Is it possible to draw an image with Dart/Flutter? - image

Question

I'm looking to find a path to generating an image (jpeg or png) from within a flutter application. The image would be composed of circles, lines, text etc.
There does appear to be a means of drawing to the screen using a canvas (https://docs.flutter.io/flutter/dart-ui/Canvas/Canvas.html), however there doesn't appear to be the equivalent for creating an image that could be presented within or sent/used outside the application.
Is there any dart library available for drawing an image? It would seem that it possible given the underlying skia framework. In the Dart-html package there is a CanvasRenderingContext2D.
Edit: Getting something like the following working (as per Richard's suggestions) would be a start:
import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';
import 'dart:ui';
import 'dart:typed_data';
import 'dart:async';
import 'dart:io';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter Demo',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
Image _image;
#override
void initState() {
super.initState();
_image = new Image.network(
'https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png',
);
}
Future<String> get _localPath async {
final directory =
await getApplicationDocumentsDirectory(); //From path_provider package
return directory.path;
}
Future<File> get _localFile async {
final path = await _localPath;
return new File('$path/tempImage.png');
}
Future<File> writeImage(ByteData pngBytes) async {
final file = await _localFile;
// Write the file
file.writeAsBytes(pngBytes.buffer.asUint8List());
return file;
}
_generateImage() {
_generate().then((val) => setState(() {
_image = val;
}));
}
Future<Image> _generate() async {
PictureRecorder recorder = new PictureRecorder();
Canvas c = new Canvas(recorder);
var rect = new Rect.fromLTWH(0.0, 0.0, 100.0, 100.0);
c.clipRect(rect);
final paint = new Paint();
paint.strokeWidth = 2.0;
paint.color = const Color(0xFF333333);
paint.style = PaintingStyle.fill;
final offset = new Offset(50.0, 50.0);
c.drawCircle(offset, 40.0, paint);
var picture = recorder.endRecording();
final pngBytes =
await picture.toImage(100, 100).toByteData(format: ImageByteFormat.png);
//Aim #1. Upade _image with generated image.
var image = Image.memory(pngBytes.buffer.asUint8List());
return image;
//new Image.memory(pngBytes.buffer.asUint8List());
// _image = new Image.network(
// 'https://github.com/flutter/website/blob/master/_includes/code/layout/lakes/images/lake.jpg?raw=true',
// );
//Aim #2. Write image to file system.
//writeImage(pngBytes);
//Make a temporary file (see elsewhere on SO) and writeAsBytes(pngBytes.buffer.asUInt8List())
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Center(
child: new Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
_image,
],
),
),
floatingActionButton: new FloatingActionButton(
onPressed: _generateImage,
tooltip: 'Generate',
child: new Icon(Icons.add),
),
);
}
}

Answer 1

PictureRecorder lets you create a Canvas, use the Canvas drawing methods and provides endRecording() returning a Picture. You can draw this Picture to other Scenes or Canvases, or use .toImage(width, height).toByteData(format) to convert it to PNG (or raw - jpeg isn't supported).
For example:
import 'dart:ui';
import 'dart:typed_data';
....
PictureRecorder recorder = new PictureRecorder();
Canvas c = new Canvas(recorder);
c.drawPaint(paint); // etc
Picture p = recorder.endRecording();
ByteData pngBytes =
await p.toImage(100, 100).toByteData(format: ImageByteFormat.png);
Make sure that you are on flutter 0.4.4, otherwise you may not have the format parameter available.
Having seen your edit, though, I suspect you are really looking for CustomPainter where a Widget gives you a Canvas on which you can draw. Here's an example from a similar question.