| # What actually breaks a 25-second clip |
|
|
| ## Not the positions — the schedule |
|
|
| `ltx_render_clips` samples with `DISTILLED_SIGMAS`, nine fixed numbers solved |
| for the clip lengths the distillation used. `LTXVScheduler` instead derives its |
| shift from the **latent's own token count**. A 25-second clip carries about 4.4x |
| the tokens of a 2-second one, and the fixed schedule does not know that. |
|
|
| The result is a sample that has not converged. On screen it is ghosting — |
| furniture and curtains rendered semi-transparent, overlapping themselves — and |
| roughly half the colour. |
|
|
| | 25 s, 512x320, same prompt and seed | saturation | contrast | ghosting | |
| |---|---:|---:|---| |
| | fixed 8 (what this project shipped) | 0.094 | 0.108 | severe | |
| | adaptive 8 | 0.127 | 0.124 | gone | |
| | **adaptive 20** | 0.108 | **0.166** | gone, hands and fabric resolved | |
| | *15 s reference* | *0.208* | *0.161* | — | |
|
|
| Final render, 25 s at 1024x640 with the second pass: **saturation 0.161, |
| contrast 0.180** — colour close to the fifteen-second reference and contrast |
| above it. |
|
|
| Long clips now switch to the adaptive schedule automatically past 15 s, in |
| `ltx_av_generate`. A caller passing `steps` explicitly still overrides it. |
|
|
| ## Three times a metric pointed the wrong way |
|
|
| Worth recording, because the same mistake kept recurring in different clothes. |
|
|
| 1. **Laplacian variance said the broken clips had improved 2.5x.** They were |
| sharper *and* broken — fused fingers, blown highlights, collapsing |
| proportions. Sharpness cannot see any of that. |
| 2. **"Nothing degrades over the clip"** came from the same measure: the |
| first-fifth to last-fifth sharpness ratio was flat. Saturation across the |
| whole clip had halved, which no sharpness statistic reports. |
| 3. **Saturation then preferred adaptive-8 over adaptive-20** (0.127 against |
| 0.108). Side by side, adaptive-20 is clearly better: cleaner hands, defined |
| dress, a legible star belt. Saturation rewards a colour cast as readily as |
| colour. |
|
|
| The common error was choosing a measure before enumerating how the thing could |
| fail. "It will get blurry" was assumed, so sharpness was measured, and the |
| actual failure was colour and anatomy. The user looking at the clips found it |
| in seconds. |
|
|
| ## Not established |
|
|
| * One prompt, one seed, one duration. Where the fixed schedule starts failing |
| was not bracketed — 15 s is where the switch was placed, from one good clip at |
| 15 s and one bad one at 25 s, not from a sweep. |
| * 20 steps was picked over 8 by eye. 12, 16, 30 were not tried. |
| * The two-pass at 25 s does not fit a 16 GB card (OOM at 601 frames); the final |
| render was done on the Space's H200. One pass at 25 s does fit, at 6.58 GiB. |
| * Position interpolation is still applied above 20 s and still measurably |
| sharpens. It is not what fixed this. |
|
|
