Drawing images to canvas with img.crossOrigin = "Anonymous" doesn't work

StackOverflow https://stackoverflow.com/questions/23123237

Pregunta

In a client-side standalone JS application, I'm trying to make it so I can call toDataURL() on a canvas on which I've drawn some images specified by a URL. Ie I can input into a textbox the url to any image (hosted on, say, imgur) that I want to draw on the canvas, click a "draw" button and it will draw on the canvas. The end user should be able to save their final render as a single image, for this I'm using toDataURL().

Anyway, until they actually fix that annoying "operation is insecure" error (gee, you're going to tell the end user what they can and can't do with their own data?) I followed a workaround that said to add the image to the DOM and set its crossOrigin property to "Anonmyous" and then draw it to the canvas.

Here's a full working simplified version of my code (but in reality there will be many more features):

<!DOCTYPE html5>
<html>
<head>
<style>
#canvas {border:10px solid green;background-color:black;}
#imgbox {border:2px solid black;}
</style>
</head>
<body>
<canvas id="canvas" width=336 height=336></canvas>
<br><br>
<input size=60 id="imgbox">
<input type="submit" value="Draw" onclick=draw()>
<script>
function draw() {
    var canvas = document.getElementById("canvas");
    var context = canvas.getContext("2d");
    var img = new Image();
    img.src = document.getElementById("imgbox").value;
    img.crossOrigin = "Anonymous";
    context.drawImage(img, 40, 40);
}
</script>
</body>
</html>

Without the img.crossOrigin = "Anonymous"; line, I could input http://i.imgur.com/c2wRzfD.jpg into the textbox and click draw and it would work. However as soon as I added that line, the whole thing broke and it won't even be drawn to the canvas at all.

What do I need to change to fix this? I really need to be able to implement the functionality for the end user to save their final image and it's extremely annoying that the people who wrote the html5 spec purposely introduced this bug.

¿Fue útil?

Solución

You must set the CORS request before the src - just swap the lines into:

img.crossOrigin = "Anonymous";
img.src = document.getElementById("imgbox").value;

You will also need to add an onload handler to the image as loading is asynchronous:

img.onload = function() {
    context.drawImage(this, 40, 40);
    // call next step in your code here, f.ex: nextStep();
};
img.crossOrigin = "Anonymous";
img.src = document.getElementById("imgbox").value;

Otros consejos

When the server requires authorization to access the images the value should be:

img.crossOrigin = "Use-Credentials";

Otherwise the browser will give up after receiving HTTP 401.

If your image disappears after setting cross origin to anonymous it means your server doesn't allow cross origin. If you're using amazon s3 to serve your images, you need to enable public access to your bucket, and then add cross origin policy (from templates). After that adding cross origin "anonymous" should work.

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