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).

seam-craft