rasterizeHTML.js by cburgmer

API

Draw a page/an HTML string/a Document to the canvas.

rasterizeHTML.drawURL( url [, canvas] [, options] )
rasterizeHTML.drawHTML( html [, canvas] [, options] )
rasterizeHTML.drawDocument( document [, canvas] [, options] )

See the full documentation here. Examples are collected here.

How it works

For security reasons rendering HTML into a canvas is severly limited. Firefox offers such a function via ctx.drawWindow(), but only with Chrome privileges (see https://developer.mozilla.org/en/Drawing_Graphics_with_Canvas).

As described in http://robert.ocallahan.org/2011/11/drawing-dom-content-to-canvas.html and https://developer.mozilla.org/en/HTML/Canvas/Drawing_DOM_objects_into_a_canvas however it is possible by embedding the HTML into an SVG image as a <foreignObject> and then drawing the resulting image via ctx.drawImage().

In addition SVG is not allowed to link to external resources and so rasterizeHTML.js will load external images, fonts and stylesheets and store them inline via data: URIs (or inline style elements respectively).

You can hear cburgmer talk on some of the details of the implementation.

Demo I

Demo II

Limitations

All resources (HTML page, CSS, images, fonts and JS) that are needed for drawing the page can only be loaded if from the same origin, unless techniques like CORS are used. I.E. drawURL() can only load pages from the same domain as the current page and all draw methods can equally only embed styling and images from that domain.

The code is tested under Firefox, Chrome & Safari. However IE up to version 11 does not honour <foreignObject> and is unsupported.

Also the individual browsers still have some issues when rendering SVGs with embedded HTML to the canvas.

The full list of limitations is here.

Example code

<!DOCTYPE html>
<html>
<head>
    <title>rasterizeHTML.js example</title>
    <script type="text/javascript" src="rasterizeHTML.allinone.js"></script>
</head>
<body>
    <canvas id="canvas" width="400" height="200"></canvas>
    <script type="text/javascript">
        var canvas = document.getElementById("canvas");

        rasterizeHTML.drawHTML('<div style="font-size: 20px;">' +
            'Some <span style="color: green">HTML</span>' +
            ' with an image <img src="someimg.png">' +
            '</div>',
            canvas);
    </script>
</body>
</html>