pixelmatch

The smallest, simplest and fastest JavaScript pixel-level image comparison library, originally created to compare screenshots in tests.

Features accurate anti-aliased pixels detection and perceptual color difference metrics.

Inspired by Resemble.js and Blink-diff. Unlike these libraries, pixelmatch is around 90 lines of code, has no dependencies, and works on raw arrays of image data, so it's blazing fast and can be used in any environment (Node or browsers).

var numDiffPixels = pixelmatch(img1, img2, diff, 800, 600, {threshold: 0.1});

Implements ideas from the following papers:

• Measuring perceived color difference using YIQ NTSC transmission color space in mobile applications (2010, Yuriy Kotsarenko, Fernando Ramos)
• Anti-aliased pixel and intensity slope detector (2009, Vytautas Vyšniauskas)

Example output

expected	actual	diff

API

pixelmatch(img1, img2, output, width, height[, options])

• img1, img2 — Image data of the images to compare (Buffer or Uint8Array). Note: image dimensions must be equal.
• output — Image data to write the diff to, or null if don't need a diff image.
• width, height — Width and height of the images. Note that all three images need to have the same dimensions.

options is an object literal with the following properties:

• threshold — Matching threshold, ranges from 0 to 1. Smaller values make the comparison more sensitive. 0.1 by default.
• includeAA — If true, disables detecting and ignoring anti-aliased pixels. false by default.

Compares two images, writes the output diff and returns the number of mismatched pixels.

Command line

Pixelmatch comes with a binary that works with PNG images:

pixelmatch image1.png image2.png output.png 0.1

Example usage

Node.js

var fs = require('fs'),
    PNG = require('pngjs').PNG,
    pixelmatch = require('pixelmatch');

var img1 = fs.createReadStream('img1.png').pipe(new PNG()).on('parsed', doneReading),
    img2 = fs.createReadStream('img2.png').pipe(new PNG()).on('parsed', doneReading),
    filesRead = 0;

function doneReading() {
    if (++filesRead < 2) return;
    var diff = new PNG({width: img1.width, height: img1.height});

    pixelmatch(img1.data, img2.data, diff.data, img1.width, img1.height, {threshold: 0.1});

    diff.pack().pipe(fs.createWriteStream('diff.png'));
}

Browsers

var img1 = img1Ctx.getImageData(0, 0, width, height),
    img2 = img2Ctx.getImageData(0, 0, width, height),
    diff = diffCtx.createImageData(width, height);

pixelmatch(img1.data, img2.data, diff.data, width, height, {threshold: 0.1});

diffCtx.putImageData(diff, 0, 0);

Install

Install with NPM:

npm install pixelmatch

To build a browser-compatible version, clone the repository locally, then run:

npm install -g browserify
browserify -s pixelmatch index.js > pixelmatch.js

Changelog