Captions Overlay

# Captions Overlay Doctrine

> **Overlay doctrine — supplements the upstream `embedded-captions` skill. Applies ON TOP of it; do not expect it folded into the upstream skill.**

Two ideas combine here. First, the **caption model** — every spoken phrase is `drop`, `rail`, or `embed`, and embed is the scarce earned peak, not the default. Second, the **overlay law** — a caption line is composited ON TOP of the film as an overlay; it is NOT a reserved zone, so you never shift content up or leave a dead band to "make room" for it. The two reinforce each other: because captions ride as an overlay (the verbatim rail in front, the occasional embed behind the subject), the composition keeps its full frame and centers on the true vertical center.

## The caption model — drop / rail / embed

Every spoken phrase is one of three things (verbatim from `embedded-captions`):

| | What | How it's shown | | --------- | ------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **drop** | filler — um/uh, stutters, self-corrections | not shown | | **rail** | the default — ordinary spoken content (verbatim) | clean lower-third subtitle, **in front**, readable. A punch word can get an inline `emphasis` highlight (accent colour / active-word pop) — it stays on the rail. | | **embed** | a promoted peak — the headline beat | one big word composited **behind the subject** (matte occlusion), designed entrance + exit |

**The rail carries most of the text; embed is the scarce, earned peak** — ≤1 per beat, never two adjacent/co-visible, spaced ≥ a beat apart. A short clip → usually one embed; a long explainer → ~one per section. Embedding every word is the common mistake.

This is the **Standard** mode shape (rail = the verbatim lower-third; embed = the climax composited behind the subject). **Cinematic** mode drops the rail and makes everything embed-style — use it only for pure-cinematic asks, never for explainer / voiceover where the words must read.

### Rail-first, embed-scarce (the load-bearing rules)

Quoted from the `embedded-captions` non-negotiables:

- **Rail-first for talking-head / explainer.** Don't embed the whole transcript — most text is the rail; embed only peaks. Embedding everything is the default mistake. - **Embed is scarce + spaced.** ≤1 embed per sentence/beat, never two adjacent or co-visible, ≥ a beat apart, at most one `apex`. climax = per-beat peak, **not** "the single payoff of the entire clip."

## The overlay law — captions are NOT a reserved band

In a generated launch composition, when captions are enabled, finalize composites a **small, minimal word-by-word caption line** as an overlay layer ON TOP of the whole film (a single text line, bottom-centered, roughly the bottom ~5-8% of canvas height). It is an overlay, not a reserved zone (verbatim from constraint #13 of the product-launch-video scene agent):

- **Center the composition on the TRUE vertical center — y = H / 2** (landscape 540, portrait 960). Do not shift content up to "make room" for captions; a composition centered at 0.42 × H with a dead lower band is the bug, not the fix. - Content may extend to the canvas bottom. Full-bleed subjects, rails, and backgrounds all welcome. - **One soft courtesy rule:** avoid parking _critical small readable text_ (a URL line, a legal line, a sub-caption) exactly in the bottom ~80px center span where the caption line sits — the overlay would fight it. Large imagery / cards / ambient content under the captions is fine; the caption skin is designed to read over content. - There is no machine keep-out gate (the old `captions.mjs keepout` check is retired). Finalize snapshot QA judges caption-over-content legibility visually.

**When captions are disabled:** identical positioning freedom — the overlay simply doesn't exist.

## Why these two rules are one doctrine

The model says the rail rides **in front** and an embed is a rare word composited **behind the subject** — both are layers added to footage that ships untouched. The overlay law says the caption line is a layer composited **on top** of the whole film, not a band carved out of the layout. So in both the captioning pipeline and the launch-video pipeline, captions are an overlay you add, not a zone you reserve:

- Keep the full frame; center on true center; let content run to the edges. - Make the rail (or the small overlay caption line) carry