canvas 中使用跨域的图片
一张跨域的图片直接在canvas中使用控制台是会给出警告的：
Error: SecurityError: Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.
原因是尽管不通过 CORS 就可以在 canvas 中使用其他来源的图片，但是这会污染画布，并且不再认为是安全的画布，这将可能在 canvas 检索数据过程中引发异常。
如果外部源的内容是 HTML <img> 或 SVG <svg> 元素，会阻止从 canvas 中读取数据。
如果外部源图片是作为 HTMLCanvasElement 或 ImageBitMap 获取的，且图片源不符合同源准则，会阻止读取 canvas 的内容。
解决方式：为图片提供设置 crossorigin 属性
代码：
<!DOCTYPE html>
<html lang="zh_CN">
<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>
</head>
<body>
<div id="imageBox"></div>
<script>
let imageURL =
"https://cdn.glitch.com/4c9ebeb9-8b9a-4adc-ad0a-238d9ae00bb5%2Fmdn_logo-only_color.svg?1535749917189";
let imageDescription = "The Mozilla logo";
downloadedImg = new Image();
downloadedImg.crossOrigin = "Anonymous"; // 核心代码
downloadedImg.addEventListener("load", imageReceived, false);
downloadedImg.alt = imageDescription;
downloadedImg.src = imageURL;
function imageReceived() {
const canvas = document.createElement("canvas");
const context = canvas.getContext("2d");
canvas.width = downloadedImg.width;
canvas.height = downloadedImg.height;
canvas.innerText = downloadedImg.alt;
context.drawImage(downloadedImg, 0, 0);
imageBox.appendChild(canvas);
try {
localStorage.setItem(
"saved-image-example",
canvas.toDataURL("image/png")
);
} catch (err) {
console.error(`Error: ${err}`);
}
}
</script>
</body>
</html>
