使用自定义画笔在Flutter中遮罩两个图像 [英] Masking two images in Flutter using a Custom Painter

问题描述

你好，谁能告诉我为什么这种掩盖尝试的背景是黑色的.这必须很近，但是我无法杀死背景.我看到其他人提到saveLayer(rect, paint)是这里的关键，因为这将整个画布推向了掩盖操作. 此问题(无屏蔽操作)和

Hello can anyone tell me why the background to this masking attempt is black. This must be close but I just can't kill the background. I've seen others reference that saveLayer(rect, paint) is the key here as that shoves the whole canvas rect in to the masking operation. This question (no masking operation) and this one (no actual answer) are similar but were no use to me.

main.dart

main.dart

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

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart' show rootBundle;

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  ui.Image mask;
  ui.Image image;

  @override
  void initState() {
    super.initState();
    load('images/squircle.png').then((i) {
      setState(() {
        mask = i;
      });
    });
    load('images/noodlejpg.jpg').then((i) {
      setState(() {
        image = i;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(backgroundColor: Colors.blue, title: Text('I am a title')),
      body: SafeArea(
        child: SizedBox(
          width: 200.0,
          height: 200.0,
          child: CustomPaint(painter: OverlayPainter(mask, image)),
        ),
      ),
    );
  }

  Future<ui.Image> load(String asset) async {
    ByteData data = await rootBundle.load(asset);
    ui.Codec codec = await ui.instantiateImageCodec(data.buffer.asUint8List());
    ui.FrameInfo fi = await codec.getNextFrame();
    return fi.image;
  }
}

class OverlayPainter extends CustomPainter {
  ui.Image mask;
  ui.Image image;

  OverlayPainter(this.mask, this.image);

  @override
  void paint(Canvas canvas, Size size) {
    var paint = new Paint();
    paint.isAntiAlias = true;

    if (image != null) {
      var rect = Rect.fromLTRB(0, 0, 200, 200);
      Size outputSize = rect.size;
      Size inputSize = Size(image.width.toDouble(), image.height.toDouble());
      final FittedSizes fittedSizes =
          applyBoxFit(BoxFit.cover, inputSize, outputSize);
      final Size sourceSize = fittedSizes.source;

      canvas.save();
      final Rect sourceRect = Alignment.center.inscribe(
        sourceSize,
        Offset.zero & inputSize,
      );
      canvas.drawImageRect(image, sourceRect, rect, paint);
      canvas.restore();
    }

    if (mask != null) {
      var rect = Rect.fromLTRB(0, 0, 200, 200);
      Size outputSize = rect.size;
      Size inputSize = Size(mask.width.toDouble(), mask.height.toDouble());
      final FittedSizes fittedSizes =
          applyBoxFit(BoxFit.cover, inputSize, outputSize);
      final Size sourceSize = fittedSizes.source;

      canvas.saveLayer(rect, Paint()..blendMode = BlendMode.dstIn);

      final Rect sourceRect = Alignment.center.inscribe(
        sourceSize,
        Offset.zero & inputSize,
      );
      canvas.drawImageRect(mask, sourceRect, rect, paint);
      canvas.restore();
    }
  }

  @override
  bool shouldRepaint(OverlayPainter oldDelegate) {
    return mask != oldDelegate.mask || image != oldDelegate.image;
  }
}

noodlejpg.jpg

noodlejpg.jpg

squircle.jpg

squircle.jpg

结果

推荐答案

关键在于何时调用saveLayer和何时调用restore.

The key is in when to call saveLayer and when to call restore.

从此处:

在使用Canvas.saveLayer和Canvas.restore时，在调用Canvas.restore时将应用赋予Canvas.saveLayer的Paint的混合模式.每次对Canvas.saveLayer的调用都会引入一个新图层，在该图层上绘制形状和图像.调用Canvas.restore时，该层将被合成到父层上，其中源是最近绘制的形状和图像，目标是父层. (对于第一个Canvas.saveLayer调用，父层是画布本身.)

When using Canvas.saveLayer and Canvas.restore, the blend mode of the Paint given to the Canvas.saveLayer will be applied when Canvas.restore is called. Each call to Canvas.saveLayer introduces a new layer onto which shapes and images are painted; when Canvas.restore is called, that layer is then composited onto the parent layer, with the source being the most-recently-drawn shapes and images, and the destination being the parent layer. (For the first Canvas.saveLayer call, the parent layer is the canvas itself.)

工作代码

  @override
  void paint(Canvas canvas, Size size) {
    if (image != null && mask != null) {
      var rect = Rect.fromLTRB(0, 0, 200, 200);
      Size outputSize = rect.size;
      Paint paint = new Paint();

      //Mask
      Size maskInputSize = Size(mask.width.toDouble(), mask.height.toDouble());
      final FittedSizes maskFittedSizes =
          applyBoxFit(BoxFit.cover, maskInputSize, outputSize);
      final Size maskSourceSize = maskFittedSizes.source;

      final Rect maskSourceRect = Alignment.center
          .inscribe(maskSourceSize, Offset.zero & maskInputSize);

      canvas.saveLayer(rect, paint);
      canvas.drawImageRect(mask, maskSourceRect, rect, paint);

      //Image
      Size inputSize = Size(image.width.toDouble(), image.height.toDouble());
      final FittedSizes fittedSizes =
          applyBoxFit(BoxFit.cover, inputSize, outputSize);
      final Size sourceSize = fittedSizes.source;
      final Rect sourceRect =
          Alignment.center.inscribe(sourceSize, Offset.zero & inputSize);

      canvas.drawImageRect(
          image, sourceRect, rect, paint..blendMode = BlendMode.srcIn);
      canvas.restore();
    }
  }

结果:

这篇关于使用自定义画笔在Flutter中遮罩两个图像的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！