# Plug-ins: the WebAssembly plug-in API (ABI v1)

VectorCraft does not host native plug-ins built for other illustration apps: that needs unsafe FFI into
arbitrary machine code and cannot work in the browser build. Instead, plug-ins are **WebAssembly modules**
run in a sandbox. One `.wasm` file works on macOS, Windows, Linux and the web. The design follows PhotoCraft's
plug-in API (same sandbox, same manifest and parameter schema), extended for vector documents.

There are three kinds of plug-in:

- **Object filters** read the selected paths and compound paths (geometry, fills, strokes, gradients, opacity)
  and return the objects to keep, change or add. A run is one undo step. Use them for recolouring, path
  clean-up, halftones, generative patterns and the like.
- **Live effects** rewrite one object's geometry each time it is drawn, from its appearance stack, like the
  built-in Distort & Transform effects. The art stays editable: move or reshape it and the effect follows.
- **Exporters** write the whole document in a file format of their own, for pen plotters, cutting machines,
  laser cutters and the like. File › Export › Export As lists them after the built-in formats, and the host writes the
  bytes they return: a plug-in still never touches a file.

This page is the contract for plug-in authors and the reference for the host (`crates/plugins`, the
`plugin.*` commands in `crates/engine/src/cmd/plugin.rs`).

## Using plug-ins

| Command | Params | What it does |
|---|---|---|
| `plugin.list` | `{}` | `{"plugins": [{id, name, version, kind, author, description, params, paramsSchema, effect?, format?, extensions?, size, source}], "folder": report}` |
| `plugin.info` | `{"id"}` | One plug-in: the `plugin.list` entry plus `manifest`, `defaults` and `lastError` (a live effect's last failure). |
| `plugin.run` | `{"id", "params"?: {…}, "ids"?: [...]}` (plug-in params may also be top-level keys) | Runs an object filter on the selection (or `ids`). One undo step; the result is selected. |
| `plugin.install` | `{"path": "/x/y.wasm"}` (desktop) or `{"dataBase64": "…", "name"?}`, `"replace": bool = true` | Validates and installs a module; returns its `plugin.list` entry. The desktop app and `vectorcraft-cli` keep it in the `Plug-ins` folder next to the preferences, so it is installed again after a restart (#2541). |
| `plugin.remove` | `{"id"}` | Uninstalls, and deletes it from the `Plug-ins` folder. |
| `plugin.reload` | `{"path"?: "folder"}` (default: the preference folder) | Installs every `*.wasm` in a folder; a bad module is reported and skipped. |

Live effects are applied like any effect: `effect.apply {"effect": "plugin.<id>", "params": {…}}`, edited with
`effect.setParams`, listed by `effect.list` (menu `Effect › Plug-ins`) and baked by Object › Expand Appearance.

An exporter adds a write-only format, `plugin.<id>`, which `document.formats` lists with its `plugin` id.
`document.export {"format": "plugin.<id>", "params"?: {…}, "path"?}` writes it, and so does a `path` ending in one of
its extensions that no built-in format has; `selectedOnly` hands it the selection, as for the built-in formats.

In the app, installed filters appear under **Object › Plug-ins**, next to **Install Plug-in…** and **Reload
Plug-ins**; live effects under **Effect › Plug-ins** and in the Appearance panel's effect menu. A plug-in with
parameters gets a dialog generated from its schema (numbers, whole numbers, checkboxes, dropdowns, text and lists
of numbers, labelled, explained and sorted into sections as its manifest says) with a live preview; without
parameters it runs at once. Parameters too many for the window's height scroll inside the dialog, which keeps its
heading and buttons in view. Exporters are formats of **File › Export › Export As**, where Use
Artboards is greyed out (the plug-in writes the whole document); one with parameters asks for them in a dialog of the
same kind once the file is picked, and its Export button writes the file. **File › Open** of a `.wasm` file installs
a plug-in too (on the web this is how plug-ins get in).

**Preference folder.** Edit › Preferences › Plug-ins & Scratch Disks › *Additional Plug-ins Folder*: when set, every
`*.wasm` in that folder (not recursive) is installed at startup and whenever the preference changes (desktop builds;
the web build installs from bytes).

Plug-ins are installed **per process**, not per document. Documents store a live effect as an ordinary effect record
(`{"id": "plugin.<id>", "params": {…}}`): while the plug-in is installed it is drawn, exported and expanded; without it
the object is drawn as if the effect weren't there, and the record is kept for when the plug-in comes back.

## The sandbox

The host is [`wasmi`](https://github.com/wasmi-labs/wasmi), a pure-Rust WebAssembly interpreter
(MIT OR Apache-2.0). It builds for `wasm32-unknown-unknown` too, so plug-ins also run in the web build. Limits
(`vectorcraft_plugins::Limits`, defaults):

| Limit | Default | On violation |
|---|---|---|
| Host imports | none: no WASI, file system, network, clock or randomness | module rejected at install |
| Module size | 32 MiB | rejected |
| Linear memory | 512 MiB per instance (input and parameters: at most half) | `memory.grow` returns -1; oversized input or initial memory fails |
| Output | 64 MiB of JSON | `Limit("output size")` |
| Instructions (fuel) | 50 M per call + 4,000 per input byte for `vc_run`; an export 10 G + 4,000 per byte | `Limit("instruction budget")` |
| Wall clock | 60 s per filter run, 5 s per live-effect run, 300 s per export (desktop; checked every 20 M instructions) | `Limit("time budget")` |
| Call depth | 1,024 frames | trap |
| Module shape | wasmi's strict limits (≤ 10,000 functions, 1 memory, …) | rejected |

An export runs where the export runs: with Background Export on, on the export's worker, whose Cancel stops the
plug-in between two of those 20 M-instruction slices; nothing is written then.

Every run gets a **fresh instance**, so no state survives between calls. Traps, exhausted budgets, malformed
modules, bad manifests, out-of-range pointers, negative return codes and invalid output all become errors, and the
document is left untouched: a filter's output is checked completely before one edit applies it. A live effect that
fails draws the object's geometry unchanged; `plugin.info` reports why (`lastError`).

wasmi is built with its *portable* (loop) dispatcher: the default tail-call dispatcher relies on LLVM
sibling-call optimisation, and when that doesn't happen a long-running module overflows the host stack, which aborts
the process instead of returning an error.

Drawing evaluates effects often, so live-effect results are cached by plug-in installation, parameters and input
geometry: a plug-in runs again only when one of them changes.

## ABI v1

A plug-in is a core WebAssembly module (MVP + the usual post-MVP features: bulk memory, sign extension, multi-value,
saturating conversions; no SIMD, no memory64) with **no imports** and these exports. Integers are `i32` unless noted;
pointers are offsets into the module's memory.

| Export | Signature | Meaning |
|---|---|---|
| `memory` | memory | The module's linear memory. |
| `vc_abi_version` | `() -> i32` | Must return `1`. |
| `vc_manifest` | `() -> i64` | `(len << 32) \| ptr` of the UTF-8 manifest JSON (≤ 64 KiB). |
| `vc_alloc` | `(size) -> ptr` | A block of `size` bytes the host may write, or `0` on failure. Never freed: each run gets a fresh instance. |
| `vc_run` | `(input, input_len, params, params_len) -> i64` | Runs the plug-in on the input JSON with the parameters JSON; returns `(len << 32) \| ptr` of the output JSON, or a negative error code. |

### Manifest

```json
{
  "id": "org.example.tidy",
  "name": "Tidy Paths",
  "version": "1.0.0",
  "kind": "filter",
  "author": "Example",
  "description": "What it does.",
  "params": {
    "amount": {"type": "number", "min": 0, "max": 100, "default": 50, "unit": "%"},
    "steps":  {"type": "int", "min": 1, "max": 16, "default": 4, "label": "Steps per curve"},
    "closed": {"type": "bool", "default": false},
    "mode":   {"type": "choice", "options": ["soft", "hard"], "default": "soft",
               "help": "Soft keeps the corners round; hard keeps them sharp."}
  }
}
```

- `id`: 1–64 characters from `A-Z a-z 0-9 . _ -` (reverse-DNS recommended). Installing a module with the same id
  replaces the old one. A live effect's effect id is `plugin.<id>`.
- `name`: the menu label, or an exporter's format name in Export As (1–64 printable characters). `kind`: `"filter"`
  (object filter), `"effect"` (live effect) or `"exporter"`. Exporters came after the first ABI v1 hosts, which refuse
  the kind when the plug-in is installed.
- `extensions` (exporters only, and required): 1–8 file extensions, lower-case letters and digits without the dot
  (`["plt"]`); the first is the one Export As gives the file. A built-in format keeps an extension both list (`svg`):
  `document.export` then writes the exporter's format only when asked for by its format id.
- `params`: in dialog order. Names are `[A-Za-z0-9_]`, not starting with `_`, and not `id`, `ids`, `params` or
  `preview`. The host validates the user's values before calling: missing values take the default, numbers are
  clamped to `min..max` (ints rounded), a wrong type or an unknown choice is an error. Numeric defaults
  are also clamped to their range, including the implicit zero default of an `int` parameter. Besides `number`,
  `int`, `bool` and `choice` there are `string` (`{"type": "string", "default": "", "maxLength": 256}`: a text field;
  longer text is cut to `maxLength`, at most 4,096 characters) and `numberList` (`{"type": "numberList", "min": 0,
  "max": 360, "default": [45, 135], "minItems": 1, "maxItems": 16}`: numbers typed separated by commas or spaces,
  each clamped to `min..max`; a list of too few or too many is an error, and `maxItems` is at most 256). A plug-in
  declares up to 256 parameters, at most 64 of them outside groups.

Each parameter may also say how dialogs show it. Hosts from before these keys ignore them, so a plug-in that
declares them still installs there:

| Key | What the dialog does with it |
|---|---|
| `label` | Labels the field (1–64 characters). Without one, the label is the name spaced out (`rowSpacing` → Row Spacing). |
| `unit` | Shows the unit the value is in after it (1–16 characters, such as `mm` or `°`). Nothing is converted: the plug-in gets the number as typed. |
| `group` | Puts the parameter in a section that folds (1–64 characters). Parameters outside any group come first, then each group in the order it first appears. |
| `help` | Shows what the parameter does when the pointer rests on its label (1–1,000 characters; line breaks allowed). |
| `helpUrl` | Opens this `https://` page (at most 512 bytes) from a button after the field. |
| `visibleWhen` | `{"pattern": ["lines", "crosshatch"], "outline": [true]}` shows the parameter only while each parameter it names (a checkbox or a choice declared before it) has one of the values listed and is shown itself. |

A hidden parameter still goes to the plug-in with its value: what the dialog showed never changes a run. The
filter, live-effect and exporter dialogs use all of these keys. The Appearance panel's editor of a plug-in effect's
parameters uses `label`, `unit`, `help` and `visibleWhen`, and lists the parameters without sections or help buttons.

### Input

`input` is UTF-8 JSON: `{"objects": [...]}`. An object filter gets the selected paths and compound paths in paint
order (bottom first): the selected ones and those inside selected groups and layers; hidden, locked, guide and clipping
paths are left out, and so are other kinds of object (type, images, symbols, live blends…), which a filter never
touches. With nothing selected the list is empty (a generator then adds its own objects). A live effect gets one
object: its geometry (`type`, `path`, `fillRule`, `bounds`) and its `fills` and `strokes`, as a filter gets them (hosts
from before these give geometry only). The paints are read only: an effect may draw as wide as a stroke or use a
fill's colour to decide its shape, but it changes only the geometry.

An **exporter** gets something else: the document as a native `.vectorcraft` file's JSON, as `document.save` writes it
but without indentation (the format is documented in [`crates/format/src/lib.rs`](../crates/format/src/lib.rs)). That is every layer, object,
appearance and effect record, artboard, swatch and symbol, so an exporter reads whatever the document holds; it checks
`version`, since a later host may write a newer one. Images of 8 KiB or more are listed in `images` with their MIME
type and `blob` but without their data, which follows the JSON in a saved file; smaller ones keep their base64 `data`.

```json
{"objects": [
  {"id": 12, "type": "path", "name": "Leaf",
   "path": {"subpaths": [{"closed": true, "anchors": [
     {"p": [10, 10]},
     {"p": [60, 10], "in": [45, 0], "out": [75, 20], "kind": "Smooth"},
     {"p": [35, 60]}]}]},
   "fillRule": "nonzero",
   "fills": [{"type": "solid", "color": {"model": "rgb", "r": 0.9, "g": 0.2, "b": 0.1}}],
   "strokes": [{"paint": {"type": "gradient", "gradient": {"kind": "Linear", "stops": [
       {"offset": 0, "color": {"model": "gray", "k": 0}},
       {"offset": 1, "color": {"model": "cmyk", "c": 0, "m": 0.5, "y": 1, "k": 0}}]}, "angle": 0},
     "width": 2, "join": "round", "cap": "butt", "miterLimit": 10}],
   "effects": [{"id": "distort.roughen", "params": {"detail": 10, "points": "smooth", "relative": true, "seed": 0,
     "size": 5}, "visible": true}],
   "opacity": 1,
   "bounds": [10, 0, 75, 60]}
]}
```

- `type`: `"path"` or `"compound"` (a compound path: `path` holds its members' subpaths, painted as one).
- `path`: VectorCraft's own path form. Each subpath is a run of anchors; `p` is the anchor, `in` / `out` its handles
  (absent: no handle), `kind` `"Smooth"` when the handles stay collinear. Coordinates are points, y down.
- `fills`, `strokes`: the object's fills and strokes in paint order (bottom first); `null` is no paint. A stroke has its
  `paint`, `width` in points, `join` (`"miter"`, `"round"` or `"bevel"`), `cap` (`"butt"`, `"round"` or `"square"`)
  and `miterLimit`. Paints use the
  native file format's form: `{"type": "solid", "color", "swatch"?, "tint"?}`, `{"type": "gradient", "gradient":
  {"kind": "Linear"|"Radial"|"Freeform", "stops": [{offset, color, opacity, midpoint}], "dither"?, "method"?: "Perceptual"}, "geom"?, "angle",
  "freeform"?}`, `{"type": "pattern", "pattern", "xf"?}`. Colours are `{"model": "rgb", r, g, b}`, `{"model": "cmyk",
  c, m, y, k}`, `{"model": "gray", k}` (ink: 0 white, 1 black) or `{"model": "lab", l, a, b}`; components 0–1 except
  Lab's.
- `effects` (filters): the object's own effects in order, as its effect records: `id` (as `effect.list` lists it, or
  `plugin.<id>` for a live-effect plug-in), `params` and `visible`. Its fills' and strokes' own effects aren't listed.
- `bounds`: `[x0, y0, x1, y1]`, the geometric bounds (read only).

### Parameters

`params` points at `params_len` bytes of UTF-8 JSON: the validated parameters, plus `_context`:

```json
{"amount": 50, "mode": "soft",
 "_context": {"mode": "filter", "bounds": [10, 0, 75, 60], "artboard": [0, 0, 612, 792], "documentData": {"pens": 4}}}
```

`mode` is `"filter"`, `"effect"` or `"export"`; `bounds` the selection's geometric bounds (a live effect: the reference
box of the effect, the object's bounds or those of the previous effect's result); `artboard` (filters) the artboard
under the selection's centre, else the first; `documentData` (filters) what the plug-in last kept with the document
(`null` until it keeps something; see Output). A live effect's `item` says where it sits: `"object"` (one of the
object's effects), or `{"fill": i}` / `{"stroke": i}` (in the own effects of the object's `i`th fill or stroke,
counted from the bottom as in `fills` and `strokes`). An export's `_context` is `{"mode": "export", "extension": "plt",
"name": "Poster"}` instead: the extension of the file being written (one the exporter lists) and its name without the
extension (the document's name when the export is not given a path), which some formats keep in their header.

### Output

The output is UTF-8 JSON in the input's shape: `{"objects": [...]}`. To decline with a message for the user, return
`{"error": "Select two paths."}`.

For an **object filter**, the output is the new state of the input objects:

- An object with the `id` of an input object updates it; keys it leaves out keep their value (`{"id": 12}` keeps the
  object as it is). `fills` and `strokes` set the object's fills and strokes in order: extra entries add fills (above
  the last one) or strokes, missing ones are removed; each stroke keeps its weight, join, cap and miter limit unless
  `width`, `join`, `cap` or `miterLimit` are given (a new one: 1 pt, miter join, butt cap, miter limit 10).
- `effects` replaces the object's own effects, in order: each `{id, params?, visible?}`. A record the object already had
  stays as it is; a new one is admitted as `effect.apply` admits it: an effect `effect.list` lists or an installed
  live-effect plug-in, with parameters it takes (filled in to their defaults; a plug-in's checked against its
  manifest), at most one 3D effect (its fills' and strokes' own effects counted), which must fit the object, and a
  Glass `textureImage` as Glass stores it (base64 of a PNG: a plug-in can't have the host read a file). A tool can
  then set up objects for a live effect in one step.
- An object without an `id` is new and needs a `path`. It goes just above the object listed before it, or, listed
  before any input object, just below the first one listed after it. New objects take a path's defaults: no fills or
  strokes, full opacity.
- Input objects missing from the output are deleted. The order of the input objects themselves doesn't change.
- `type` turns a path into a compound path (one member per subpath) or back; `name: ""` clears the name.
- A `path` returned as it was keeps a live shape live (a rectangle keeps its corner widgets); a changed one makes it a
  plain path.

The host validates everything before the document changes: coordinates must be finite and within ±4,000,000 pt, at
most 100,000 objects and 4,000,000 anchors, 256 fills, strokes or gradient stops; colours, opacities and offsets are
clamped into range, stroke weights to 0–1,000 pt and miter limits to 1–500; a pattern must exist in the document. A
paint returned unchanged keeps its swatch link; any other paint is unlinked. One refused effect record refuses the
whole output.

A filter's output may also hold `documentData`: any JSON of at most 1 MiB, which replaces what the plug-in keeps with
the document (`null` removes it), in the same undo step as its objects. Settings that belong to the whole document
rather than to one object go there, such as the pens a plotter holds. A filter reads and writes only its own data. It
is the document's foreign data, as the perspective grid is: the native file keeps it in `document.unknown.pluginData`
under the plug-in's id, and hosts from before `documentData` keep it when they save. An exporter is given the whole
document, so it can read what every plug-in keeps there, not only its own. A live effect isn't given it: its geometry
is worked out from the object alone.

For a **live effect**, the geometry of every returned object's `path` (their subpaths, in order) replaces the
object's; the rest is ignored (the object keeps its own appearance). An empty list removes the geometry.

An **exporter** returns `{"dataBase64": "…", "warnings"?: ["…"]}`: the file, in base64, which the host writes as it
is, and what the export left out or changed, for the user (at most 100 warnings of at most 500 characters each, shown
with the export's own). A decline (`{"error": …}`), a missing `dataBase64` or data that isn't base64 writes nothing.

## Writing a plug-in in Rust

[`crates/plugins/example`](../crates/plugins/example) is a complete object filter, **Desaturate**: it moves every
colour of the selection towards grey, in each colour's own model, using `serde_json` (no imports, no WASI). Build it
with:

```sh
rustup target add wasm32-unknown-unknown
crates/plugins/example/build.sh      # cargo build --release --target wasm32-unknown-unknown
```

The script also refreshes `crates/plugins/tests/fixtures/desaturate.wasm`, the test fixture built from that source.
The crate is not a workspace member. Any language that targets core WebAssembly works (C, Zig, AssemblyScript,
WAT…), as long as the module has no imports: in Rust, `std` on `wasm32-unknown-unknown` is fine as long as it never
touches WASI. `vectorcraft_plugins::wat::module` is the skeleton of a plug-in in WebAssembly text, which the tests
build their plug-ins from; `wat::echo_exporter` is an exporter whose file is its input or its parameters (with a
base64 encoder written out in WebAssembly text), which the tests read back what the host hands a plug-in with, and
`wat::when_given` a plug-in whose output depends on whether its input or parameters hold a given text, which shows in a
live effect's geometry.

## Performance

The interpreter costs roughly 10–100× native speed per instruction. Filters handle a selection in one call, so a
plug-in that parses and writes JSON with `serde_json` takes milliseconds for hundreds of paths. Live effects run once
per change of their input, parameters or paints (results are cached), not per frame.

## Not in v1

- Tool plug-ins (pointer events on the canvas, a guide line or rectangle, geometry back), panel plug-ins, host
  callbacks (a plug-in reporting progress or asking whether it was cancelled), importers (plug-in formats that open
  files).
- Live effects that change paint (they reshape geometry; an object filter recolours), and filters on type, images or
  symbols.
- Apply Last Effect and Last Effect work for plug-in effects; there is no "last filter" for object filters yet.