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.body.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 */
    });

Get the raw pixel data as a Uint8Array with every 4 array elements representing the RGBA data of a pixel:

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

domtoimage.toPixelData(node)
    .then(function (pixels) {
        for (var y = 0; y < node.scrollHeight; ++y) {
          for (var x = 0; x < node.scrollWidth; ++x) {
            pixelAtXYOffset = (4 * y * node.scrollHeight) + (4 * x);
            /* pixelAtXY is a Uint8Array[4] containing RGBA values of the pixel at (x, y) in the range 0..255 */
            pixelAtXY = pixels.slice(pixelAtXYOffset, pixelAtXYOffset + 4);
          }
        }
    });

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

Rendering options

filter

A function taking DOM node as argument. Should return true if passed node should be included in the output (excluding node means excluding it's children as well). Not called on the root node.

bgcolor

A string value for the background color, any valid CSS color value.

height, width

Height and width in pixels to be applied to node before rendering.

style

An object whose properties to be copied to node's style before rendering. You might want to check this reference for JavaScript names of CSS properties.

Browsers

It's tested on latest Chrome and Firefox (49 and 45 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. Internet Explorer is not (and will not be) supported, as it does not support SVG <foreignObject> tag

Dependencies

Source

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

• Promise
• 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:

1. Clone the original DOM node recursively

2. 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

3. 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

4. Embed images

   • embed image URLs in <img> elements

   • inline images used in background CSS property, in a fashion similar to fonts

5. Serialize the cloned node to XML

6. Wrap XML into the <foreignObject> tag, then into the SVG, then make it a data URL

7. Optionally, to get PNG content or raw pixel data as a Uint8Array, 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

8. Done!

Things to watch out for

• if the DOM node you want to render includes a <canvas> element with something drawn on it, it should be handled fine, unless the canvas is tainted - in this case rendering will rather not succeed.

• at the time of writing, Firefox has a problem with some external stylesheets (see issue #13). In such case, the error will be caught and logged.

Authors

Anatolii Saienko, Paul Bakaus (original idea)

License

MIT