node.js Image Comparison with GIF output

screenshot.js

/* Takes two images and compares them.
 * If they're different, a GIF with the differences will be created.
 *
 * @requires ImageMagick to be installed to generate the GIF :(
 * Install it via Homebrew: brew install imagemagick
 *
 * @usage: node screenshot.js file1.jpg file2.jpg
 */
var pngparse = require('pngparse');
var exec = require('child_process').exec;
var args = process.argv;

var image1,
    image2;

function compare() {
    image1 = args[2];
    image2 = args[3];

    if (!image1 || !image2) {
        throw new Error('You need 2 files.');
    }

    parseImage(image1, function (img1) {
        parseImage(image2, function (img2) {
            compareFiles(img1, img2)
        });
    });
}

function parseImage(filePath, callback) {
    pngparse.parseFile(filePath, function (err, res) {
        if (err) {
            throw new Error('Error parsing image1', err);
        }

        if (callback) {
            callback(res);
        }

        return res;
    });
}

function compareFiles(first, second) {
    var i = 0,
        len = first.data.length;

    for (i; i < len; i += 4) {
        if (first.data[i] !== second.data[i] ||
            first.data[i + 1] !== second.data[i + 1] ||
            first.data[i + 2] !== second.data[i + 2]) {

            return createDiffGif();
        }
    }

    console.log('No difference between files');
    return false;
}

function createDiffGif() {
    console.log('Files are different! Creating GIF...');
    exec('convert -delay 50 -loop 0 ' + image1 + ' ' + image2 + ' difference.gif');
}

// For CLI only with this
module.exports = compare();