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How to solve cross-domain problems encountered when drawing on canvas

不言Original: 2018-06-14 11:25:333973browse

This article mainly introduces relevant information that explains in detail the cross-domain problems encountered when drawing canvas. The content is quite good. I will share it with you now and give it as a reference.

When drawing an external link image in canvas, we will encounter a cross-domain problem.

The example is as follows:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>crossorigin</title>
</head>
<body>
    <canvas width="600" height="300" id="canvas"></canvas>
    <img id="image" alt="">
    <script>
        var canvas = document.getElementById(&#39;canvas&#39;);
        var ctx = canvas.getContext(&#39;2d&#39;);
        var image = new Image();
        image.onload = function() {
            ctx.drawImage(image, 0, 0);
            document.getElementById(&#39;image&#39;).src = canvas.toDataURL(&#39;image/png&#39;);
        };
        image.src = &#39;https://ss0.bdstatic.com/70cFvHSh_Q1YnxGkpoWK1HF6hhy/it/u=3497300994,2503543630&fm=27&gp=0.jpg&#39;;
    </script>
</body>

When you open this page in the browser, you will find this problem:

Uncaught DOMException: Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.

This is limited by the CORS policy and there will be cross-domain problems, although it can be used Image, but drawing it to the canvas will pollute the canvas. Once a canvas is polluted, the data of the canvas cannot be extracted. For example, the canvas toBlob(), toDataURL(), or getImageData() methods cannot be used; when using these methods, it will The above security error is thrown

This is a distressing problem, but fortunately img has added the crossorigin attribute. This attribute determines whether the CORS function is enabled during the image acquisition process:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>crossorigin</title>
</head>
<body>
    <canvas width="600" height="300" id="canvas"></canvas>
    <img id="image" alt="">
    <script>
        var canvas = document.getElementById(&#39;canvas&#39;);
        var ctx = canvas.getContext(&#39;2d&#39;);
        var image = new Image();
        image.setAttribute(&#39;crossorigin&#39;, &#39;anonymous&#39;);
        image.onload = function() {
            ctx.drawImage(image, 0, 0);
            document.getElementById(&#39;image&#39;).src = canvas.toDataURL(&#39;image/png&#39;);
        };
        image.src = &#39;https://ss0.bdstatic.com/70cFvHSh_Q1YnxGkpoWK1HF6hhy/it/u=3497300994,2503543630&fm=27&gp=0.jpg&#39;;
    </script>
</body>

Comparing the above two pieces of JS code, you will find this extra line:

image.setAttribute(&#39;crossorigin&#39;, &#39;anonymous&#39;);

It’s that simple, Perfect solution!

The above is the entire content of this article. I hope it will be helpful to everyone’s study. For more related content, please pay attention to the PHP Chinese website!

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