add 'threshold' option for comparison

The values of threshold must be between 0 and 100 and represents
percentage of different pixels.

Author: Nick Georgiev

lib/image-diff.js

@@ -73,16 +73,20 @@ ImageDiff.createDiff = function (options, cb) {
     // DEV: According to http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=17284
     // DEV: These values are the total square root mean square (RMSE) pixel difference across all pixels and its percentage
     // TODO: This is not very multi-lengual =(
-    var resultInfo = stderr.match(/all: (\d+\.?\d*) \((.*?)\)/);
+    var resultInfo = stderr.match(/all: (\d+\.?\d*) \((\d+\.?\d*e?\-?\d*)\)/);
 
     // If there was no resultInfo, throw a fit
     if (!resultInfo) {
       return cb(new Error('Expected `image-diff\'s stderr` to contain \'all\' but received "' + stderr + '"'));
     }
 
     // Callback with pass/fail
-    var totalDifference = resultInfo[2]; // this is the percentage difference
-    return cb(null, parseFloat(totalDifference) <= options.threshold, options.diffPath);
+    var percentDifference = parseFloat(resultInfo[2], 10);
+    var threshold = (typeof(options.threshold) === 'undefined' || isNaN(parseFloat(options.threshold, 10))) ? 0.0 : parseFloat(options.threshold);
+    if (isNaN(percentDifference)) {
+      return cb(new Error('Attempted to parse percentage difference from `image-diff\'s stderr` but got `NaN` from "' + resultInfo[2] + '"'));
+    }
+    return cb(null, percentDifference <= threshold);
   });
 };
 ImageDiff.prototype = {
@@ -190,9 +194,8 @@ ImageDiff.prototype = {
           expectedPath: expectedTmpPath,
           diffPath: diffPath,
           threshold: threshold
-        }, function saveResult (err, _imagesAreSame, _diffPath) {
+        }, function saveResult (err, _imagesAreSame) {
           imagesAreSame = _imagesAreSame;
-          diffPath = _diffPath;
           cb(err);
         });
       }
@@ -205,7 +208,7 @@ ImageDiff.prototype = {
         fs.unlink(filepath, cb);
       }, function callOriginalCallback (_err) {
         // Callback with the imagesAreSame
-        callback(err, imagesAreSame, diffPath);
+        callback(err, imagesAreSame);
       });
     });
   }