add diff image output functionality and tests

.gitignore

@@ -2,6 +2,7 @@
 node_modules
 examples/imgs/*
 !examples/imgs/original.jpg
+!examples/imgs/noise1.png
 !examples/imgs/original.png
 !examples/imgs/original.gif
 !examples/imgs/originalSideways.jpg

lib/compare.js

@@ -1,7 +1,7 @@
 // compare
 
 var exec = require('child_process').exec;
-var utils = require('./utils')
+var utils = require('./utils');
 
 /**
  * Compare two images uses graphicsmagicks `compare` command.
@@ -22,6 +22,22 @@ module.exports = exports = function compare (orig, compareTo, tolerance, cb) {
   orig = utils.escape(orig);
   compareTo = utils.escape(compareTo);
 
+  // outputting the diff image
+  if (typeof tolerance === 'object') {
+    var diffOptions = tolerance;
+    if (typeof diffOptions.file !== 'string') {
+      throw new TypeError('The path for the diff output is invalid');
+    }
+    // graphicsmagick defaults to red
+    var highlightColorOption = diffOptions.highlightColor
+      ? ' -highlight-color ' + diffOptions.highlightColor + ' '
+      : ' ';
+
+    return exec('gm compare' + highlightColorOption + orig + ' ' + compareTo +
+      ' -file ' + diffOptions.file, cb);
+  }
+
+  // else, output the mean square error (mse)
   if ('function' == typeof tolerance) {
     cb = tolerance;
     tolerance = 0.4;
@@ -38,5 +54,5 @@ module.exports = exports = function compare (orig, compareTo, tolerance, cb) {
 
     var equality = parseFloat(match[1]);
     cb(null, equality <= tolerance, equality, stdout);
-  })
+  });
 }

test/compare.js

@@ -0,0 +1,22 @@
+var assert = require('assert');
+var fs = require('fs');
+
+module.exports = function (gm, dir, finish, GM) {
+  GM.compare(dir + '/original.jpg', dir + '/original.png', function(err, same) {
+    if (err || !same) finish(err);
+    else outputDiff();
+  });
+
+  function outputDiff() {
+    var options = {
+      highlightColor: 'yellow',
+      file: dir + '/diff.png'
+    };
+    GM.compare(dir + '/original.jpg', dir + '/noise1.png', options, function(err) {
+      if (err) return finish(err);
+      fs.exists(options.file, function(exists) {
+        finish(!exists);
+      });
+    });
+  }
+};