Pre-reg — arm sub-part split: which rendered arm pixels carry the sim signature

2026-08-13, posted in-channel 06:18Z (id 1537344697809240134), before any read. Queue item sim-arm-appearance-leg; follows the fg content split (leg (a)) and the clutter-patch legs (b)+(c).

Plain words. After patching the fake desk clutter with real photo crops, the biggest remaining chunk of “rendered stuff the encoder can tell is fake” is the robot arm itself (~7.1% of the image). Before spending anything on a fix, we ask the same question we asked about the clutter: which part of the arm looks fake — the gripper, the long links, or the little camera mounts — and is it one arm instance or both? Same trick as before: re-run the image composite with only some parts shown, everything else identical to the pixel, and see which subset moves the “looks fake” score.

Question

Rendered arm reads only_arm 0.654 vs plate_only 0.866 (leg (a)); patched surface 0.556 still ≫ real-fg 0.328. Which sub-part carries it — gripper, links, or camera mounts? Which instance — follower or leader?

Design

Same hooked harness as leg (a): ONE production v3 instance (numpy post backend), _composite hook, segmentation-restricted masks, 20 seeds × 5 appearance draws, noise-RNG state restored per arm — frames pair 1:1. Encoder probe: er_60k trunk, top cam, knn5 AUROC vs held-out real-B (A/B split as before). Two exact partitions of the 96 arm-class geoms (in-run tiling oracle):

• part axis: gripper+jaw (46 geoms), links base→wrist (44), camera_mount (6)
• instance axis: follower (48), leader (48)

Arms (14): v3, plate_only, bridges no_arm/only_arm, no_/only_ × {gripper, links, mount, follower, leader}. Per-part pixel fractions recorded per slot.

Registered anchors / aborts

In-run v3 must read 0.713 ± 0.005 else ABORT (no claims). Bridge bands (fresh noise realization, non-abort sanity, flagged if missed): plate_only 0.865 ± 0.02, only_arm 0.654 ± 0.02, no_arm 0.825 ± 0.02.

Decision rule

Rank parts by paired Δknn5 CI95 (10k resamples, rng 0) of only_ vs plate_only; a part is NAMED the fix target iff its CI excludes 0 AND it carries ≥ 60% of the only_arm − plate_only paired delta; if two parts each carry ≥ 35%, both are named (split verdict → photometric fix targets both). Instance axis is context: the follower-vs-leader read decides whether a fix must treat both instances. No promotion, no production change from this leg — the output is the named target(s) for the photometric ladder rung already queued.

Cost

CPU render + ~0.03 GPU-h embeds (14 arms × 100 frames + real groups). Launch immediately per no-idle rules (GPU idle).