rasterizeHTML.js

Renders HTML into the browser's canvas.

See the API.

Example

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

See the examples page and the examples shipped with the code.

How does it work

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).

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.

At the time of writing it seems that the individual browsers still have some issues with rendering SVGs with embedded HTML to the canvas. See the wiki for a list of known issues and do add your findings there.

Development

For linting, tests and minification install Node.js and run

$ ./go

For the integration test under Chrome and Safari open test/manualIntegrationTestForWebkit.html (under Chrome you will either need to start the browser passing in the option --allow-file-access-from-files or load the page through a local webserver).

Where is it used?

• CSS Critic, a lightweight framework for regression testing of Cascading Style Sheets
• ...

Author

Christoph Burgmer. Licensed under MIT. Reach out on Twitter.