Cross Origin Resource Sharing (CORS) is what allows XHR (i.e. Ajax) requests to go cross domain. It’s a simple header response to the XHR request that says “yes, you can complete your request”, looking like this (if we allowed any client to place an XHR request against our service):
Should you be adding this header to your existing API services? Yes.
What use is CORS for images?
(As far as I know) there’s one major use case: when you’re creating a dynamic canvas image and you want to export it. The canvas element has a method called
toDataURL which gives you a base64 string representing the content of the canvas.
Perhaps you’re creating a collage of your flickr photos and want to print that collage out. More specifically, in my case, I’ve been taking a photo a day for 3 years, and I wanted to print the entire collection out on a material canvas. I use the canvas API to generate my image, but I can’t export it.
That’s to say, if you use canvas API method
drawImage to paint a flickr photo (perhaps even your own) on to a canvas element, you can’t use
getImageData to further manipulate the pictures, nor can you use
toDataURL because the url your script executed on is not the same origin* as the flickr photo.
* origin, which is made up of the protocol, host and port number.
The solution to allowing us to export our canvas creations is to enable CORS from the source origin. Simply adding the
Access-Control-Allow-Origin header when an image requests origin rights.
So in my case, where I wanted to generate a file to be sent to the printers, I moved my code to using Node.js (which I’ll post about in detail another day). Couldn’t do the export in the browser – but I should have been able to.
How to request images with CORS
A blank value (as an HTML attribute) has the default value of
anonymous – which is likely what you want. Alternatively you can set this to
use-credentials which sets the credentials request header – which the server can use to decide whether you have rights to the content.
When the CORS headers are sent back from the server for an image, and that image is used on a canvas, the origin-clean flag is true, and we are able to remix the content as we please.
Below you can see a simple working example. The script reads the image element, draws and scales it to a new canvas, then creates a circular version of the picture, and it’s then exported to a new real image element, and finally the original image is replaced with the cirlified image. The source image is hosted on http://rem.io (the
<img> element with the
crossOrigin attribute at the top of the HTML) but the image remixing is happening on http://jsbin.com (as an embed on this site via an iframe). This is all possible with CORS support.
So if you happen to work at a company, or know someone that works at a company that hosts user images – please please add CORS support to your images (if an origin header is sent in the request).
Go, go make that change, now!