CORS enabled image

The HTML specification introduces a crossorigin attribute for images that, in combination with an appropriate CORS header, allows images defined by the <img> element that are loaded from foreign origins to be used in canvas as if they were being loaded from the current origin.

See CORS settings attributes for details on how the crossorigin attribute is used.

What is a "tainted" canvas?

Although you can use images without CORS approval in your canvas, doing so taints the canvas. Once a canvas has been tainted, you can no longer pull data back out of the canvas. For example, you can no longer use the canvas toBlob(), toDataURL(), or getImageData() methods; doing so will throw a security error.

This protects users from having private data exposed by using images to pull information from remote web sites without permission.

Example: Storing an image from a foreign origin

You must have a server hosting images with the appropriate Access-Control-Allow-Origin header.  Adding crossOrigin attribute makes a request header. You can use this excerpt from the HTML5 Boilerplate Apache server configs to appropriately respond with this response header:

<IfModule mod_setenvif.c>
    <IfModule mod_headers.c>
        <FilesMatch "\.(cur|gif|ico|jpe?g|png|svgz?|webp)$">
            SetEnvIf Origin ":" IS_CORS
            Header set Access-Control-Allow-Origin "*" env=IS_CORS
        </FilesMatch>
    </IfModule>
</IfModule>

Given that's all sorted, you will be able to save those images to DOM Storage as if they were served from your domain.

var img = new Image,
    canvas = document.createElement("canvas"),
    ctx = canvas.getContext("2d"),
    src = "http://example.com/image"; // insert image url here
img.crossOrigin = "Anonymous";
img.onload = function() {
    canvas.width = img.width;
    canvas.height = img.height;
    ctx.drawImage( img, 0, 0 );
    localStorage.setItem( "savedImageData", canvas.toDataURL("image/png") );
}
img.src = src;
// make sure the load event fires for cached images too
if ( img.complete || img.complete === undefined ) {
    img.src = "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==";
    img.src = src;
}

Browser compatibility

See Also

• Using Cross-domain images in WebGL and Chrome 13
• HTML Specification - the crossorigin attribute