@kovaci/html-to-image

Fork from html-to-image which adds support for web components

html-to-image

✂️ Generates an image from a DOM node using HTML5 canvas and SVG.

Fork from dom-to-image with more maintainable code and some new features.

Install

npm install --save html-to-image

Usage

/* ES6 */
import * as htmlToImage from 'html-to-image';
import { toPng, toJpeg, toBlob, toPixelData, toSvg } from 'html-to-image';

/* ES5 */
var htmlToImage = require('html-to-image');

All the top level functions accept DOM node and rendering options, and return a promise fulfilled with corresponding dataURL:

• toPng
• toSvg
• toJpeg
• toBlob
• toCanvas
• toPixelData

Go with the following examples.

toPng

Get a PNG image base64-encoded data URL and display it right away:

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

htmlToImage.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 base64-encoded data URL and download it (using download):

htmlToImage.toPng(document.getElementById('my-node'))
  .then(function (dataUrl) {
    download(dataUrl, 'my-node.png');
  });

toSvg

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

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

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

toJpeg

Save and download a compressed JPEG image:

htmlToImage.toJpeg(document.getElementById('my-node'), { quality: 0.95 })
  .then(function (dataUrl) {
    var link = document.createElement('a');
    link.download = 'my-image-name.jpeg';
    link.href = dataUrl;
    link.click();
  });

toBlob

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

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

toCanvas

Get a HTMLCanvasElement, and display it right away:

htmlToImage.toCanvas(document.getElementById('my-node'))
  .then(function (canvas) {
    document.body.appendChild(canvas);
  });

toPixelData

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');

htmlToImage.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);
      }
    }
  });

Options

filter

(domNode: HTMLElement) => boolean

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.

backgroundColor

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

width, height

Width and height 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.

quality

A number between 0 and 1 indicating image quality (e.g. 0.92 => 92%) of the JPEG image.

Defaults to 1.0 (100%)

cacheBust

Set to true to append the current time as a query string to URL requests to enable cache busting.

Defaults to false

imagePlaceholder

A data URL for a placeholder image that will be used when fetching an image fails.

Defaults to an empty string and will render empty areas for failed images.

pixelRatio

The pixel ratio of the captured image. Defalut use the actual pixel ratio of the device. Set 1 to use as initial-scale 1 for the image.

Browsers

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

• Promise
• SVG <foreignObject> tag

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.

Safari is not supported, as it uses a stricter security model on <foreignObject> tag. Suggested workaround is to use toSvg and render on the server.

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 dataURLs
   • 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.
• Rendering will failed on huge DOM due to the dataURI limit varies.

Contributing

Pull requests and stars are highly welcome.

For bugs and feature requests, please create an issue.