Contributing

Language: Rust only. No JavaScript or TypeScript. On the web, wasm-bindgen generates a small loader; never hand-write JS.
Licence: contributions are MIT OR Apache-2.0 (LICENSE-MIT, LICENSE-APACHE, NOTICE). The ArtCraft logos in docs/brand/ are not open source (docs/brand/LICENSE-brand.txt).
Clean-room: do not copy code, shaders, icons, ICC profiles or other assets from proprietary software (such as Photoshop). Match behaviour and look by observation and public specs. Only use assets with permissive licences, keep their license next to the asset (e.g. assets/fonts/OFL-*.txt, assets/icons/LICENSE-lucide.txt), and list every asset in ATTRIBUTION.md.
Commands, not handlers: new features are engine commands with tests, and the UI calls them (checklist below).
Layering: cargo xtask layers must pass. Register new crates in xtask/src/layers.rs.
Tests: required for every change. Format code needs round-trip and malformed-input tests. Pixel code is tested at 8, 16 and 32-bit.
Style: cargo fmt, and cargo clippy -- -D warnings. Match surrounding code. Comments explain why.
UI: use theme::Tokens and widgets::*. Verify visually (offscreen snapshot example or the control channel) before submitting, and attach before/after screenshots to PRs.
Commits: small, focused, with a clear subject line.

Adding a command

Find the id. Search crates/ui-egui/src/menu_catalog.rs for the Photoshop menu item. Using its id makes the menu item live with no UI work. Commands without a Photoshop menu entry use a descriptive id in the same style (layer.smartFilter.delete) and an empty menu path.
Algorithm goes in the lowest crate that fits (algo for imaging, paint, vector, text, cms), with unit tests. It takes depth-agnostic surfaces (photocraft-raster), works per tile, and is deterministic (seeded randomness).
Command goes in an engine module (crates/engine/src/<area>_cmds.rs) exposing specs(), registered with v.extend(...) in commands.rs. Fill in:
- id, label, menu path and Photoshop's default shortcut,
- a params doc string such as {"radius":px=4,"mode":"a|b"="a"} (this is what agents read through commands / command_list),
- an enabled predicate (greys the menu item out),
- run, which returns a JSON result and records exactly one history step (use coalesce for drags and typing sessions).
Respect the context: active selection (feathered), layer vs mask target, locks, the colour model and depth.
Tests in the module: behaviour, undo/redo, disabled states, bad params, several depths.
Dialog (if it has parameters): filters and adjustments get schema-driven dialogs with live preview (ui-egui/src/filter_dialog.rs, dialogs.rs).
Run cargo xtask parity and commit the updated docs/parity.md.