Flutter：如何将 Canvas/CustomPainter 保存到图像文件中？

Question

我正在尝试从用户那里收集签名并将其保存到图像中。我已经做到了可以在屏幕上绘图的程度，但现在我想单击一个按钮以保存到图像并存储在我的数据库中。

这是我目前所拥有的：

import 'package:flutter/material.dart';

class SignaturePadPage extends StatefulWidget {
  SignaturePadPage({Key key}) : super(key: key);

  @override
  _SignaturePadPage createState() => new _SignaturePadPage();
}
class _SignaturePadPage extends State<SignaturePadPage> {

  List<Offset> _points = <Offset>[];

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: GestureDetector(
        onPanUpdate: (DragUpdateDetails details) {
          setState(() {
            RenderBox referenceBox = context.findRenderObject();
            Offset localPosition =
            referenceBox.globalToLocal(details.globalPosition);
            _points = new List.from(_points)..add(localPosition);
          });
        },
        onPanEnd: (DragEndDetails details) => _points.add(null),
        child: new CustomPaint(painter: new SignaturePainter(_points)),
      ),
    );
  }
}

class SignaturePainter extends CustomPainter {
  SignaturePainter(this.points);
  final List<Offset> points;
  void paint(Canvas canvas, Size size) {
    Paint paint = new Paint()
      ..color = Colors.black
      ..strokeCap = StrokeCap.round
      ..strokeWidth = 5.0;
    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null)
        canvas.drawLine(points[i], points[i + 1], paint);
    }
  }
  bool shouldRepaint(SignaturePainter other) => other.points != points;
}

不知道从那里去哪里......

Answer 1

您可以使用PictureRecorder 捕获CustomPainter 的输出。将PictureRecorder 实例传递给Canvas 的构造函数。然后，PictureRecorder.endRecording 返回的 Picture 可以通过 Picture.toImage 转换为 Image。最后，使用Image.toByteData提取图像字节。

这是一个例子：https://github.com/rxlabz/flutter_canvas_to_image

Answer 2

在您的小部件中添加渲染方法

  ui.Image get rendered {
    // [CustomPainter] has its own @canvas to pass our
    // [ui.PictureRecorder] object must be passed to [Canvas]#contructor
    // to capture the Image. This way we can pass @recorder to [Canvas]#contructor
    // using @painter[SignaturePainter] we can call [SignaturePainter]#paint
    // with the our newly created @canvas
    ui.PictureRecorder recorder = ui.PictureRecorder();
    Canvas canvas = Canvas(recorder);
    SignaturePainter painter = SignaturePainter(points: _points);
    var size = context.size;
    painter.paint(canvas, size);
    return recorder.endRecording()
        .toImage(size.width.floor(), size.height.floor());
  }

然后使用状态获取渲染图像

var image = signatureKey.currentState.rendered

现在，您可以使用toByteData(format: ui.ImageByteFormat.png) 生成png 图像并使用asInt8List() 存储

var pngBytes = await image.toByteData(format: ui.ImageByteFormat.png)
File('your-path/filename.png')
    .writeAsBytesSync(pngBytes.buffer.asInt8List())

有关如何将画布导出为 png 的完整示例，请查看此示例 https://github.com/vemarav/signature

Answer 3

现有的解决方案对我有用，但我使用 PictureRecorder 捕获的图像与屏幕上呈现的图像相比总是模糊不清。我最终意识到我可以使用一些基本的 Canvas 技巧来实现这一目标。基本上，在创建PictureRecorder 的Canvas 之后，将其大小设置为所需比例的数倍（这里我将其设置为4x）。然后只需canvas.scale它。 Boom - 与现代分辨率屏幕上显示的图像相比，您生成的图像不再模糊！

您可能希望将_overSampleScale 的值调高以用于打印或可能会被放大/扩展的图像，或者如果您大量使用此值并希望提高图像预览加载性能，则可以调低该值。在屏幕上使用它，您需要使用 实际 宽度和高度的 Container 约束您的 Image.memory 小部件，与其他解决方案一样。理想情况下，这个数字是 Flutter 在其虚假“像素”中的 DPI（即 PictureRecorder 捕获的内容）与屏幕的实际 DPI 之间的比率。

static const double _overSampleScale = 4;
Future<ui.Image> get renderedScoreImage async {
    final recorder = ui.PictureRecorder();
    Canvas canvas = Canvas(recorder);
    final size = Size(widget.width * _overSampleScale, widget.height * _overSampleScale);
    final painter = SignaturePainter(points: _points);
    canvas.save();
    canvas.scale(_overSampleScale);
    painter.paint(canvas, size);
    canvas.restore();
    final data = recorder.endRecording()
      .toImage(size.width.floor(), size.height.floor());
    return data;
  }