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The edit model — ED.1

crates/edit is the editor's spine: a pure, serializable model of an edit with no GPU, media, or UI dependencies — just data and the arithmetic that maps timeline time to source-clip time. Everything the editor does (trim, split, speed, crop, zoom, undo/redo, export) is an operation on, or a render of, this model. Keeping it dependency-light is what makes it exhaustively unit-testable on any machine.

The shape

classDiagram
  class EditProject {
    +ClipRef source
    +Vec~TimelineSegment~ segments
    +Vec~ZoomSegment~ zooms
    +BackgroundConfig background
    +CursorConfig cursor
    +Option~CropRect~ crop
    +AspectRatio aspect
    +u32 project_fps
    +project_duration() Frame
    +source_time(project_frame) Option~Frame~
  }
  class TimelineSegment {
    +Frame source_start
    +Frame source_end
    +f64 timescale
  }
  class ZoomSegment {
    +ZoomId id
    +Frame start
    +Frame end
    +f64 amount
    +ZoomMode mode
    +EditEase ease
  }
  EditProject "1" o-- "many" TimelineSegment
  EditProject "1" o-- "many" ZoomSegment
  EditProject "1" o-- "1" ClipRef

The edited video is the ordered concatenation of the segments. Trimming adjusts a segment's source_start / source_end; splitting replaces one segment with two adjacent ones sharing the cut frame; changing speed sets a segment's timescale. Zoom regions, the framing config, and the crop ride alongside — the renderer applies them per frame.

Project time → source time

Project time is frame-indexed at project_fps (default 30). The core of the model is source_time(project_frame): walk the segment list, accumulating each segment's project length, until you find the one containing the frame, then map the within-segment offset to a source frame using that segment's timescale.

flowchart TD
  A[project_frame] --> B{walk segments\nacc += project_len}
  B -->|frame < acc + len| C[offset = frame - acc]
  C --> D[source_start + offset × timescale]
  D --> E[source frame to decode]
  B -->|past last segment| F[None — end of timeline]

A timescale of 2.0 means 2× speed: a 100-source-frame slice occupies only 50 project frames, and project offset p maps to source frame 2p. Worked example for a three-segment project:

segmentsource framestimescaleproject frames
0[0, 300)1.00..300
1[300, 600)2.0300..450
2[600, 900)1.0450..750

So source_time(375) lands in segment 1 at offset 75 → source frame 300 + 75×2 = 450, and the whole timeline is 750 project frames long.

timescale is sanitized, sped-up clips never vanish

A non-finite or non-positive timescale falls back to real time, so the mapping can never divide by zero or produce NaN. A non-empty slice always occupies at least one project frame, so even a 100× speed-up of a single frame stays visible.

See the edit rustdoc for the full API — EditProject, TimelineSegment, and ZoomSegment.