Document scale parameter for Image and RawImage

[mmcdon20]

Steps to reproduce

If you set the scale parameter on an Image or RawImage to 2 the image gets smaller, if you set it to .5 the image gets larger. This is the opposite effect that I was expecting, it is also opposite to Transform.scale() which works the way I expect.

Expected results

I expected the scale parameter in Image and RawImage to have the same effect as the scale parameter in Transform.scale().

Actual results

The scale parameter in Image and RawImage have the opposite effect as the scale parameter in Transform.scale().

Code sample

EDIT: Combined code into a single demo

import 'dart:async';
import 'dart:typed_data';
import 'dart:ui' as ui;

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() async {
  const url = 'https://upload.wikimedia.org/wikipedia/en/b/b7/Google1998.png';
  final Completer<ui.Image> completer = Completer<ui.Image>();
  final Uint8List bytes = (await http.get(Uri.parse(url))).bodyBytes;

  ui.decodeImageFromList(bytes, completer.complete);

  runApp(MaterialApp(
    debugShowCheckedModeBanner: false,
    theme: ThemeData(
      sliderTheme: const SliderThemeData(
        showValueIndicator: ShowValueIndicator.always,
      ),
    ),
    home: ScaleDemo(
      rawImage: await completer.future,
      bytes: bytes,
    ),
  ));
}

class ScaleDemo extends StatefulWidget {
  const ScaleDemo({
    required this.rawImage,
    required this.bytes,
    super.key,
  });

  final ui.Image rawImage;
  final Uint8List bytes;

  @override
  State<ScaleDemo> createState() => _ScaleDemoState();
}

class _ScaleDemoState extends State<ScaleDemo> {
  double scale = 1;

  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      length: 3,
      child: Scaffold(
        appBar: AppBar(
          title: const Text('Scale Demo'),
          actions: [
            Padding(
              padding: const EdgeInsets.all(15),
              child: Text(
                'Scale: ${scale.toStringAsFixed(2)}',
                style: Theme.of(context).textTheme.titleLarge,
              ),
            ),
          ],
          bottom: const TabBar(
            tabs: [
              Tab(text: 'Image scale'),
              Tab(text: 'RawImage scale'),
              Tab(text: 'Transform.scale'),
            ],
          ),
        ),
        body: TabBarView(
          children: [
            Image.memory(widget.bytes, scale: scale),
            RawImage(image: widget.rawImage, scale: scale),
            Transform.scale(scale: scale, child: Image.memory(widget.bytes)),
          ],
        ),
        bottomNavigationBar: BottomAppBar(
          child: Slider(
            value: scale,
            max: 2,
            min: .1,
            label: scale.toStringAsFixed(2),
            onChanged: (v) => setState(() => scale = v),
          ),
        ),
      ),
    );
  }
}

Screenshots or Video

No response

Logs

No response

Flutter Doctor output

Doctor output

[√] Flutter (Channel master, 3.22.0-35.0.pre.7, on Microsoft Windows [Version 10.0.19045.4412], locale en-US)
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• No issues found!

[darshankawar]

@mmcdon20
Thanks for the report. Can you provide runnable code sample (ex: main.dart) that we can directly copy paste to verify this further ?

[mmcdon20]

@darshankawar I have already provided runnable code samples. See the section titled "Code Sample". Each of those is a complete program.

[christophsj]

Hi,

I tried to reproduce your issue and I wasn't able to do so with your sample code, the images were the same size no matter what scale. However I did manage to reproduce it with the following example where the issue can easily be seen, maybe that helps :)

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  double _scale = .5;

  void _changeScale() {
    setState(() {
      _scale = _scale == 0.5 ? 2 : .5;
    });
  }

  static const url =
      'https://upload.wikimedia.org/wikipedia/en/b/b7/Google1998.png';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: SingleChildScrollView(
          child: Column(
            children: <Widget>[
              Text(_scale.toString()),
              Image.network(url, scale: _scale),
              const SizedBox(
                height: 100,
              ),
              Transform.scale(scale: _scale, child: Image.network(url))
            ],
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: _changeScale,
          child: const Icon(Icons.edit),
        ),
      ),
    );
  }
}

[darshankawar]

Thanks for the code @christophsj That's what I was looking for, runnable code.
I do see the images being rendered as reported, so keeping the issue open and labeling for team's attention.

stable, master flutter doctor -v

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[goderbauer]

This is working as intended and documented on other parts of the API:

The scale argument specifies the linear scale factor for drawing this image at its intended size and applies to both the width and the height. For example, if this is 2.0, it means that there are four image pixels for every one logical pixel, and the image's actual width and height (as given by the dart:ui.Image.width and dart:ui.Image.height properties) are double the height and width that should be used when painting the image (e.g. in the arguments given to Canvas.drawImage).

We should also include this note in the other Image constructors (and RawImage).