1.刮刮乐（橡皮擦）效果的核心api
ctx.globalCompositeOperation = type;设置要在绘制新形状时应用的合成操作的类型。
我们这里需要用到的类型是 destination-out
此属性的详细信息：MDN文档
2.基础版刮刮乐功能
canvs 覆盖在图片上
<style>
body {
margin: 0;
}
img {
width: 400px;
height: 300px;
left: 200px;
position: absolute;
z-index: -1;
}
canvas {
margin-left: 200px;
}
</style>
<img src="./test.jpg" alt="pic"/>
<canvas id="canvas" width="400" height="300"></canvas><script>
let canvas = document.querySelector('#canvas');
let context = canvas.getContext('2d');
// 绘制涂层
context.beginPath();
context.fillStyle = 'grey';
context.fillRect(0, 0, 400, 300);
// 监听鼠标移动事件
canvas.addEventListener('mousemove', (e) => {
// 当鼠标左键按下&&移动鼠标时，清除鼠标附近涂层
if (e.which === 1 && e.button === 0) {
const x = e.clientX, y = e.clientY;
context.globalCompositeOperation = 'destination-out';
context.beginPath();
// 清除以鼠标位置为圆心，半径为10px的圆的范围
context.arc(x - 200, y, 10, 0, Math.PI * 2);
context.fill();
}
})
</script>3.进阶版刮刮乐功能
进阶功能：
点击时，以当前位置为圆心刮开一部分区域；
刮开x百分比（可以自定义）后后显示全部，并且使用动画逐渐变淡；
调用第一次刮的回调方法和刮完的回调方法，可以传入或不传；
涂层上面可以显示自定义文字；
首先改为class形式，方便多次创建刮刮乐。
class Scratch {
constructor(id, { maskColor = 'grey', cursorRadius = 10 } = {}) {
this.canvas = document.getElementById('canvas');
this.context = this.canvas.getContext('2d');
this.width = this.canvas.clientWidth;
this.height = this.canvas.clientHeight;
this.maskColor = maskColor; // 涂层颜色
this.cursorRadius = cursorRadius; // 光标半径
this.init();
}
init() {
// 添加涂层
this.addCoat();
let bindEarse = this.erase.bind(this);
this.canvas.addEventListener('mousedown', (e) => {
// 按下左键
if (e.which === 1 && e.button === 0) {
// 擦掉涂层
this.canvas.addEventListener('mousemove', bindEarse);
}
})
document.addEventListener('mouseup', () => {
this.canvas.removeEventListener('mousemove', bindEarse);
})
}
addCoat() {
this.context.beginPath();
this.context.fillStyle = this.maskColor;
this.context.fillRect(0, 0, this.width, this.height);
}
erase(e) {
const x = e.clientX, y = e.clientY;
this.context.globalCompositeOperation = 'destination-out';
this.context.beginPath();
this.context.arc(x - this.width / 2, y, this.cursorRadius, 0, Math.PI * 2);
this.context.fill();
}
}
new Scratch('canvas');然后，记录鼠标位置，mouseup时判断是点击还是点击&移动鼠标，如果是点击则以当前位置为圆心刮开一部分区域；
this.canvas.addEventListener('mousedown', (e) => {
this.posX = e.clientX;
this.posY = e.clientY;
...
})
document.addEventListener('mouseup', (e) => {
if (this.posX === e.clientX && this.posY === e.clientY) {
this.erase(e);
}
...
})然后，判断刮开面积是否超过一半，如果是清空涂层；
ImageData ctx.getImageData(sx, sy, sw, sh);sx：将要被提取的图像数据矩形区域的左上角 x 坐标。
sy：将要被提取的图像数据矩形区域的左上角 y 坐标。
sw：将要被提取的图像数据矩形区域的宽度。
sh：将要被提取的图像数据矩形区域的高度。
ImageData 对象，包含canvas给定的矩形图像数据。可以用来判断是否被刮开。
每4个元素表示一个像素点的rgba值，所以可以判断第4个的值是否小于256的一半即128，如果小于128即可视为透明（被刮开）。
清空指定区域内容：
void ctx.clearRect(x, y, width, height);document.addEventListener('mouseup', (e) => {
this.getScratchedPercentage();
if (this.currPerct >= this.maxEraseArea) {
this.context.clearRect(0, 0, this.width, this.height);
}
})
getScratchedPercentage() {
const pixels = this.context.getImageData(0, 0, this.width, this.height).data;
let transparentPixels = 0;
for (let i = 0; i < pixels.length; i += 4) {
if (pixels[i + 3] < 128) {
transparentPixels++;
}
}
this.currPerct = (transparentPixels / pixels.length * 4 * 100).toFixed(2);
}然后，设置第一次刮的回调方法和刮完的回调方法；
constructor(id, { maskColor = 'grey', cursorRadius = 10, maxEraseArea = 50,
firstEraseCbk = () => { }, lastEraseCbk = () => { } } = {}) {
...
this.firstEraseCbk = firstEraseCbk; // 第一次刮的回调函数
this.lastEraseCbk = lastEraseCbk; // 刮开的回调函数
}
this.canvas.addEventListener('mousedown', (e) => {
if (this.currPerct === 0) {
this.firstEraseCbk();
}
})
document.addEventListener('mouseup', (e) => {
if (this.currPerct >= this.maxEraseArea) {
this.context.clearRect(0, 0, this.width, this.height);
this.lastEraseCbk();
}
})然后，刮开全部时慢慢清空涂层，设置背景色渐变效果；requestAnimationFrame 做出来的动画更流畅
回调函数用闭包形式可以给回调函数传参
document.addEventListener('mouseup', (e) => {
if (this.currPerct >= this.maxEraseArea) {
this.done = true;
requestAnimationFrame(this.fadeOut(255));
this.lastEraseCbk();
}
})
fadeOut(alpha) {
return () => {
this.context.save();
this.context.globalCompositeOperation = 'source-in';
this.context.fillStyle = this.context.fillStyle + (alpha -= 1).toString(16);
this.context.fillRect(0, 0, this.width, this.height);
this.context.restore();
// 到210已经看不到涂层了
if (alpha > 210) {
requestAnimationFrame(this.fadeOut(alpha));
}
}
}然后，初始化涂层的时候，再涂层上显示自定义文字；
addCoat() {
...
if (this.text) {
this.context.font = 'bold 48px serif';
this.context.fillStyle = '#fff';
this.context.textAlign = 'center';
this.context.textBaseline = 'middle';
this.context.fillText(this.text, this.width / 2, this.height / 2);
}
}完整代码
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
body {
margin: 0;
}
img {
width: 400px;
height: 300px;
left: 200px;
position: absolute;
z-index: -1;
}
canvas {
margin-left: 200px;
}
</style>
</head>
<body>
<img src="./test.jpg" alt="pic" />
<canvas id="canvas" width="400" height="300"></canvas>
<script>
class Scratch {
constructor(id, { maskColor = 'grey', cursorRadius = 10, maxEraseArea = 50, text = '',
firstEraseCbk = () => { }, lastEraseCbk = () => { } } = {}) {
this.canvasId = id;
this.canvas = document.getElementById(id);
this.context = this.canvas.getContext('2d');
this.width = this.canvas.clientWidth;
this.height = this.canvas.clientHeight;
this.maskColor = maskColor; // 涂层颜色
this.cursorRadius = cursorRadius; // 光标半径
this.maxEraseArea = maxEraseArea; // 刮开多少后自动清空涂层
this.text = text;
this.firstEraseCbk = firstEraseCbk; // 第一次刮的回调函数
this.lastEraseCbk = lastEraseCbk; // 刮开的回调函数
this.currPerct = 0; // 当前刮开多少百分比
this.done = false; // 是否刮完
this.init();
}
init() {
// 添加涂层
this.addCoat();
let bindEarse = this.erase.bind(this);
this.canvas.addEventListener('mousedown', e => {
if (this.done) {
return;
}
this.posX = e.clientX;
this.posY = e.clientY;
// 按下左键
if (e.which === 1 && e.button === 0) {
// 擦掉涂层
this.canvas.addEventListener('mousemove', bindEarse);
}
if (this.currPerct === 0) {
this.firstEraseCbk();
}
})
document.addEventListener('mouseup', e => {
if (this.done) {
return;
}
if (e.target.id !== this.canvasId) {
return;
}
if (this.posX === e.clientX && this.posY === e.clientY) {
this.erase(e);
}
this.canvas.removeEventListener('mousemove', bindEarse);
this.getScratchedPercentage();
if (this.currPerct >= this.maxEraseArea) {
this.done = true;
requestAnimationFrame(this.fadeOut(255));
this.lastEraseCbk();
}
})
}
// 添加涂层
addCoat() {
this.context.beginPath();
this.context.fillStyle = this.maskColor;
this.context.fillRect(0, 0, this.width, this.height);
// 绘制涂层上的文字
if (this.text) {
this.context.font = 'bold 48px serif';
this.context.fillStyle = '#fff';
this.context.textAlign = 'center';
this.context.textBaseline = 'middle';
this.context.fillText(this.text, this.width / 2, this.height / 2);
}
}
// 擦除某位置涂层
erase(e) {
const x = e.clientX, y = e.clientY;
this.context.globalCompositeOperation = 'destination-out';
this.context.beginPath();
this.context.arc(x - this.width / 2, y, this.cursorRadius, 0, Math.PI * 2);
this.context.fill();
}
// 计算被擦除的部分占全部的百分比
getScratchedPercentage() {
const pixels = this.context.getImageData(0, 0, this.width, this.height).data;
let transparentPixels = 0;
for (let i = 0; i < pixels.length; i += 4) {
if (pixels[i + 3] < 128) {
transparentPixels++;
}
}
this.currPerct = (transparentPixels / pixels.length * 4 * 100).toFixed(2);
}
// 清空涂层时淡出效果
fadeOut(alpha) {
return () => {
this.context.save();
this.context.globalCompositeOperation = 'source-in';
this.context.fillStyle = this.context.fillStyle + (alpha -= 1).toString(16);
this.context.fillRect(0, 0, this.width, this.height);
this.context.restore();
// 到210已经看不到涂层了
if (alpha > 210) {
requestAnimationFrame(this.fadeOut(alpha));
}
}
}
}
new Scratch('canvas', { text: '刮一刮', maxEraseArea: 10 });
</script>
</body>
</html>
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