极客教程HTML 如何允许跨源使用图像和画布
要允许跨源使用图像和画布，服务器必须在其HTTP响应中包括适当的CORS（跨源资源共享）标头。这些标头可以被设置为允许特定的来源或方法，或者允许任何来源访问资源。
HTML画布
HTML5画布是网页上的一个矩形区域，由JavaScript代码控制。任何东西都可以在画布上绘制，包括图像、形状、文本和动画。画布是创建游戏、图形和网络应用的绝佳工具。
办法
允许跨源使用图像和画布的方法是在标题中添加以下内容 –
Access-Control-Allow-Origin – *
这将允许所有的图像和画布元素被跨源使用。
例子
下面是一个完整的例子，说明如何允许跨源地使用图像和画布。要运行它，只需在网页浏览器中打开HTML文件。
<!DOCTYPE html>
<html>
<head>
<script>
function allowCrossOrigin(img, url) {
if (url.indexOf('https://') !== 0 && url.indexOf('http://') !== 0) {
// only allow cross-origin requests for images that are hosted on a secure
// (HTTPS/HTTP) server
return;
}
// create a new Image object and set its src property to the url of the image
// that we want to load
var image = new Image();
image.src = url;
// when the image has loaded, set the src property of the img element to the
// url of the image
image.onload = function() {
img.src = url;
};
}
</script>
</head>
<body>
<!-- define an img element and set its src property to a local image -->
<img id='local-image' src='https://cdn.pixabay.com/photo/2012/08/27/14/19/mountains-55067__340.png' width='200' height='200'>
<!-- define another img element and try to set its src property to
an image that is hosted on a different domain -->
<img id='remote-image' width='200' height='200'>
<script>
// get a reference to the img element with id="remote-image"
var remoteImage = document.getElementById('remote-image');
// set the src property of the img element to the url of the image that we want
// to load
remoteImage.src = 'https://i.natgeofe.com/n/2a832501-483e-422f-985c-0e93757b7d84/6_square.jpg';
// call the allowCrossOrigin function, passing in the img element and the url
// of the image that we want to load
allowCrossOrigin(remoteImage, 'https://i.natgeofe.com/n/2a832501-483e-422f-985c-0e93757b7d84/6_square.jpg');
</script>
</body>
</html>