Blame for 13_pixel_arm/README.md

# Pixel arm: the best pixel-root memory (window model + 8-step update head; 2026-10-31)

The current best pixel (root) memory on the orbit benchmark. It replaces the previous pixel arm (Bk, formerly `12_pixel_arm_bk/`,
removed 2026-11-01): same window model, a head retrained for 8 update steps, and a lighter evaluation configuration. It matches Bk's
PSNR at the end of the stream (−0.00 ± 0.03 dB on test33) with better LPIPS (−.010), 3.75 x less memory and about 3.5 x the speed.

- `model_001000.pt`: the window model (3D-Belief trunk, proposal head, Gaussian decoder; EMA) of `outputs/joint_j1keptlr`, step 1000. The
  same file as Bk's (sha256 unchanged); `args['decoder_from']` is repo-relative (the only change from the cluster original).
- `upd_000800.pt`: the learned root update head (EMA, `['ema']`), retrained for 8 update steps (`outputs/opt_it8`, `command_head.json`):
  head only, from Bk's head, 800 steps, head lr 2.5e-5, seed 3, rollouts over 400 DL3DV training orbits (`data/orbit_cache/dl3dv_train`,
  disjoint from val33 / test33) with the 5 deg / .08 pose store, replay 8 views at a time and kept targets. Its `['args']` still carries
  absolute cluster paths inherited from older heads; loading reads only `['ema']` and `inner_lpips`.
- `command_window.json` / `command_j1.json`: the window model's training (J1, then Bk's +1000 steps at half lr); `command_eval_test33.json`:
  the evaluation that produced the numbers below (shard 0 of 4).

**Needs, besides these files** (run from the repo root, branch amodal-completion, commit b7f7e0a or later):
- the decoder construction files `free_completion_20260927/checkpoints/ray/step002000` from `03_checkpoints_streaming.zip`, at
  `outputs/free_completion_20260927/checkpoints/ray/step002000` (the snapshot's weights are loaded over it);
- the 3D-Belief prior `3d_belief_re10k_256.pt` at `$FLC_BELIEF_CKPT` (read when the trunk is built; not in this repo);
- for `--sky-layer`: the sky segmenter `nvidia/segformer-b0-finetuned-ade-512-512` at `checkpoints/segformer_b0_ade`;
- the orbit caches `data/orbit_cache/dl3dv_test33` (and `dl3dv_val33`) from `11_orbit_eval_data/`.

## How it is run

Each 6-frame window (advancing 2 frames) is written into the root memory; the update head then refines the roots for 8 steps, fitting the
window's frames plus a self-replay of every stored past pose that overlaps the update. A pose is stored when it is > 10 deg or > 0.15 x scene
depth from every stored pose; its replay target is **kept**: the memory's own render right after the last window that observed that pose or
its cell (uint8, 256 px). Lossless speed-ups (half-precision state and roots, a lean bookkeeping TSDF, a cheaper exact gradient), every render
on a 5 x 5 Gaussian lattice per root (25 Gaussians), pruning by opacity mass, a short lifetime for the update state of roots not updated
recently, and a sky layer (a direction-only panorama of observed sky behind the Gaussians; far views skip roots that render mostly on sky).

**No past real frame is stored**: each window's frames are used for its write and update and dropped. Kept per stream (test33, ~348 MB at the
end): roots, update-head state, bookkeeping TSDF, replay poses (+ 32 x 32 caches rendered from the memory), kept replay targets (the memory's
own renders), and the sky panorama + masks (one fused colour per observed sky direction, the only stored content derived from real pixels).

    python scripts/unified/eval_long_stream.py --snapshot 13_pixel_arm/model_001000.pt --root-updater 13_pixel_arm/upd_000800.pt \
      --replay-chunk 8 --replay-store cell --fast-grad --state-half --roots-half --tsdf-lean --tsdf-carve 0 \
      --lod 5 --prune-mass .4 --state-ttl 4 --tsdf-size .1 --sky-layer --sky-thr 2 --sky-far 16 \
      --admit-angle 10 --admit-dist .15 --ru-iters 8 \
      --data data/orbit_cache/dl3dv_test33 --clips 33 --loops 2 --async-period 0 --out <dir>

For far overview renders only, `--far-cap 4 --far-depth .15` (with `--sky-far`) also caps Gaussians at 4 source pixels and skips those in
front of the TSDF surface at that camera; render-only, the memory and near-path renders are unchanged.

## Results

dl3dv_test33 (33 DL3DV test orbits), walked twice (6 orbits); after every window the memory renders the first orbit's held-out frames; all
pixels (holes count); one seed; one process per GPU (three H100, one H200). `test33_orbit.png` (orbit curves, per-scene changes, tiles) and
`test33_orbit.json` (the numbers below).

| test33, 6 orbits, all pixels | PSNR orbit 1 / 3 / 6 | orbit 6 − orbit 1 | walk 2 (6 − 3) | LPIPS at 6 | MB at the end | s per frame |
|---|---|---|---|---|---|---|
| **pixel arm (this)** | **17.36 / 18.08 / 18.12** | **+0.75 ± 0.10 (29 / 33 rise)** | +0.04 (19 / 33) | .394 | 348 | 2.09 |
| frame memory (Context-as-Memory style, our window model) | 16.27 / 16.52 / 16.52 | +0.26 ± 0.25 | 0.00 | .382 | 17 | 0.04 |
| Gen3R, frame buffer (8 scenes) | 19.29 / 17.30 / 17.43 | −1.86 | +0.13 | .301 | | |
| SPMem TSDF (same window predictions) | 14.20 / 14.73 / 14.51 | +0.31 ± 0.20 | −0.22 | .627 | 147 | 0.05 |
| plain point accumulation | 12.35 / 11.85 / 11.65 | −0.69 ± 0.09 | −0.20 | .753 | 78 | 0.15 |

LPIPS of this arm at orbits 1 / 3 / 6: .417 / .389 / .394. Bk on the same scenes: 17.42 / 18.15 / 18.12, +0.69, 1.1-1.3 GB, 5.1 s per frame.
dl3dv_val8 (8 scenes, the selection set): 16.44 / 17.10 / 17.08, +0.64, LPIPS .435, 367 MB, 2.37 s per frame (Bk 8.23 s, matched GPUs).
A window takes ~4.4 s on test33 (6-frame windows advancing 2, two walks), most of it the replay of ~44 stored poses per window.

**Known limits:** the mean over the first orbit's views rises for about 1.5 orbits and then stays flat; views the memory already rendered well
slowly lose detail (0.7-1.1 dB below their own peak) while weaker views catch up. Far views show haze from a few hundred runaway roots; the
far-view filter hides it, it is not removed. Runs are not bit-reproducible (repeat runs of the same flags differ by up to ~0.1 dB per scene).

sha256 model_001000.pt: 08e30bdaac8ddb560151384df06e16bc88606cbe86915bb92d587e16e522110e
sha256 upd_000800.pt: 7a1d6c3a76c68102f278048a8869ab589e4bab48e80250e9805eaaa428d99d5d