How can I read (from disk) and resize an image, in Flutter/Dart

It's not a very good way to resize picture via Image library, since it blocks ui thread, and it brings very bad UX. There is a a maxWidth argument in image_picker lib, you can set it, so these writing files manipulation will be unnecessary in some cases.

To resize an image that is defined in pubspec.yaml use "BoxFit":

@override
Widget build(BuildContext context) {
  return (new Container(
    width: 250.0,
    height: 250.0,
      alignment: Alignment.center,
      decoration: new BoxDecoration(

      image: DecorationImage(
          image: AssetImage('assets/Launcher_Icon.png'),
          fit: BoxFit.fill
      ),
    ),
  ));
}

also reference how to access images: https://flutter.io/assets-and-images/

Use the ResizeImage image provider.

Using a separate package is nice if you want to use many of the functionality, or if you can't do otherwise. But just to depend on something instead of what the framework itself (and its underlying graphics engine) can do easily... :-)

If you have an ImageProvider now, say, to display an image from the bytes in memory:

Image(image: MemoryImage(bytes))

Just wrap it inside a ResizeImage:

Image(image: ResizeImage(MemoryImage(bytes), width: 50, height: 100))

And if you want even more control, just create your own image provider based on the source code of this one.

Here's an example Thumbnail widget which does this on the flight

It uses Isolate to offload CPU-intensive work to background thread and have UI thread jank-free

import 'dart:io';
import 'dart:isolate';
import 'package:flutter/material.dart';
import 'package:image/image.dart' as IMG;
import 'package:path/path.dart';

class Thumbnail extends StatefulWidget {
  final Size size;
  final File image;

  const Thumbnail({Key key, this.size, this.image}) : super(key: key);
  @override
  _ThumbnailState createState() => _ThumbnailState();
}

class _ThumbnailState extends State<Thumbnail> {
  List<int> imgBytes;
  Isolate isolate;

  @override
  void initState() {
    _asyncInit();

    super.initState();
  }

  static _isolateEntry(dynamic d) async {
    final ReceivePort receivePort = ReceivePort();
    d.send(receivePort.sendPort);

    final config = await receivePort.first;

    print(config);

    final file = File(config['path']);
    final bytes = await file.readAsBytes();

    IMG.Image image = IMG.decodeImage(bytes);
    IMG.Image thumbnail = IMG.copyResize(
      image,
      width: config['size'].width.toInt(),
    );

    d.send(IMG.encodeNamedImage(thumbnail, basename(config['path'])));
  }

  _asyncInit() async {
    final ReceivePort receivePort = ReceivePort();
    isolate = await Isolate.spawn(_isolateEntry, receivePort.sendPort);

    receivePort.listen((dynamic data) {
      if (data is SendPort) {
        if (mounted) {
          data.send({
            'path': widget.image.path,
            'size': widget.size,
          });
        }
      } else {
        if (mounted) {
          setState(() {
            imgBytes = data;
          });
        }
      }
    });
  }

  @override
  void dispose() {
    if (isolate != null) {
      isolate.kill();
    }
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return SizedBox(
      height: widget.size.height,
      width: widget.size.width,
      child: imgBytes != null
          ? Image.memory(
              imgBytes,
              fit: BoxFit.cover,
            )
          : Container(
              decoration: BoxDecoration(
                gradient: LinearGradient(
                  colors: [Colors.grey[100], Colors.grey[300]],
                  begin: Alignment.centerLeft,
                  end: Alignment.centerRight,
                ),
              ),
            ),
    );
  }
}

There are many solutions :

Use ResizeImage class

ResizeImage class instructs Flutter to decode the image at the specified dimensions instead of at its native size.

Usage : Just wrap your ImageProvider with ResizeImage class

Example :

Image(image: ResizeImage(AssetImage('eg.png'), width: 70, height: 80)),

ImageProvider includes AssetImage , NetworkImage , FileImage and MemoryImage.

Use cacheHeight and cacheWidth property in your Image widget

These properties create a widget that displays an [ImageStream] obtained from an asset , network , memory or file .

Example :

Image.asset('assets/image.png', cacheHeight:120 , cacheWidth: 150),

There are these properties in Image.asset ,Image.network , Image.file and Image.memory

you can use the image class from dart ui library, get the image object with your desired width and height using the frameInfo from intantiateImageCodec and then save it in your desired path

 import 'dart:ui' as ui;

        Uint8List m = File(path).readAsBytesSync();
        ui.Image x = await decodeImageFromList(m);
        ByteData bytes = await x.toByteData();
        print('height is ${x.height}'); //height of original image
        print('width is ${x.width}'); //width of oroginal image

        print('array is $m');
        print('original image size is ${bytes.lengthInBytes}');

            ui.instantiateImageCodec(m, targetHeight: 800, targetWidth: 600)
            .then((codec) {
          codec.getNextFrame().then((frameInfo) async {
            ui.Image i = frameInfo.image;
            print('image width is ${i.width}');//height of resized image
            print('image height is ${i.height}');//width of resized image
            ByteData bytes = await i.toByteData();
            File(path).writeAsBytes(bytes.buffer.asUint32List());
            print('resized image size is ${bytes.lengthInBytes}');
          });
        });

 final pickedFile = await picker.getImage(
        source: ImageSource.gallery,
        imageQuality: 25,
        maxHeight: 1024,
        maxWidth: 1024);

I would say use the dart image package.

import 'package:image/image.dart' as image show
  decodedImage, copyResize //, encodePng
;
import 'dart:convert' as convert show
  base64encode
;

void resizeImage() async {
  List<int> fileBytes = await file.readAsBytes();
  image.Image decodedImage = image.decodeImage(fileBytes) as image.Image;
  image.Image thumbnail = image.copyResize(decodedImage, width: 60);
  List<int> resizedIntList = thumbnail.getBytes();
  // Or compress as a PNG image
  // List<int> resizedIntList = image.encodePng(thumbnail, level: 6);

  String resizedBase64Image = convert.base64Encode(resizedIntList);
}

You could also reference the following code, if you don't want the overhead of the package.

import 'dart:ui' as ui show
  Codec, instantiateImageCodec, FrameInfo;
import 'dart:typed_data' as typedData show
  ByteData, Uint8List
;
import 'dart:convert' as convert show
  base64Encode
;

void resizeImage() async {
  typedData.Uint8List fileBytes = await file.readAsBytes();

  // Resize image
  // ----------
  ui.Codec codec = await ui.instantiateImageCodec(
    fileBytes,
    targetWidth: 60
  );
  ui.FrameInfo frameInfo = await codec.getNextFrame();
  ui.Image resizedImage = frameInfo.image;
  // ----------

  // Convert to List<int>
  // ----------
  typedData.ByteData resizedByteData = await resizedImage.toByteData() as typedData.ByteData;
  typedData.Uint8List resizedUint8List = resizedByteData.buffer
    .asUint8List(resizedByteData.offsetInBytes, resizedByteData.lengthInBytes);
  List<int> resizedIntList = resizedUint8List.cast<int>();
  // ----------

  String resizedBase64Image = convert.base64Encode(resizedIntList);
}

You can use the dart image package: https://pub.dartlang.org/packages/image.

The package provide various services such as resize, crop and rotate.

While this package does work, unfortunately it is very slow.

See discussion: https://github.com/brendan-duncan/image/issues/55

You can use the Image widget in the Scaffold widget,

First of all you need to create assets folder in the root and add an images folder, after that add,

    flutter:
      assets:
        - assets/images/

to the pubspec.yaml file, after that

new Image(
          image: AssetImage('assets/images/pizzaFont.png'),
          height: 12,
          width:12, ......
   )

You can use width and height to change the size of the image.

For more information follow,

https://medium.com/@suragch/how-to-include-images-in-your-flutter-app-863889fc0b29