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):

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 the verbatim words.
• Promote a word to an embed only at a genuine peak — scarce, spaced, never two at once.
• Reserve nothing; judge legibility of captions-over-content visually, not by a keep-out gate.