Image diffing using JavaScript

Recently I had need of a way to find how “similar” a collection of images were. A classic way to diff images is to take one, invert it’s colors, then draw it over the second at 50% opacity. It’s pretty easy to do this in CSS. If both images are the same, the resulting combination will be perfectly grey (rgb(128, 128, 128)).

Since we’re JavaScripting all the things these days, I figured that this technique could be implemented in JS. Fortunately, HTML’s <canvas> element has APIs that allow for easy access to raw pixel data of an image. You can call getImageData on a canvas context, which will return an object that contains an array of RGBA values for each pixel in the image. Using that data, it’s easy to invert the image a canvas displays (subtract each color value from 255), and also compare the resulting combined image.

The only real annoying bit is synchronizing loading each image, and also determining how long the entire diff process took. To that end, I created a giant array of promises, each of which was itself a promise for loading/comparing two images. The resolution of the giant promise array concludes with a timestamp comparision.

Check out the full source code. Usage is simple:

diff(sources).then(results => {;

Where sources is an array of image URLs, and results is an array of integers, indicating similarity between two images in the source array. A value close to zero indicates the two images are very similar, while a value closer to 128 shows that they are very different.

The only downside to implementing this type of algorithm in JavaScript is that it can take a very long time to compare two large images, as you are iterating over each pixel in the image, twice. For two 8 megapixel images (e.g. taken by your iPhone 5), that is 32 million pixels. As a proof of concept/toy/small image diffing tool, it works pretty nicely though.