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Architecture

FilmCraft is a Cargo workspace of small crates with strictly enforced layering. The engine is headless: every feature can be reached without a window, and the egui UI is one client among the CLI, the JSON control channel and the MCP server.

Design principles:

Engine-first. Project-changing actions go through Session::execute(id, params).
Everything is a command. Stable id, label, menu path, shortcut, parameter doc, enabled() predicate with a human-readable reason, and run().
Exact time. Integer ticks, rational frame rates. No f64 seconds in edit math.
Copy-on-write snapshots. The project is an Arc<Project>. Undo is a stack of snapshots, and background readers (playback, export) hold a snapshot without locking.
CPU reference, GPU fast path. The CPU compositor is the oracle. The GPU path is tested against it.
Pure Rust, clean-room. Codecs and containers are written from public specifications (see AGENTS.md).

1. Layers

 L6  apps/filmcraft · apps/filmcraft-cli
 L5  ui-egui · automation
 L4  engine
 L3  render · gpu · export · golden (test-only)
 L2  edit · codecs · interchange · captions
 L1  frame · media · project · audio-dsp · text
 L0  foundation: time · geom · color · bitstream · testkit (dev-dependency only)
     codecs/containers: isobmff · matroska · h264 · h264enc · hevc · prores · aac

Crates are named filmcraft-<dir> (crates/time is filmcraft-time). The apps are filmcraft and filmcraft-cli.

Crate	Layer	Purpose
`time`	L0	`Tick`, `FrameRate`, `TimeRange`, timecode parse/format (NDF/DF, frames, feet+frames, samples)
`geom`	L0	`Vec2`, `Rect`, `Affine`, Motion-transform composition
`color`	L0	colour spaces, transfer functions, YUV↔RGB matrices, LUTs
`bitstream`	L0	bit reader/writer, Exp-Golomb, emulation prevention
`isobmff`	L0	MP4/MOV demux and mux
`matroska`	L0	MKV/WebM demux
`h264`, `h264enc`	L0	H.264 decoder; H.264 encoder
`hevc`	L0	H.265 Main/Main 10 decoder
`prores`	L0	ProRes decoder and encoder
`aac`	L0	AAC-LC decoder and encoder
`testkit`	L0	test-only helpers, used only as a dev-dependency: ffmpeg/ffprobe discovery, fixture dirs, golden images (testing.md)
`frame`	L1	`VideoFrame` (planar YUV / RGBA8 / linear RGBA f32, colour metadata), `AudioBuffer`
`media`	L1	`MediaSource` trait, probing/openers, frame cache, generators, stills, WAV
`project`	L1	document model, effect definitions, keyframes
`audio-dsp`	L1	loudness metering (BS.1770 / R128) and audio effects; no dependencies
`text`	L1	text engine: font database (bundled OFL fonts + system fonts), shaping (harfrust), bidi, line breaking, paragraph layout, glyph/path rasteriser, strokes (crates/text/README.md)
`edit`	L2	pure edit algebra (insert, overwrite, razor, ripple, roll, slip, slide, rate stretch…)
`codecs`	L2	container + codec hub: MP4/MOV and MKV sources, GOP-aware seeking, decoder registry, audio decoding
`interchange`	L2	EDL, FCP7 XML, FCPXML and OTIO import/export (no file I/O)
`render`	L3	sequence evaluation, CPU compositor, video effects, transitions, audio mix
`gpu`	L3	wgpu compositor (WGSL)
`golden`	L3	test-only: golden-image tests of the CPU renderer and GPU-vs-CPU parity; empty library, dev-dependencies only
`export`	L3	render → encode → mux pipeline, progress/cancel
`engine`	L4	`Session`, command registry, undo history, media pool, jobs, interchange glue
`ui-egui`	L5	the egui frontend: docking, panels, timeline, monitors, playback, control-channel handlers
`automation`	L5	MCP server (`rmcp`, stdio), headless or bridged to the running app
`filmcraft`	L6	desktop binary: eframe/wgpu window, cpal audio output, file dialogs, native macOS menu, TCP control server
`filmcraft-cli`	L6	headless CLI: `probe`, `commands`, `render`, `run`, `mcp`

What `cargo xtask layers` enforces

The table of layers lives in xtask/src/main.rs (LAYERS). It also reserves names for planned crates. The check reads cargo metadata and looks at normal and build dependencies (dev-dependencies are exempt):

Rule	Detail
Every crate has a layer	A new crate fails the check until it is added to `LAYERS`.
Only downward edges	A crate may not depend on a crate in a higher layer.
Same-layer edges are listed	From L1 up, a same-layer edge must be in `SAME_LAYER`: `media→frame`, `project→media`, `project→frame`, `gpu→render`, `export→render`, `cli→filmcraft`, plus a few reserved for planned crates.
L0 codecs stay standalone	L0 crates other than `time`, `geom`, `color`, `bitstream`, `testkit` may depend on no workspace crate except `filmcraft-bitstream`. External crates such as `thiserror` and `rayon` are allowed.
No UI/OS crates below L5	`egui`, `eframe`, `egui-wgpu`, `winit`, `rfd`, `cpal`, `muda` are allowed only in L5 and L6.

cargo xtask wasm runs cargo check --target wasm32-unknown-unknown on every L0–L4 crate, so everything up to the engine stays web-portable. unsafe_code = "deny" applies workspace-wide.

2. Time base

All time is filmcraft_time::Tick(i64) at TICKS_PER_SECOND = 254_016_000_000.

That number divides evenly into the frame duration of every broadcast rate (23.976, 24, 25, 29.97, 30, 48, 50, 59.94, 60, 120…) and the sample duration of every common audio rate (8 kHz to 192 kHz, including the 44.1 kHz family). So frame and sample positions are exact integers, edits never drift at 29.97, and audio and video line up to the sample.

Type	Use
`Tick`	timeline and media positions and durations
`FrameRate { num, den }`	`frame_duration()`, `tick_of(frame)`, `frame_at(tick)`, `snap(tick)`
`TimeRange`	half-open `start + duration`
Timecode	display only (SMPTE NDF/DF, frames, feet+frames, samples); `parse_timecode` and `format_time`

Commands take time as time (ticks), frame, seconds or timecode; the engine converts once at the boundary.

3. Data model (`filmcraft-project`)

Project
├─ root: Bin                         tree of bins
├─ items: map ItemId → ProjectItem   flat
│    kind: Media(MediaClip) | Sequence(Sequence) | Subclip{..} | AdjustmentLayer{..} | Graphic{..}
└─ next_id

Sequence
├─ settings: frame rate, size, sample rate, …
├─ video_tracks / audio_tracks: Vec<Track>
├─ markers, mark_in / mark_out
Track
├─ locked, sync lock, targeting, mute/solo/visibility
├─ items: Vec<TrackItem>             sorted, never overlapping
├─ transitions: Vec<Transition>
└─ audio: volume_db, pan, effects (mixer inserts), mixer: MixerStrip
     (automation mode + lanes, sends, output, record arm, solo safe, input map)
Sequence (audio) ─ submix_tracks: Vec<Track>, master_volume_db / master_effects / master_mixer
TrackItem (a clip instance)
├─ item: ItemId, start (timeline ticks), source_in (media ticks), duration, speed
├─ link group, label, enabled
└─ effects: Vec<EffectInstance>      intrinsic Motion/Opacity/Volume… first, then standard effects
EffectInstance
└─ effect id, enabled, params: id → constant value or keyframe track

Graphic clips (graphics.md) reference a Graphic canvas item; their text and shape layers are hidden graphic_text / graphic_shape effect instances on the track item.
Everything is plain serde data. Sequence::check() validates the invariants (no overlaps, unique ids), and the engine runs it after every sequence edit.
Timeline positions are sequence ticks; source_in and keyframes are in media time, so trims and splits never move keyframes.
Effect definitions are data. project::effect::effect_defs() lists every video effect, audio effect and transition with its parameter schema. The Effects panel tree, the Effect Controls rows and the parameter docs agents see are all generated from it.
Project files (.fcproj) are the project serialised as JSON. Saves are atomic: write a temporary sibling file, then rename it over the target.

4. Command system (`filmcraft-engine`)

pub struct CommandSpec {
    pub id: &'static str,                 // "sequence.addEdit"
    pub label: &'static str,              // "Add Edit"
    pub menu: &'static [&'static str],    // ["Sequence"]; empty = not in menus
    pub shortcut: Option<&'static str>,   // "Cmd+K" (Cmd = ⌘ on macOS, Ctrl elsewhere)
    pub params: &'static str,             // r#"{"time":ticks?}"#, shown to agents
    pub enabled: fn(&Session) -> Result<(), String>,  // Err carries the reason
    pub run: fn(&mut Session, &Value) -> Result<Value>,
    pub journal: bool,                    // false for read-only queries
}

All commands are in crates/engine/src/commands.rs (cmd! for actions, query! for read-only queries such as project.inspect, sequence.inspect, effects.list, jobs.list). Ids follow the menu structure: file.*, edit.*, clip.*, sequence.*, markers.*, timeline.*, effects.*…
Session::execute(id, params) finds the spec, checks enabled, runs it and appends it to the journal.
Undo. Edits go through Session::edit(label, |project, state| …) or Session::edit_sequence(label, |seq, ctx, state| …). These clone the project, apply the closure, and on success push the old Arc<Project> with the label onto the undo stack (200 entries). On error nothing changes. edit.undo and edit.redo swap snapshots. Thanks to structural sharing a snapshot costs little.
Editor state (EditorState: active sequence, playheads, selection, targeting, edit points…) is serde, so agents can read it with state.inspect.
Events (ProjectChanged, Toast, OpenSequence, OpenSource) are drained by frontends each frame.
UI-only commands (tools, playback, zoom, panels, workspaces) live in crates/ui-egui/src/menus.rs (UI_COMMANDS). The menu bar is built from the engine registry plus this table, and menus::invoke is the single entry point for menus, shortcuts and the control channel.

5. Media, render and playback pipeline

file ──► codecs (MP4/MOV, MKV, audio)        demux + decode, GOP-aware seek
          │   decoder registry: h264, hevc, prores, mjpeg (+ any registered first)
          ▼
        media::MediaSource ──► frame cache (byte-budgeted LRU, shared)
          ▼
        render::render_sequence(project, seq, t, scale)        CPU reference
          per track bottom→top: map timeline t → media t (speed), fetch frame,
          standard effects → Motion → Opacity/blend, transitions, composite
          in linear-light premultiplied f32
          │
          └─ render::plan::plan_frame → gpu::GpuCompositor     GPU path
               layers = decoded YUV/RGBA frames + matrix + opacity;
               anything the shaders don't cover is pre-rendered on the CPU
          ▼
        ui-egui frames.rs worker pool ──► monitors (program/source), thumbnails, prefetch

Sources. media::MediaSource yields video_frame(FrameRequest) and audio(start, frames, rate) in media time. Sources are Send + Sync and shared by monitors, thumbnails, playback and export. The engine's MediaPool creates one per project item, lazily, through registered openers (codecs::openers(): MP4/MOV, MKV/WebM, audio files).
Offline media and proxies. The pool caches each item's source per reference (path, offline flag), so relinking and undo take effect at once. Media that can't be opened renders the offline slate (render::offline) instead of failing. With proxies enabled, an item with a proxy reads it through ProxySource, which reports the original's size. Export always uses MediaPool::full_res_provider. See project-files.md.
Seeking. codecs::Mp4Source seeks to the preceding sync sample and decodes forward, caching every frame of the GOP. Sequential playback reuses the decoder. Decoders implement codecs::VideoDecoder. register_video_decoder puts a factory in front of the built-in ones, so a hardware decoder can take precedence. The GOP cache never holds its lock while decoding. Decoders run slices on rayon, so an export worker waiting inside a decode can pick up another frame of the same source. A request that finds the shared decoder busy decodes with a private decoder.
Compositor. render is the reference for monitors, thumbnails and export. render::plan turns a frame into GPU layers. Non-Normal blend modes, standard effects, adjustment layers, nested sequences and non-dissolve transitions are rendered on the CPU for that layer or frame and handed to the GPU as an image, so both paths give the same picture. Setting FILMCRAFT_CPU_COMPOSITE=1 forces the CPU path in the desktop app.
Frame scheduling. crates/ui-egui/src/frames.rs runs a small pool of worker threads with prioritised jobs: the frame on screen first, then playback prefetch, then thumbnails. The UI never decodes. It shows the exact frame when it is ready and holds the nearest cached frame meanwhile.
Audio clock. The desktop app passes a cpal output (apps/filmcraft/src/audio.rs) to the UI as AudioOut. While playing, the samples played by the sound card drive the playhead and video follows. Without an audio device, playback falls back to the wall clock. Dropped frames are counted. Sequence audio goes through the mixer graph (render::mixer, §5.1); clip audio effects run on audio-dsp via render::audio_fx.

5.1 Audio mixer

clip: gain → clip effects → Volume / Channel Volume / Panner (clip keyframes, media time)
      → audio transitions → summed per track                           render::audio::track_input
track / submix strip:  input map, mono fold → pre-fader inserts → pre-fader sends → mute
      → fader (volume) → meter → post-fader inserts → post-fader sends → pan / balance → output
Mix:  bus sum → pre-fader inserts → fader → meter → post-fader inserts → out    render::mixer

Model (project::mixer). Every audio track, submix and the Mix has a MixerStrip. Static values stay in Track::volume_db, pan, muted and the send/effect parameters; automation is keyframes in sequence ticks: lanes volume, pan, mute (hold), send.<i>.level in MixerStrip::lanes, and insert parameters (fx.<slot>.<param>) in the effect's own keyframes. Up to 5 inserts (EffectInstance::post_fader picks the side) and 5 sends per strip. Submixes feed the Mix or a submix after them (no feedback). All fields have serde defaults, so older projects load unchanged.
Graph (render::mixer::mix_graph). Lanes are evaluated per sample; effect parameters update on an absolute 64-sample grid, so the output does not depend on how callers cut the timeline into requests (export batches and device callbacks give identical samples). Inserts that report latency delay their strip; each route into a bus gets a compensation delay and the graph is read ahead by its total latency. Graph state (DSP, delay lines) is cached per structure and continued by sequential readers; other requests start fresh with a pre-roll (effect tails, ≤ 3 s). Tracks run in parallel (rayon), buses in order. Mono tracks pan with the −3 dB constant-power law; stereo tracks and sends use balance. Solo keeps soloed and solo-safe strips plus everything feeding them or fed by them. 24 tracks × (EQ + Dynamics + Studio Reverb) + a compressed submix renders at ~6× realtime on one core (release).
Automation modes (Premiere semantics). Off ignores lanes; Read plays them; Latch records from the first touch and holds the last value until playback stops; Touch records while held and ramps back to the existing automation over the automatch time (Preferences ▸ Audio, 1 s); Write records every control from playback start (then switches to Touch unless "Switch to Touch after Write" is off).
Recording (engine::mixer). Playback start runs mixer.recordStart, stop runs mixer.recordStop. Fader and knob drags send mixer.touch (value, playhead) while held and mixer.release when let go. Held values go to render::mixer::LiveMix, which the playing mix reads, so moves are heard at once. At stop each gesture stream is thinned (linear keyframe thinning, optional minimum time interval) and written over its time range as one undo step, with boundary keyframes that keep the automation outside the range unchanged.
Live state. PreviewStore::live (LiveMix) also carries per-strip meter peaks posted by the mix (Track Mixer and Audio Meters read them) and the newest project snapshot, which the audio callback uses, so edits made during playback are heard.

6. Export jobs (`filmcraft-export`)

file.exportMedia {path, format, scale, audio, quality}
  → engine creates a Job {id, label, progress, result} and runs it on a background thread
  → export: render frames in parallel batches → encode in order → mux; audio mixed per batch
  → jobs.list shows progress; jobs.cancel sets the shared cancel flag

Format	Encoder	Container
`h264`	`filmcraft-h264enc` + `filmcraft-aac`	MP4 (`isobmff`)
`prores`	`filmcraft-prores`	MOV
`mjpeg`	built in	MOV
`png`, `gif`, `wav`	built in	image sequence / GIF / WAV

Video encoders implement export::VideoEncoder. Codec crates plug in with register_encoder and register_audio_encoder.

Timelines can be exchanged as EDL, FCP7 XML, FCPXML or OTIO. file.import detects these formats and merges the result into the project as one undoable step, and file.exportInterchange, file.exportEdl, file.exportFcpxml and file.exportOtio write them.

7. Automation surfaces

All of these dispatch the same command ids.

Surface	Where	Scope
UI	`ui-egui` menus, shortcuts, panels	`menus::invoke` → engine or UI command
CLI	`filmcraft-cli`	`commands`, `run script.jsonl` (one `{"id","params"}` per line), `render`, `probe`
Control channel	`filmcraft --control <port>`	JSON lines on loopback TCP: engine commands plus synthetic input, inspection and screenshots of the live UI
MCP	`filmcraft-cli mcp`	stdio MCP server: headless in-process session, or `--bridge` to the control channel

Automation ids. Every interactive widget calls app.auto.add(id, rect, label) each frame (crates/ui-egui/src/automation.rs). Agents click by id, e.g. tools.Razor, timeline.clip.<id>, effects.item.gaussian_blur, panel.Timeline.
UI state that is not project data (tool, workspace, dock layout, zoom, scroll, monitor settings) is in crates/ui-egui/src/state.rs as serde structs, so ui.inspect and ui.set can read and write it.

Protocol reference: control-protocol.md. Agent guide: agents.md.

8. Not built yet

The layer table reserves names for crates that don't exist yet: riff, mjpeg, dnx, keyframe, effects, audio, scopes, playback, format and platform. Until they exist, that work lives elsewhere: keyframes and effect definitions in project, effects and the audio mix in render, scopes and playback in ui-egui, and OS integration (cpal, rfd, native menus) in apps/filmcraft. ROADMAP.md has the milestone status.

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