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lightcraft/docs/control-protocol.md
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Brandon ThomasandClaude Opus 5.5 37ab239b7b M0.10: headless UI rendering — CPU rasterizer for egui, ui.screenshot headless + fallback, lightcraft-cli snapshot
- ui-egui/softpaint: pure-Rust rasterizer for tessellated egui output (gamma-space premultiplied
  blending like egui-wgpu, bilinear texel-centre sampling, scissor clips, tie-break rule so shared
  edges blend once, parallel row bands, deterministic) + CPU texture mirror from TexturesDelta.
- ui-egui/headless: HeadlessView (offscreen context with our fonts/theme) and Headless (windowless
  app session: logic+ui per frame, answers viewport Screenshot/InnerSize/Close, settle loop,
  control requests through the shared handler).
- ui.screenshot {headless: true} in the running app renders from the logic tick (works while the
  window is occluded / display asleep); windowed captures fall back to headless after 2 s.
  Screenshots now wait for in-flight renders, not just queued ones.
- lightcraft-cli snapshot [--demo|--library DIR] [--script f.jsonl] -o out.png --size WxH --scale S.
- Fixes found with it: dialog backdrop was painted over the dialog itself (now an area below the
  window); dialog content padding; crop overlay options use our buttons (registered widget ids).
- Docs: control-protocol.md (headless section), AGENTS.md (prefer headless when unattended).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-01 01:48:48 -04:00

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# 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). 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 |
| `ui.inspect` | — | UI state, window, canvas/image rects, active photo, selection, perf, status |
| `ui.widgets` | `{filter?}` | On-screen widgets `{id, rect: [x, y, w, h]}` (screen points) |
| `ui.clickWidget` / `ui.dragWidget` | `{id, count?, fx?, fy?}` / `{id, toX?, toY?, dx?, dy?, steps?}` | Real egui input on a widget |
| `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 |
| `ui.render` | `{id?, size?, path?}` | Render a photo (PNG to `path`), `{width, height}` |
| `app.quit` | — | Close the app |
## 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).