samples/long/README.md

# Past 20 seconds: what the temporal RoPE does, and what to do about it

## There is a hard number, and it is in the config

`LTXAVModel` carries `positional_embedding_max_pos=[20, 2048, 2048]`, and
`get_fractional_positions` divides each coordinate by its entry. The temporal
one is **seconds, not frames** — both `_prepare_positional_embeddings`
(`model.py:841`) and the AV cross-attention path (`av_model.py:865`) multiply
the temporal coordinate by `1 / frame_rate` first.

So the temporal rotary embedding was built for **20 seconds**. Past that, the
fraction exceeds 1.0 and the frequencies extrapolate off the end of anything the
model saw in training. This is the failure
[Train Short, Inference Long (arXiv 2602.14027)](https://arxiv.org/html/2602.14027v1)
attributes long-horizon degradation to.

`fit_temporal_positions` raises `max_pos[0]` to the clip's own duration, mapping
it back onto [0, 1] — position interpolation, the same trick used to extend a
language model's context, and training-free. It is safe to patch at runtime
because `generate_freq_grid_np`'s cache is keyed on theta, dimension count and
device, never on `max_pos`.

## Measured at 25 seconds

601 frames, 1024x640, two-pass, same prompt and seed, on the Space's H200.

| | segment sharpness (5 equal parts) | last/first |
|---|---|---:|
| untreated | 0.00086 0.00130 0.00143 0.00139 0.00096 | 1.12 |
| **interpolated** | **0.00247 0.00302 0.00286 0.00318 0.00198** | 0.80 |

The interpolated render is **2.2-2.9x sharper in every segment**.
`crop_ab.png` at 1:1: hair strands, eyelashes and background props are resolved
on the treated side and waxy on the untreated one.

## What this did not find

**No progressive collapse.** The premise being tested was that quality decays as
the clip runs on. It does not, in either version — the last-fifth to first-fifth
sharpness ratio is 1.12 untreated and 0.80 treated, and neither drifts in
character, wardrobe or room across 25 seconds. Most of the long-video literature
found in this search — compounding error, history forgetting, attribute drift —
describes **autoregressive or sliding-window** generation, where each window
conditions on the last. LTX-2.5 as driven here samples the whole clip at once,
so that class of failure does not apply and searching for it would have been
looking in the wrong place.

What extrapolation costs here is **detail across the whole clip**, not stability
at the end.

## Limits

* The two renders are different trajectories, not the same trajectory rendered
  twice, so the sharpness gap is what changed and not a controlled isolation of
  cause. Same prompt, seed, size, sampler and refine strength; only `max_pos[0]`
  differs.
* One duration (25 s), one prompt, one seed. 30 s, 40 s and beyond were not
  tried, and the point where interpolation itself starts to hurt — it compresses
  temporal detail by construction — was not located.
* Below 20 s this changes nothing and is not applied.
* Laplacian variance tracks contrast as well as sharpness. It is used here
  between two renders of the same scene, which is the only place it is fair.

## Files

`idol25-fit.mp4`, `idol25-nofit.mp4`, `ab.png` (timeline), `crop_ab.png` (1:1).