lightcraft/docs/web.md

LightCraft in the browser

apps/lightcraft-web runs the same egui UI as the desktop app (crates/ui-egui) in the browser, compiled to WebAssembly and drawn with WebGL2 (eframe's glow backend).

Build and run locally

One-time setup:

rustup target add wasm32-unknown-unknown
cargo install wasm-bindgen-cli --version <the wasm-bindgen version in Cargo.lock> --locked

cargo xtask web checks the CLI against Cargo.lock and prints the exact install command if it's missing or a different version.

Build, then serve:

cargo xtask web            # → <target>/web/{index.html, worker.js, lightcraft_web.js, lightcraft_web_bg.wasm}
cargo xtask web --serve    # build, then serve on http://127.0.0.1:8080/ (or `--serve 9000`)
cargo xtask web --dev      # unoptimized build with debug info (faster to compile, slow to run)

<target> is target/ unless CARGO_TARGET_DIR is set. Any static HTTP server works, as long as it serves .wasm as application/wasm. The bundle can't be opened from file:// (module workers and OPFS need an HTTP(S) origin; localhost/127.0.0.1 count as secure). If wasm-opt (binaryen) is on PATH, the release build also runs it over the module.

The release build uses the web Cargo profile (release + thin LTO, no debug info).

Deploying: headers

cargo xtask web --serve sends these on every response, and a production server should too:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Resource-Policy: same-origin

COOP + COEP make the page cross-origin isolated (crossOriginIsolated === true). That is required for SharedArrayBuffer, i.e. for a future wasm-threads build (+atomics, nightly build-std), and gives full-resolution performance.now(). The current render workers (below) don't share memory, so the app also runs without them; with COEP on, every subresource must be same-origin or send CORP/CORS headers (the bundle has no third-party resources).

What works

• A persistent library. The library lives in the browser's storage for the page's origin: OPFS when the main thread can write it (FileSystemFileHandle.createWritable: Chrome, Edge, Firefox, recent Safari), otherwise IndexedDB. Both hold the same layout:

  • library/catalog.snap, library/catalog.log: the same crash-safe journal as the desktop app (lightcraft-catalog), plus presets.json, view.json, prefs.json and ui.json (panel layout). The catalog Store is a memory mirror loaded at start-up; every change is flushed in the background within a frame or two, each file replaced atomically, in modification order (apps/lightcraft-web/src/files.rs). View state and UI prefs are saved every second when they change (a tab can close without notice).
  • originals/<content hash>: the bytes of every imported photo. The catalog refers to them as web/<hash>/<file name>; the main thread keeps recently used originals in memory (≤ 768 MB) and loads the rest on demand (the active photo is prefetched).
  • thumbs/<key>.jpg + thumbs/index.json: the rendered-thumbnail cache, with the desktop's budget (2 GB, least recently used pruned to 80 %).

  A new library starts with the procedural demo photos. The app asks for persistent storage (navigator.storage.persist()); without it the browser may evict the data under storage pressure. URL options: ?store=idb forces IndexedDB, ?store=memory keeps nothing, ?reset deletes the stored library first.

• Importing photos with no filesystem. File ▸ Add Photos… (⌘⇧I) opens the browser's file picker. You can also drop files anywhere on the page. The bytes are written to storage, then imported and decoded by the same engine code as the desktop app (lightcraft_engine::files::{probe_bytes, load_bytes}): JPEG/PNG/TIFF/WebP and the supported raw formats. "Copy into library" is the same as "Add" here.

• Rendering in Web Workers. Renders don't run on the main thread: up to four dedicated workers (hardwareConcurrency − 1, ?workers=N to override, ?workers=0 for the old inline path) each run a second instance of the same wasm module. index.html compiles the module once and posts the compiled WebAssembly.Module to worker.js. A job crosses as JSON (the photo's source, develop settings, request); the worker reads the original or the cached thumbnail from storage itself, keeps decoded sources and the loupe's stage cache, renders, and transfers the RGBA bytes back. Jobs for a photo go back to the worker that decoded it. This needs no wasm threads or SharedArrayBuffer. If no worker starts, rendering falls back to the main thread.

• Every develop control (sliders, curves, mixer, grading, masking, crop…) works as it does on the desktop, since it's the same crate.

• Export downloads the file. Export… (⌘⇧E) runs the same app.export path as the desktop app (lightcraft_engine::export: JPEG/PNG/TIFF/WebP, sizing, naming). The host's write service hands each file to the browser as a download instead of writing it to disk.

• Automation. await lightcraft.command("library.info", "{}") runs any engine or UI command by id on the next frame and resolves to the JSON result (web.stats reports storage, workers and the render queue). This is how the headless-Chrome checks drive the page.

Not yet

• Exports and auto-adjustments run on the main thread (they need the original's pixels there). Right after a reload, the first such command on a photo whose original isn't in memory yet fails with "still loading" and works a moment later.
• Durability is "a frame later", not fsync-before-return: a crash or tab kill in the few milliseconds between a command and its flush loses that command.
• Preset files (import/export .json) aren't wired to browser pickers yet.
• Control channel / MCP. These are desktop-only, because they need a TCP socket.
• AVIF export is untested in the browser; it's the one encoder that may not be wasm-safe.

Measuring

Open http://127.0.0.1:8080/?bench to run a scripted measurement:

1. Wait for the grid thumbnails.
2. Open the first photo in Detail.
3. Drag Exposure through 8 steps, exactly as a slider drag does (begin interaction → develop.set ×8 → end).

The page then logs one console line:

lightcraft-bench {"first_frame_ms":…,"thumbs_done_ms":…,"slider_draft_ms":[…],"slider_draft_median_ms":…,"slider_job_ms":[…],"release_full_ms":…}

• slider_draft_ms: time from the develop.set command until the new loupe texture is ready.
• slider_job_ms: the pipeline's share of that time.
• release_full_ms: the full-quality render after the drag ends.