A javascript function that takes two images and returns a new image highlighting the differences between the two images. Uses the Longest-Common-Subsequence algorithm (LCS) to align the two images (vertically). This will prevent unnecessarily big diffs for images where content has shifted up or down. Works in the browser and in Node.

Image 1	Image 2	Image diff	Traced

npm install lcs-image-diff

Pro tip: You're best off using this module in a web worker, to offload heavy image manipulation from the main thread.

const imageDiff = require('lcs-image-diff');

// `image1` and `image2` are instances of `ImageData`
// https://developer.mozilla.org/en-US/docs/Web/API/ImageData
// https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/getImageData
const { data, width, height, diff } = imageDiff(image1, image2);

// `data` is a `UInt8ClampedArray`
// `width` and `height` is the resulting size of the diff image
// `diff` is a number between 0 and 1 describing how much the two images differ.

Usage is mostly the same as in the browser, you just have to pass in a custom hashFunction. Here's an example using images loaded with Jimp and a hash function using the crypto module.

const crypto = require('crypto');
const Jimp = require('jimp');

const imageDiff = require('lcs-image-diff');

function createHash(data) {
  return crypto
    .createHash('md5')
    .update(data)
    .digest('hex');
}

const image1 = (await Jimp.read('1.jpg')).bitmap;
const image2 = (await Jimp.read('2.jpg')).bitmap;

const { data, width, height, diff } = imageDiff(image1, image2, {
  hashFunction: createHash,
});

When presenting an image diff to a user, it can be helpful to highlight diff areas. The diff image returned by the imageDiff function will do some of that, but in some cases when only a few pixels have changed it can be useful to further trace the diff. For that purpose, imageDiff will return a trace object that can be used to generate an SVG image with paths tracing the diff.

const imageDiff = require('lcs-image-diff');

const { data, width, height, trace } = imageDiff(image1, image2);
const svg = trace.toSVG();

document.getElementById('#trace-svg').innerHTML = svg;

The SVG image is slightly larger than the diff image so that it can properly highlight edges and corners. For that reason, you need to place the SVG in a container that bleeds out a little to account for the extra size.

<div id="trace-svg" style="margin: 0 -10px"></div>

...or if you hate magic numbers, use the constant attached to the imageDiff function:

document.getElementById('#trace-svg').style.margin = `0 ${
  imageDiff.DIFF_TRACE_PADDING
}px`;

• Henric Trotzig (@trotzig)
• Joe Lencioni (@lencioni)

Make sure to check out happo.io - the cross-browser screenshot testing tool