Painting with power

WindowPaint lets you pan, zoom and paint over any other widget.

It uses the new InteractiveViewer coupled with CustomPainter; giving you the bare minimum to get you started.

Getting Started

For the general use case you should manage to get up and running by playing around with example/lib/main.dart.

If you want to add your own types of drawing tools, I'd advise you to take a look at DrawObjectAdapter and DrawObject and their subclass reference implementations that ship with this library.

This library does not ship with UI controls as part of its core library, but you can copy the example/lib/window_paint_control.dart file and use that as a starting point.

Adapters and Objects

DrawObjectAdapter is responsible for creating, updating, (de)serializing and selecting their DrawObject counterpart. These classes are documented well enough to be able to write your own implementations.

This library ships with four reference implementations:


Additionally, the example project ships with a fifth:


State Restoration

This library has full state restoration support with the RestorableWindowPaintController:

void main() {

class MyApp extends StatelessWidget {
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
      // Remember to set this, otherwise state restoration will be disabled.
      restorationScopeId: 'root', 

class MyHomePage extends StatefulWidget {
  _MyHomePageState createState() => _MyHomePageState();

class _MyHomePageState extends State<MyHomePage> with RestorationMixin { // <- Add the RestorationMixin.
  final adapters = <DrawObjectAdapter>[
    // Add your adapters here...
    // Remember to always provide the same adapters,
    // otherwise not all objects will manage to be restored!

  // This is your friend.
  late final RestorableWindowPaintController _controller;

  void initState() {
    _controller = RestorableWindowPaintController(

  // Register your controller for state restoration.
  void restoreState(RestorationBucket? oldBucket, bool initialRestore) {
    registerForRestoration(_controller, 'controller');

  // Give your component a unique restorationId in the enclosing RestorationScope.
  String? get restorationId => 'main_page';

  void dispose() {
    // Of course, remember to dispose of it.

Path Simplification

Take a look at DrawPencil.finalize() to see simplifyPoints() in action.

This function takes hundreds of individual points and reduces them down to the bare minimum to approximately represent the same path with the given tolerance. The results are rather impressive.

Credit for this algorithm goes to Vladimir Agafonkin.


Canvas-based path rendering is rather dynamic. Thus, if we want to do object selection, we're forced to manage hit-testing ourselves.

Luckily someone else already figured out the math for us!

For simple AABB hit-testing, we can utilize Flutter's Rect.contains(Offset) method.

For more complex hit-testing, like OBB, we've implemented the Line(Offset start, Offset end, double extent) class. This allows us to hit-test against diagonal lines (like the pencil strokes or the rectangle inner cross).

For even more possibilities, check out the excellent vector_math.


The name window paint comes from my childhood memories.

Me and my siblings used to paint the windows in our bedrooms with "art", the kind of stunning art you're able to create with this library 👨‍🎨. After drying, our works overlayed the outside world — which is exactly what this package does.