Deferred export frame generator — ED.20

The optical printer was the lab's export stage: a camera and a projector locked together, re-photographing the cut negative one frame at a time onto fresh stock, honoring every splice and speed change as it advanced. ED.20 is that printer in software. Given an EditProject, the ExportFrameGenerator walks the project frames 0..project_duration and, for each, prints the right source frame onto the output.

flowchart LR
  F["project frame f"] --> ST["source_time(f)\n(trim · split · speed)"]
  ST --> DEC["EditorVideoStream.frame\n(seek + decode)"]
  DEC --> COMP["EditorPreview.render_frame\n(same compose as preview)"]
  COMP --> OUT["ExportFrame { bgra, pts, source_frame }"]
  OUT --> ENC["encoder (ED.21)"]

The key is that the timeline edits are already baked into the frame selection: source_time maps each project frame to its source frame, so a trimmed clip starts later, a split is invisible (two segments, one continuous source walk), and a 2× segment advances the source twice as fast. The generator composes through the same EditorPreview path the live preview uses, so the exported file matches the cut you scrubbed. Each ExportFrame carries a PTS computed with the same formula the live recorder's encoder feed uses, so timestamps line up when ED.21 hands the stream to the encoder.