核心代码：
<script type="text/javascript">
// Converts image to canvas; returns new canvas element
function convertImageToCanvas(image) {
var canvas = document.createElement("canvas");
canvas.width = image.width;
canvas.height = image.height;
canvas.getContext("2d").drawImage(image, 0, 0);
return canvas;
}
// Converts canvas to an image
function convertCanvasToImage(canvas) {
var image = new Image();
image.src = canvas.toDataURL("image/png");
return image;
}
</script>
需要注意：
跨域的图片需要设置crossOrigin="Anonymous"属性，然后等图片加载完成后才能使用 toDataURL() 方法
否则会报错：
Uncaught DOMException: Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported. 图片转canvas画布的时候(convertImageToCanvas)
需要注意的是：
如果图片原尺寸是200px*200px, 通过样式改变图片的宽高，则 image.width 和 image.height 获取的是改变后的宽高
如果改变后的宽高小于原尺寸，那么将不能完整的把图片完整的绘制出来
因为drawImage(image, 0, 0)是根据图片原尺寸进行绘制的
示例：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
/*img {
width: 100px;
}*/
</style>
</head>
<body>
<!--<img id="tulip" src="https://ss2.bdstatic.com/70cFvnSh_Q1YnxGkpoWK1HF6hhy/it/u=2256920272,2188849592&fm=26&gp=0.jpg" alt="" crossOrigin="Anonymous">--> <img id="tulip" src="https://ss2.bdstatic.com/70cFvnSh_Q1YnxGkpoWK1HF6hhy/it/u=158393800,2846304514&fm=26&gp=0.jpg" alt="" crossOrigin="Anonymous">
</body>
</html>
<script>
// Converts image to canvas; returns new canvas element
function convertImageToCanvas(image) {
var canvas = document.createElement("canvas");
canvas.width = image.width;
canvas.height = image.height;
canvas.getContext("2d").drawImage(image, 0, 0);
return canvas;
} // Converts canvas to an image
function convertCanvasToImage(canvas) {
var image = new Image();
image.src = canvas.toDataURL("image/png");
return image;
} function drawImage(img) {
var cs = convertImageToCanvas(img);
var image = convertCanvasToImage(cs);
document.body.appendChild(image);
} window.onload = function () {
var img = document.getElementById("tulip");
img.onload = drawImage(img);
}
</script>
参考链接：
[link](https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image)
[link](https://*.com/questions/20424279/canvas-todataurl-securityerror)
[link](https://www.jianshu.com/p/6fe06667b748)
