# Control protocol

`lightcraft --control 7980` (or `LIGHTCRAFT_CONTROL_PORT=7980`) starts a JSON-lines server on
`127.0.0.1:7980` (loopback only). One request per line, one reply per line, in order:

```text
→ {"id": 1, "method": "engine.execute", "params": {"command": "photo.rate", "params": {"rating": 4}}}
← {"id": 1, "ok": true, "result": null}
← {"id": 2, "ok": false, "error": "unknown command `nope`"}
```

Requests are answered on the UI thread between frames (timeout 60 s).

**Only requests are read.** Every line must be a JSON object with a string `method` (`id` and `params` are
optional; blank lines are skipped). Anything else — text that isn't JSON, a JSON array or number, an object without
`method`, invalid UTF-8, or a line longer than 4 MiB — gets one error reply
(`{"ok": false, "error": "… closing the connection"}`) and the server **closes the connection**; nothing sent after
it on that connection runs. This keeps an HTTP request (for example a web page's cross-origin `fetch` to
`127.0.0.1:<port>`) from smuggling a command in its body: its request line is rejected first. Junk never reaches the
UI thread, and at most 16 connections are served at once (further ones get an error line and are closed). Clients
that hit an error reply should reconnect. The port has no authentication, so only enable it when you need it. The MCP server's connect
mode ([mcp.md](mcp.md)) is a thin layer over this channel. Implementation:
`crates/ui-egui/src/control.rs` (methods) and `apps/lightcraft/src/control_server.rs` (transport).

## Methods

| Method | Params | Result |
|---|---|---|
| `engine.execute` (alias `ui.menu.invoke`) | `{command, params?}` | Run any engine or UI command (see `engine.commands`) |
| `engine.commands` | — | Engine + UI commands: id, label, menu, shortcut, params doc, enabled |
| `ui.menu.list` | — | Menu entries (flat: id, label, menu path, shortcut, enabled) |
| `ui.menu.tree` | — | The menu bar as shown (File … Help): items `{id, params?, label, shortcut?, enabled, checked?}`, separators, submenus — the model behind the native macOS menu bar and the in-window menus |
| `ui.inspect` | — | UI state, window, canvas/image rects, `scroll: {grid, filmstrip}` (scroll offsets in points, `null` until drawn), active photo, selection, perf (`frameMs` = layout, `logicMs` = per-frame logic before it, `updateMs` = both, `maxUpdateMs`, `fps`, render queue …, `gpu` = adapter in use, `gpuReason` = why renders don't use the GPU, `gpuFallback` = latest render redone on the CPU and why — see `docs/gpu-pipeline.md`), status, memory (bytes per cache, see `library.memory`; plus stage caches and textures), `export: {running: {total, done, current} \| null, last}`, `notices` (warnings waiting to be shown, e.g. a damaged settings file; OK = `button:noticeOk`), `quitPrompt` (why quitting was stopped: unsaved changes; `button:quitRetry` / `button:quitAnyway` / `button:quitCancel`), `import: {done, total, imported, cancelled} \| null` (an import runs on a worker thread), `tasks` (other background work: `Find Missing Photos`, `Auto Import`) |
| `ui.widgets` | `{filter?}` | On-screen widgets `{id, rect: [x, y, w, h]}` (screen points) |
| `ui.clickWidget` / `ui.dragWidget` / `ui.hoverWidget` | `{id, count?, fx?, fy?}` / `{id, toX?, toY?, dx?, dy?, steps?}` / `{id, fx?, fy?}` | Real egui input on a widget (hover: the pointer rests on it, e.g. for preset/profile previews) |
| `ui.move` / `ui.click` / `ui.drag` | `{x, y, count?, button?}` / `{x, y, toX, toY, steps?}` | Raw pointer input, screen points |
| `ui.pointer` | `{events: [{kind: down\|drag\|up, x, y}], alt?, shift?, cmd?}` | Gesture in normalized image coordinates (Detail view) |
| `ui.key` | `{key, cmd?, shift?, alt?, ctrl?}` | Key press |
| `ui.text` | `{text}` | Text input |
| `ui.scroll` | `{dx, dy}` | Mouse wheel |
| `ui.set` | partial UI state, e.g. `{"view": "detail"}` | Resulting UI state |
| `ui.dialog.confirm` / `ui.dialog.cancel` | — | Close the open dialog |
| `ui.resize` | `{width, height}` | Resize the window |
| `ui.screenshot` | `{path?, headless?}` | `{path, width, height}` once the frame (with finished renders) is captured. `headless: true` draws the UI on the CPU (no compositor needed); a windowed capture that gets no frame within 2 s falls back to headless automatically |
| `engine.execute {command: "app.export", params}` | export params (see `docs/mcp.md`), plus `preset`, `dir` / `path`, `ids`, `background` | Writes the files and returns `{files}`; with `background: true` (what the Export dialog and menus use) it returns `{background: true, total}` at once and the batch runs on a worker thread — poll `ui.inspect` → `export` |
| `ui.render` | `{id?, size?, path?}` | Render a photo (PNG to `path`), `{width, height}` |
| `app.quit` | — | Close the app |

### When the library can't be saved

With a persistent library, every command that changes something is written to the catalog journal (fsynced) before
it replies. If that write fails (disk full, volume gone, permissions), the command replies `ok: false` with
`"saved in memory but not written to disk: <reason>; LightCraft will retry"`. The change itself **is** applied (and
undoable) and stays queued: the next command, and the app's frame loop every couple of seconds, retry the write, so
nothing is lost once the disk is writable again — unless the app quits first. Meanwhile `ui.inspect` → `unsaved`
is `{ops, error}` (else `null`), `library.info` reports `unsavedOps` / `unsavedError`, and the top bar's cloud icon
shows a warning (widget `indicator:unsaved`). Queries and commands that change nothing still succeed. A failed
compaction (snapshot) is not a failed command — the log is kept whole — and only shows in `library.info` →
`lastError`.

### When the library can't be opened

If the desktop app can't open its library at launch (another program has it open, an unreadable or newer-format
catalog, a missing drive), the session starts empty and in memory — never with demo photos — and a window asks what
to do: `ui.inspect` → `libraryProblem` is `{path, error, temporarySession, pendingImport}` (else `null`); its buttons
are `button:libraryRetry`, `button:libraryChoose`, `button:libraryTemporary` (Continue Without Saving) and
`button:libraryQuit`. A temporary session shows a banner (`indicator:temporarySession`, `button:libraryReopen`) and
writes nothing. `app.openLibrary` opening a library ends it.

## Headless rendering (no window, no GPU)

The egui UI can be rasterized on the CPU (`crates/ui-egui/src/softpaint.rs`, driven by
`crates/ui-egui/src/headless.rs`): same tessellation, gamma-space premultiplied blending and
scissor clipping as egui's GPU backends, so the image matches the window (minus GPU dithering).

- **In the running app:** `{"method": "ui.screenshot", "params": {"path": "a.png", "headless": true}}`.
  The UI is drawn into an offscreen context from the app's logic tick, which keeps running when the
  window is occluded or the display sleeps/locks. Windowed screenshots fall back to this after 2 s.
- **Without any app window:** `lightcraft-cli snapshot` runs a whole app session headlessly and
  answers the same control requests (same handler) from a JSON-lines script:

```text
lightcraft-cli snapshot --demo -o grid.png --size 1600x1000 [--scale 2]
lightcraft-cli snapshot --library DIR --script tour.jsonl -o shot.png
```

  `tour.jsonl` holds one request per line (`#` comments allowed), e.g.
  `{"method": "ui.set", "params": {"view": "detail", "right": "edit", "openSections": ["optics"]}}`
  then `{"method": "ui.screenshot"}`. Replies are printed to stdout. A `ui.screenshot` without
  `path` writes `-o` (then `OUT-2.png`, `OUT-3.png`, …); `ui.settle {timeoutMs?}` waits until no
  renders are in flight. Each request runs frames until it is answered and its injected input
  (clicks, keys, drags) has played out. Widget ids for `ui.clickWidget` come from `ui.widgets`
  (e.g. `button:upright-auto`). A 1600×1000 demo snapshot takes ~1–2 s (debug build).