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The zoom lane — ED.12

The rostrum operator never improvised a push-in. Every camera move was planned on an exposure sheet — ruled paper where each row was a frame and a margin column noted the moves: zoom in, frames 120–180; hold; zoom out. The move list was authored on paper, then executed by the stand. ED.12 is that exposure sheet as a timeline lane: each block is one planned push-in, and the zoom engine is the stand that executes it at preview and export.

flowchart LR
  ADD["+ Zoom at the playhead"] --> OP["EditOp::AddZoom"]
  OP --> H["edit::History.apply"]
  H --> ZL["project.zooms"]
  ZL --> LANE["ZoomLane blocks\n(laid out by fraction)"]
  LANE -->|select / ×| H
  ZL --> ENG["ED.16 engine compiles\neach to a push-in at render"]

The lane authors ZoomSegment values; it never renders pixels. A block is laid out by zoom_spans with the same fraction-of-duration math as the filmstrip, so the zoom lane lines up frame-for-frame with the video and audio tracks above and below it. + Zoom drops a default ~1.5 s, 1.6× region at the playhead through EditOp::AddZoom; clicking a block selects it; its × removes it — all via the proptest-verified History, so every add and remove is undoable from the trim bin.

Authoring is separate from rendering

The lane and the engine are deliberately split. The lane (app-ui, reactive Leptos) only mutates the project's zoom list; the engine (edit, pure arithmetic) turns that list into a transform per frame. Neither knows about the other's medium — which is why the zoom model is unit-testable without a GPU and the lane is testable without a renderer. Drag a block's body to move it, or its edges to retime, in the deferred gesture pass (alongside clip-edge trim); for now the authoring verbs are add, select, and remove.