Seam Craft

# Seam Craft — render prerequisites for scene-to-scene transitions

This is the **render-correctness doctrine** for PLV scene-to-scene seams: the prerequisites and master-timeline mechanics that make any transition composite correctly, independent of which specific transition is chosen. The per-transition catalog (crossfade, push-slide, zoom-through, cut-the-curve, …) lives in the transition registry — this page is the doctrine that sits underneath all of them.

The transitions this doctrine governs are **Tier-B-ready**: pure transform / opacity / filter on the two scene **clip wrappers** (`#el-<sid>`), no injected overlay DOM, no per-scene cooperation. Overlay families (staggered blocks, blinds, light leak, grid dissolve, page burn) and shader transitions are deferred to later phases.

## Stage ground prerequisite (white-flash guard)

Several templates open a window where the two wrappers' summed opacity < 1 (the cut-the-curve mid-window cut, zoom-through's 0.15 floor, plain crossfade's power-curve dip). Whatever is BEHIND the wrappers shows through during that window. If the assembled `index.html` `#root` has no opaque background, the renderer composites the dip over its default **white** page → a white flash at every seam, glaring on dark films (observed on two Spotify runs before the fix). **The assembler must paint the stage:** `#root { background: var(--canvas-deep, var(--canvas, #000)) }` — `assemble-index.mjs` now emits this; any other consumer of these templates owns the same guarantee.

## How the injector applies a transition

At a `break` boundary between scene _i_ (`from`) and scene _i+1_ (`to`), the injector:

1. Extends `#el-<from>` wrapper `data-duration` by `duration_s` (holds its final frame — verified: `core/src/runtime/init.ts:1393-1410` external-slot branch). 2. Pulls `#el-<to>` wrapper `data-start` earlier by `duration_s` (creates the overlap window). 3. Reassigns **all** clip `data-track-index` as a 0/1 ping-pong so the two overlapping wrappers never share a track (same-track overlap is illegal — `core/src/lint/rules/composition.ts`). Higher track composites on top. 4. Stamps the `gsap_template` into `window.__timelines["main"]` at `T = overlap-start`.

Verified by prototype render (2026-05-31): the master-timeline wrapper tween is seeked and rendered (no double-seek with the sub-comp's own paused timeline — the runtime drives them independently), the extended wrapper holds scene _i_'s final frame, and the higher-track incoming wrapper composites over + blends with the outgoing one.

## Template placeholders

The injector substitutes these tokens in each `gsap_template` line:

| Token | Meaning | | ---------------------------------- | ------------------------------------------------------------------------ | | `__OLD__` | `"#el-<from>"` — outgoing clip wrapper selector (quoted) | | `__NEW__` | `"#el-<to>"` — incoming clip wrapper selector (quoted) | | `__T__` | overlap-start time in seconds (master clock) | | `__DUR__` | `duration_s` for this boundary | | `__DX__` | horizontal travel for directional types: `-1920` (LEFT) / `1920` (RIGHT) | | `__DY__` | vertical travel: `-1080` (UP) / `1080` (DOWN) | | `__ORIGIN_OUT__` / `__ORIGIN_IN__` | transformOrigin pair for `squeeze` |

`filter` / `scaleX` / `transformOrigin` are lint-clean on the master timeline (verified: `core/src/lint/rules/gsap.ts` has no per-property whitelist and scopes its checks to `data-composition-id` ranges; the x/y/scale/rotation/opacity whitelist is a _scene-worker_ prompt rule only — it does not bind index.html).