DOM to Image

What is it

dom-to-image is a library which can turn arbitrary DOM node into a vector (SVG) or raster (PNG) image, written in JavaScript. It's based on domvas by Paul Bakaus and has been completely rewritten, with some bugs fixed and some new features (like web font and image support) added.

Usage

All the top level functions accept DOM node and rendering options, and return promises, which are fulfilled with corresponding data URLs.
Get a PNG image base64-encoded data URL and display right away:

var node = document.getElementById('my-node');

domtoimage.toPng(node)
    .then(function (dataUrl) {
        var img = new Image();
        img.src = dataUrl;
        document.appendChild(img);
    })
    .catch(function (error) {
        console.error('oops, something went wrong!', error);
    });

Get a PNG image blob and download it (using FileSaver, for example):

domtoimage.toBlob(document.getElementById('my-node'))
    .then(function (blob) {
        window.saveAs(blob, 'my-node.png');
    });

Get an SVG data URL, but filter out all the <i> elements:

function filter (node) {
    return (node.tagName !== 'i');
}

domtoimage.toSvg(document.getElementById('my-node'), {filter: filter})
    .then(function (dataUrl) {
        /* do something */
    });

All the functions under impl are not public API and are exposed only for unit testing.

Browsers

It's tested on latest Chrome and Firefox (44 and 40 respectively at the time of writing), with Chrome performing significantly better on big DOM trees, possibly due to it's more performant SVG support, and the fact that it supports CSSStyleDeclaration.cssText property.

Dependencies

Source

Only standard lib is currently used, but make sure your browser supports:

Promise
const keyword
SVG <foreignObject> tag

Tests

Most importantly, tests depend on:

js-imagediff, to compare rendered and control images
ocrad.js, for the parts when you can't compare images (due to the browser rendering differences) and just have to test whether the text is rendered

How it works

There might some day exist (or maybe already exists?) a simple and standard way of exporting parts of the HTML to image (and then this script can only serve as an evidence of all the hoops I had to jump through in order to get such obvious thing done) but I haven't found one so far.

This library uses a feature of SVG that allows having arbitrary HTML content inside of the <foreignObject> tag. So, in order to render that DOM node for you, following steps are taken:

Clone the original DOM node recursively
Compute the style for the node and each sub-node and copy it to corresponding clone

and don't forget to recreate pseudo-elements, as they are not cloned in any way, of course

Embed web fonts

find all the @font-face declarations that might represent web fonts
parse file URLs, download corresponding files
base64-encode and inline content as data: URLs
concatenate all the processed CSS rules and put them into one <style> element, then attach it to the clone

Embed images

embed image URLs in <img> elements
inline images used in background CSS property, in a fashion similar to fonts

Serialize the cloned node to XML
Wrap XML into the <foreignObject> tag, then into the SVG, then make it a data URL
Optionally, to get PNG content, create an Image element with the SVG as a source, and render it on an off-screen canvas, that you have also created, then read the content from the canvas
Done!

Authors

Anatolii Saienko, Paul Bakaus (original idea)

License

MIT

begrossi / dom-to-image