craft-libs/crates/fig/README.md

craft-fig

A clean-room reader and writer for the .fig container: the local copies Figma Design saves (.fig) and FigJam whiteboards (.jam, the same archive with another canvas header). No unsafe.

This crate is the wire layer. It opens the archive, decompresses the canvas, hands the schema and the message to craft-kiwi, finds images, and reads and writes the two blob encodings (vector networks and outline geometry). What the decoded node changes mean, and how they map onto an application's document, stays with the caller: the message is a kiwi::Kv tree read by field name, in the file's own terms.

Provenance

Written by FigCraft's contributors, from public format knowledge only: public descriptions of the container, the kiwi format's published specification, and black-box observation of files made from synthetic documents. Nothing inside the incumbent's application bundle, web bundle or network traffic was read, disassembled or copied, and no GPL, LGPL or AGPL code was consulted. Written files were checked by opening them in other software. Moved here from figcraft/crates/format/src/fig.rs, fig/network.rs, fig/export.rs (the container half) and fig/map/layer.rs (the geometry blob).

The format

• A .fig file is a ZIP archive: canvas.fig (the document), meta.json (file_name, client_meta.background_color, client_meta.thumbnail_size), thumbnail.png and images/<hash> (the hash in hex, no extension). A bare canvas.fig is a file too: the clipboard carries one.
• canvas.fig is an 8-byte magic (fig-kiwi; fig-jam. for a whiteboard), a little-endian u32 version, then length-prefixed chunks, each compressed with raw deflate, zlib or zstd. The first chunk is a kiwi binary schema, the second a message encoded with it.
• The message's nodeChanges list the layers (each with its id, its parent's id and its position among the siblings). Its blobs hold vector networks (vectorData.vectorNetworkBlob), outline geometry (fillGeometry[].commandsBlob) and, in files written by plugins and other tools, the bytes of images (image.dataBlob).

Reading

let archive = craft_fig::Archive::open(&bytes)?;             // a zip or a bare canvas
let packed = archive.canvas.len();
let (schema, message) = craft_fig::decode_canvas(&archive.canvas)?;
let root = schema.find("Message").ok_or("no Message definition")?;
let msg = schema.decode_message_within(root, &message, craft_fig::value_budget(packed))?;
let embedded = craft_fig::embedded_copies(&msg, craft_fig::Limits::default().max_image);
let image = archive.image("0a1b…").or_else(|| embedded.get("0a1b…").map(|b| b.to_vec()));
let network = craft_fig::network::decode(blob_bytes);        // Option<Network>
let outline = craft_fig::path::decode(blob_bytes);           // Option<Vec<Command>>

Writing

let canvas = craft_fig::encode_canvas(&kiwi::encode::schema(&schema), &kiwi::encode::message(&schema, root, &msg)?)?;
let bytes = craft_fig::write_archive(&craft_fig::Package {
    file_name: "Untitled", background: [1.0, 1.0, 1.0, 1.0], canvas: &canvas, images: &images, thumbnail: None,
})?;

Limits

Input is hostile. Nothing panics; every limit has a test.

Limit	Value
canvas.fig read or written (MAX_CANVAS)	256 MiB
A decompressed chunk (MAX_CHUNK)	512 MiB
The decompressed schema chunk (MAX_SCHEMA)	16 MiB
Chunks read (MAX_CHUNKS)	16
meta.json	1 MiB
Values decoded from the message (value_budget)	4 per compressed canvas byte, at least 20 million, at most 128 million (32 million on 32-bit targets)
An image (Limits::max_image)	256 MiB
Image bytes read in all, failed reads included (Limits::max_image_total)	4 GiB (1 GiB on 32-bit targets)
An image hash (MAX_IMAGE_HASH)	64 bytes
Vertices, segments, regions or loop entries in a vector network (network::MAX_ITEMS)	200 000
Commands in a geometry blob (path::MAX_COMMANDS)	200 000
A written file (canvas and images)	2 GiB

The value budget follows the compressed size, which the file itself bounds, not what it unpacks to: a small file can't claim a large tree however far its chunks inflate.

ZIP

The archive is read with the zip crate directly, with the bounds above applied to each member. A shared bounded ZIP reader is planned (craft-libs #126); this crate moves to it when it lands.

The message as REST file JSON (rest)

rest::to_rest maps a decoded message onto the REST API file JSON that craft-figma-rest models: pages, the layer tree, geometry, paints, strokes, effects, text with styled runs, auto layout and grids, components, component sets and instances (expanded from their main, with overrides and property values applied), shared styles, variables, prototype interactions and flows. Layer kinds the REST shape doesn't have (sticky notes, connectors, tables, …) come out as the closest frames, text and vectors. What isn't understood is skipped with a warning.

let mapped = craft_fig::rest::to_rest(&msg, "Untitled", &craft_fig::rest::Options {
    max_nodes: craft_fig::rest::layer_budget(packed),
    ..Default::default()
});
let file = craft_figma_rest::File::from_value(mapped.json, usize::MAX)?;  // or read the JSON
// mapped.networks: the vector networks layers name by `vectorNetworkRef`
// mapped.warnings: what was skipped or approximated

• Geometry the caller owns. A connector's route between its ends, and whether a text path can be measured, come from Options::geometry (the rest::Geometry trait): an editor routes imported connectors as it routes its own. rest::Straight draws straight lines and accepts every text path.
• Extensions. Besides the public fields, the JSON carries the extensions craft-figma-rest documents and private _fig* handoffs (each layer's provenance, cached library definitions, mains the layer cap kept out, region paints, saved line breaks, text paths) for an importer that wants them.
• Budgets. At most Options::max_nodes layers are produced or scanned, instance expansion included (rest::layer_budget follows the compressed canvas: 8 bytes a layer, 200 000 to 2 000 000); nesting is followed 256 levels deep (the walk recurses once per level in small frames: an unoptimised build maps past that limit on 512 KiB, half the 1 MiB stack of a WebAssembly build, and a test at the limit runs on a 1 MiB thread); conditional actions 16, expressions 32; grid tracks 1 000; region paint elements 200 000 per file; scale factors between 0.001 and 1000.

The mapping was written in FigCraft (crates/format/src/fig/map) by its contributors, from the same public format knowledge as the rest of this crate.