rplace

A collaborative pixel art canvas inspired by Reddit's r/place. Place pixels, create art together in real-time.

Features

• 4096×4096 canvas with a 256-color palette (16-step grayscale + 240-hue HSL wheel)
• Real-time updates via WebSocket (Cloudflare Durable Objects)
• Batch pixel placement up to 2048 pixels per request
• Rate limit — 1 request per second per user (batch size independent)
• Zoom/pan with mouse wheel + drag (desktop) and pinch-zoom + drag (mobile)
• Long-press to place on touch devices
• Image importer — upload, dither, and auto-paint images onto the canvas

Tech Stack

Layer	Technology
Frontend	Svelte 5 (runes) + HTML5 Canvas
Backend	Hono on Cloudflare Workers
Real-time	WebSocket via Cloudflare Durable Objects (Hibernation API)
Storage	Durable Object SQLite — chunked BLOB rows for canvas, TTL rows for cooldowns
Build	Vite

Architecture

Browser (Svelte SPA + WebSocket)
  |  GET  /api/canvas   → full canvas binary (16 MB, edge-cached 10s)
  |  POST /api/place    → batch pixel placement (validated at edge)
  |  WS   /api/ws       → CanvasRoom Durable Object broadcast
  v
Cloudflare Worker (Hono — thin proxy)
  └─▶ CanvasRoom Durable Object  (single instance, idFromName('main'))
        ├── canvas_chunks   SQLite BLOB rows × 256 (64 KB each = 16 MB)
        ├── cooldowns       SQLite TTL rows (1s rate-limit, lazy GC)
        └── WebSocket hub   Hibernation API broadcasts pixel deltas

CHUNK_COUNT = ceil(CANVAS_WIDTH × CANVAS_HEIGHT / CHUNK_BYTES) — bumping canvas dimensions in src/lib/constants.js and redeploying lazy-allocates new chunks on first read. See docs/canvas-resize-procedure.md.

Getting Started

Prerequisites

• Node.js 18+
• Cloudflare account (free tier works — Workers Free + DO SQLite Free)

Setup

git clone <repo-url>
cd rplace
pnpm install

No external storage to configure. The canvas and rate-limit state live inside the Durable Object.

Development

# Run worker locally (serves API + static frontend)
pnpm dev

# Or split frontend + worker
pnpm dev:client   # Vite dev server on :5173 (proxies /api to :8787)
pnpm dev          # Wrangler dev server on :8787

Deploy

pnpm deploy   # Builds frontend + deploys worker to Cloudflare

Project Structure

src/
├── worker.js                          # Hono entry — thin proxy + edge validation
├── durable-objects/
│   ├── canvas-room.js                 # DO: storage + cooldown + WS hub
│   └── lib/
│       ├── schema.js                  # Idempotent CREATE TABLE
│       ├── chunk-storage.js           # BLOB chunk read/write
│       └── cooldown-store.js          # Rate-limit acquire + lazy GC
├── lib/
│   ├── constants.js                   # CANVAS_WIDTH/HEIGHT, CHUNK_BYTES, palette
│   ├── canvas-decoder.js              # Raw bytes → RGBA (client-side)
│   ├── cookie.js                      # parseCookie + formatSetCookie
│   ├── get-user-id.js                 # Cookie+IP rate-limit identity
│   ├── pixel-buffer.js                # Pending-stroke buffer (undo/redo)
│   ├── image-uploader.js              # Browser-side batched uploader
│   ├── image-pipeline.js              # Image-to-canvas processing
│   ├── image-pipeline-client.js       # Client-side queue
│   ├── image-pipeline-worker.js       # Web Worker handler
│   ├── image-job-storage.js           # IndexedDB job persistence
│   ├── image-resize.js                # Resampling
│   ├── image-transform.js             # Rotation / flip
│   ├── image-to-palette.js            # Palette quantization
│   ├── image-color-correction.js      # Brightness / contrast / saturation
│   └── dither-kernels.js              # Dithering algorithms
├── client/
│   ├── main.js                        # Svelte mount
│   ├── App.svelte                     # Root + WebSocket
│   ├── app.css                        # Global styles
│   └── components/
│       ├── CanvasRenderer.svelte      # Canvas + zoom/pan + touch
│       ├── ColorPicker.svelte         # Favorites + 256-color grid + custom picker
│       ├── CanvasControls.svelte      # Zoom buttons + coordinates
│       ├── DrawToolbar.svelte         # Paint / submit / undo / redo
│       ├── HelpOverlay.svelte         # Keyboard shortcut help
│       └── ImageImporter.svelte       # Image-to-canvas uploader
└── index.html                         # Vite entry

API

GET /api/canvas

Returns the full canvas as raw binary (1 byte per pixel, 16 MB — Cloudflare gzips it on the edge). Cached for 10s at the edge.

POST /api/place

Place pixels on the canvas.

{
  "pixels": [
    { "x": 100, "y": 200, "color": 27 }
  ]
}

Response: { "ok": true }

Errors:

• 400 — invalid pixel data or batch > 2048
• 413 — request body too large
• 429 — rate limited (includes retryAfter seconds)

WS /api/ws

WebSocket for real-time pixel updates. Messages are JSON:

{ "type": "pixels", "seq": 42, "pixels": [{ "x": 100, "y": 200, "color": 27 }] }

seq is a monotonic broadcast counter; the client uses it to detect missed frames and refetch the canvas to resync.

Configuration

Key constants in src/lib/constants.js:

Constant	Default	Description
CANVAS_WIDTH	4096	Canvas width in pixels
CANVAS_HEIGHT	4096	Canvas height in pixels
MAX_COLORS	256	Number of palette entries
MAX_BATCH_SIZE	2048	Max pixels per placement request
REQUEST_COOLDOWN_SEC	1	Minimum seconds between requests per user
CHUNK_BYTES	65536	Bytes per SQLite BLOB chunk (must stay ≤ 2 MB CF DO row cap)
CHUNK_COUNT	derived	ceil(TOTAL_PIXELS / CHUNK_BYTES) — auto-recomputed on resize

Credits & References

• Reddit on Building & Scaling r/place (Fastly)
• Engineering Behind r/place (Sai Kumar Chintada)
• Cloudflare Durable Objects
• SQLite-backed Durable Object Storage
• reddit-plugin-place-opensource
• rPlace by anthonytedja
• rplace.live — original 32-color palette reference (since superseded by our 256-color HSL wheel)

License

MIT