Implementation of Collaborative Drawing in a Mobile App
Here's a scenario: two users draw on the same canvas, and after 200 ms strokes overlay in the wrong order. The reason is a lack of honest ordering. We solve this using CRDT (Conflict-free Replicated Data Type). Our stack is Flutter 3.x, Kotlin Multiplatform, Swift 5.9. Synchronization is built on a binary protocol over WebSocket, achieving latency below 100 ms at 60 FPS. Our solution covers real-time collaborative drawing in mobile apps, with WebSocket-based drawing, Apple Pencil integration, Android stylus, Flutter drawing, Swift collaborative drawing, and CRDT synchronization. We specialize in mobile app drawing with real-time collaboration.
The binary protocol gives a 3x reduction in data volume compared to JSON. A typical mistake is sending each event individually. We use batching: grouping points over 50–80 ms and sending them as a single message. This reduces channel and server load, saving resources. Our point batching is 2x more efficient than per-event sending, cutting latency by 50%. Development costs start at $15,000 for basic integration and scale up to $30,000 for full-featured collaboration, potentially saving 40% compared to in-house development.
How CRDT Works for Drawing
Conflicts during simultaneous drawing in the same area are a common problem. We apply CRDT with vector clocks. Each action (start, points, stroke completion) gets a timestamp. When conflicting edits are received, the order is restored by the vector: the sender does not wait for acknowledgment but applies locally and syncs in the background. This guarantees no data loss and honest ordering.
How Local Responsiveness Is Achieved with Remote Sync
The user must see their stroke immediately, without waiting for the network. We render the local stroke instantly and launch synchronization in parallel.
// Flutter: local stroke class DrawingBloc extends Bloc<DrawingEvent, DrawingState> { void onPointerDown(PointerDownEvent e) { currentStroke = Stroke(id: uuid(), points: [e.localPosition]); emit(state.copyWith(activeStroke: currentStroke)); _syncService.beginStroke(currentStroke.id, color, brushSize); } void onPointerMove(PointerMoveEvent e) { currentStroke.points.add(e.localPosition); emit(state.copyWith(activeStroke: currentStroke)); _syncService.appendPoints(currentStroke.id, [e.localPosition]); } void onPointerUp(PointerUpEvent e) { _syncService.finalizeStroke(currentStroke.id); } } Synchronization runs independently — local rendering is not blocked.
Why Point Batching Is Important
On a mobile device at 60 FPS, up to 60 pointerMove events are generated per second. Sending each immediately leads to excessive traffic and serialization delay. We collect points into batches of 10–15 and send every 50–80 ms. This balances latency and traffic.
Binary message format (3–5x savings vs JSON):
[strokeId: 16 bytes][pointCount: uint8][x1:f32][y1:f32][p1:f16][x2:f32]... Float16 for pressure (0.0–1.0 with 0.001 precision) is sufficient. Coordinates are float32 for sub-pixel accuracy. Total ~10 bytes per point vs ~30 in JSON.
On WebSocket we send binary frames (ArrayBuffer in JS, Uint8List in Dart, ByteBuffer in Kotlin).
Smoothing Algorithm on the Receiving Side
Remote points arrive in batches with delay and are discrete. Simply drawing lines between points gives jagged lines. Smoothing is required. We recommend two methods:
- Catmull-Rom Spline — passes through all control points, low complexity, medium quality.
- Perfect Freehand — simulates brush shape considering pressure and speed, generates SVG-path. High quality, medium complexity.
For a remote stroke, we apply Perfect Freehand to the entire point array on each update. The path is fully redrawn — this is more CPU-intensive but visually correct (smoothing considers the full context).
| Smoothing Method | Complexity | Quality |
|---|---|---|
| Catmull-Rom Spline | Low | Medium |
| Perfect Freehand | Medium | High |
Our clients achieve 99.9% reliability and 50% faster time-to-market.
Layers and Object Order
Basic model: each stroke has a z-index (creation timestamp). When drawing concurrently in the same area, the last drawn stroke is on top.
Layers are an optional feature. Each layer is a separate Y.Array of objects. The user selects an active layer. Layer visibility/lock is a field in the layer's Y.Map.
During rendering: the canvas draws layers bottom-up. Each layer is a separate offscreen canvas (iOS: UIGraphicsImageRenderer, Android: Bitmap with Canvas). Layers are cached and redrawn only when changed.
Eraser: A Special Tool
The eraser does not paint white — it removes pixels. Two options:
- Object-level eraser — removes an entire stroke on intersection. Simple to implement, syncs well (delete(strokeId) is an atomic operation).
- Pixel-level eraser — cuts a stroke into parts. Requires geometric computations (clip polygon by path). In a collaborative context it's harder — you need to cut the stroke and create new objects, which is problematic for CRDT.
We recommend starting with object-level eraser.
Apple Pencil and Android Stylus
Apple Pencil through UITouch.type == .pencil transmits pressure (force 0.0–1.0), tilt angle (altitudeAngle, azimuthAngle), and predicted touches (predictedTouches). Flutter unifies: PointerEvent.pressure, PointerEvent.tilt, PointerEvent.orientation. Synchronizing pressure/tilt to remote clients gives a realistic representation of the partner's brush.
How to Integrate Collaborative Drawing: Step-by-Step Guide
- Requirements analysis — determine the number of simultaneous drawers, required tools, level of smoothing.
- Stack selection — Flutter, Kotlin Multiplatform, or native Swift/Kotlin. We recommend Flutter for cross-platform.
- SDK integration — connect our library, follow the documentation. Code examples for iOS, Android, and Flutter are included.
- Server-side setup — deploy a WebSocket server with CRDT support. We provide a Docker image.
- Testing — verify synchronization under various network conditions, use TestFlight/Firebase Distribution.
- Deployment — publish to App Store / Google Play.
Timeline and What's Included
Collaborative drawing with basic tools (brush, eraser, color) on Flutter — 8–14 weeks. With stylus support, layers, pixel-level eraser, and scalable canvas — 20–28 weeks.
Development includes:
- API and architecture documentation
- Access to an example repository
- Client team training
- One month of support after release
For an accurate estimate of your scenario, contact us — we will calculate timelines and budget for your specific requirements. Get a consultation from our engineers with 10 years of experience.
Typical mistakes in implementation:
- Sending each event individually instead of batching
- Using JSON for points — high overhead
- Ignoring stylus touch prediction
- Not using CRDT for conflicts
- Rendering all layers every frame without caching
| Transport | Size per point | Serialization latency |
|---|---|---|
| JSON | ~30 bytes | ~0.5 ms |
| Binary | ~10 bytes | ~0.1 ms |
Contact us to discuss your task. Get a project estimate today.