openpencil/packages/docs/reference/file-format.md

---
title: File Format
description: Technical reference for OpenPencil .fig and .pen document formats, Kiwi binary structure, import pipeline, and export behavior.
---

# File Format

## .fig File Structure

A `.fig` file is a ZIP archive containing a Kiwi-encoded binary message:

| Offset | Content |
|--------|---------|
| 0 | Magic header `fig-kiwi` (8 bytes) |
| 8 | Version (4 bytes, uint32 LE) |
| 12 | Schema length (4 bytes, uint32 LE) |
| 16 | Compressed Kiwi schema |
| … | Message length (4 bytes, uint32 LE) |
| … | Compressed Kiwi message — `NodeChange[]` (entire document) |
| … | Blob data — images, vector networks, fonts |

## Import Pipeline

```
.fig file → parse header → decompress Zstd → decode Kiwi schema
  → decode message → NodeChange[] → build SceneGraph
  → resolve blob refs → render on canvas
```

## Export Pipeline

```
SceneGraph → NodeChange[] → Kiwi encode → compress (Zstd/deflate)
  → build ZIP (header + schema + message + thumbnail.png)
  → write .fig file
```

Export uses <kbd>⌘</kbd><kbd>S</kbd> (Save) and <kbd>⇧</kbd><kbd>⌘</kbd><kbd>S</kbd> (Save As) with native OS dialogs on the desktop app. The exported file includes a `thumbnail.png` required by Figma for file preview.

Compression uses Zstd via Tauri Rust command on desktop, with deflate fallback in the browser.

## Kiwi Binary Codec

The codec handles Figma's 194-definition Kiwi schema with `NodeChange` as the central type (~390 fields). Key components:

| Module | Purpose |
|--------|---------|
| `kiwi-schema` | Kiwi parser (from [evanw/kiwi](https://github.com/nicolo-ribaudo/kiwi)), patched for ESM and sparse field IDs |
| `codec.ts` | Encode/decode messages using the Kiwi schema |
| `protocol.ts` | Wire format parsing and message type detection |
| `schema.ts` | 194 message/enum/struct definitions |

### Sparse Field IDs

Figma's schema uses non-contiguous field IDs (e.g. 1, 2, 5, 10 with gaps). The kiwi-schema parser handles this correctly.

### Compression

`.fig` files use Zstd compression for both the schema and message payloads. Decompression uses the `fzstd` library. For export, Zstd compression is offloaded to a Tauri Rust command on the desktop app (better performance, correct frame headers). In the browser, deflate via `fflate` is used as a fallback.

## Supported Formats

| Format | Open / Read | Save / Write | Export |
|--------|-------------|--------------|--------|
| `.fig` (Figma) | ✅ | ✅ | ✅ |
| `.pen` (Pencil) | ✅ | — | — |
| `.png` | — | — | ✅ |
| `.jpg` | — | — | ✅ |
| `.webp` | — | — | ✅ |
| `.svg` | — | — | ✅ |
| `.jsx` | — | — | ✅ |

## Clipboard Format

Copy/paste uses the same Kiwi binary encoding:

1. **Copy** — encode selected `NodeChange[]` to Kiwi binary, compress, write to clipboard as `application/x-figma-design` MIME type
2. **Paste** — read clipboard, decompress, decode Kiwi binary, create nodes in scene graph

Encoding happens synchronously in the copy event handler (not async Clipboard API) for browser compatibility. This enables bidirectional clipboard between OpenPencil and Figma.