before posting).

Instrument (probes/probe_sign_convention_stage2.py, to be written)

Per repo, per episode:

1. Joint velocity from the state stream (follower = physical): v_d(t) = state_d(t+1) − state_d(t), deg/frame at 30 fps.
2. Isolated-motion pairs for target dim d: |v_d| ≥ 0.5 deg/frame AND |v_d| ≥ 2·|v_j| for every other non-gripper dim j. If a repo yields < 30 pairs, relax dominance to 1.5× once; still < 30 ⇒ that read is inconclusive-by-data (reported, not forced). Pairs capped at 400/repo by uniform stride (deterministic, no RNG in selection).
3. Flow: decode frames t, t+1 (torchcodec), grayscale, downscale to 320×240, Farneback (pyr_scale .5, levels 3, winsize 21, iters 3, poly_n 7, poly_sigma 1.5 — frozen here).
4. Flow statistics: ω(t) = least-squares image-plane angular velocity about center, Σ[(x−c)×f]/Σ|x−c|² (for wrist_roll); t_y(t) = mean vertical flow (wrist_flex, shoulder_lift); t_x(t) = mean horizontal flow (shoulder_pan).
5. Ego-cam rule (cams are unlabeled): over all motion pairs of the repo, ego cam = argmax Spearman corr(mean |flow|, Σ_d |v_d|), requiring margin ≥ 0.15 over the other cam. No margin ⇒ compute the read on both cams; verdicts only if both agree in sign, else inconclusive-by-camera.
6. Signed read per (repo, dim): Spearman ρ between v_d(t) and the dim’s flow statistic over the isolated pairs, ego cam.
7. Stream-consistency check (no video; classifies mirror type): Spearman corr(action_d(t) − state_d(t), state_d(t+3) − state_d(t)) over pairs with |action−state| ≥ 0.5°. Positive = servo-consistent (state and action share the convention — a calibration-level mirror); negative = action-stream-only flip (worse: contradictory supervision against honest pixels+state).

Reference population (defines the healthy sign per dim, deterministic): all so100 repos with ≥ 8 stage-1 panel frames and stage-1 MAE ratio ≤ 2.0 on the target dim, sorted lexicographically, first 15 yielding ≥ 30 isolated pairs each. Population validity gate: ≥ 80% sign agreement (≥ 12/15) AND median |ρ| ≥ 0.2. Population not sign-consistent ⇒ the dim’s read is invalid-by-population — no mirror verdict, and the diversity itself escalates to the owner (it would mean sign conventions vary corpus-wide, a bigger finding than three repos).

Decision rules (per candidate cell)

• MIRRORED: sign(ρ_cand) opposite to sign(median ρ_ref), |ρ_cand| ≥ 0.3, and an episode-level bootstrap (resample episodes with replacement, 1000 draws, SeedSequence(13)) puts ≥ 90% of mass on the flipped sign.
• NORMAL: sign matches the reference, |ρ_cand| ≥ 0.3, ≥ 90% bootstrap mass.
• INCONCLUSIVE otherwise (reported as such — no verdict forcing).

Hard validation gate (before any candidate cell is opened)

Synthetic-flip oracle: pick the lexicographically-first valid reference repo, negate its state stream on the target dim in-memory. The probe must read MIRRORED on the doctored copy and NORMAL on the original for each of the three statistics (ω, t_y, t_x). Fails ⇒ fix the instrument, no candidate reads. (Implementation + this gate + reference population may all run before candidate cells are opened; candidate verdicts come last, in one shot.)

Pre-declared consequences

• ≥ 1 MIRRORED ⇒ ideas #13 repair arm becomes eligible: flip-corrected derived corpus (through the #18.8 leakage-cert path) + paired screen — its own pre-reg, GPU only at a quiet boundary. Repo list flagged for any curated-v1 exclusion set and reported to the owner as a transferable data-quality finding.
• All three NORMAL/refuted ⇒ the stage-1 mirror signatures were model failures on hard repos, not data faults; #13 → falsified for these candidates, repair arm dies unlaunched.
• Inconclusive-by-data/camera on any cell ⇒ recorded honestly; no repair eligibility from that cell.
• Controls misbehave ⇒ instrument fault, debug post, no verdicts.

Cost & babysitting

~18 repos × ≤ 400 pairs × 2 cams of Farneback at 320×240 ≈ 20–40 min CPU on spare cores (nice/ionice’d — the box batch and local draws chain own the GPUs and are untouched). Runs in a later session as a normal work item; results post carries the verdict table, per-cell ρ with bootstrap intervals, the population histogram, and the two control reads.