'flags' object can be set as a property of compare's options paramete…

…r. This can be used to provide any valid command-line option.

README.md

@@ -568,6 +568,24 @@ gm.compare('/path/to/image1.jpg', '/path/to/another.png', options, function (err
 })
 ```
 
+You can also provide a `flags` property, which you can use to directly set
+[command-line options](http://www.imagemagick.org/script/compare.php):
+
+```js
+var options = {
+  file: '/path/to/diff.png',
+  highlightColor: 'yellow',
+  tolerance: 0.02,
+  flags: {
+    '-compare': 'src',
+    '-colorspace': 'CMYK'
+  }
+}
+gm.compare('/path/to/image1.jpg', '/path/to/another.png', options, function (err, isEqual, equality, raw) {
+  ...
+})
+```
+
 ##composite
 
 GraphicsMagick supports compositing one image on top of another. This is exposed through `gm.composite()`. Its first argument is an image path with the changes to the base image, and an optional mask image.

lib/compare.js

@@ -23,7 +23,7 @@ module.exports = exports = function (proto) {
     var isImageMagick = this._options && this._options.imageMagick;
     var appPath = this._options && this._options.appPath || '';
     var bin = isImageMagick
-      ? appPath + 'compare' 
+      ? appPath + 'compare'
       : appPath + 'gm'
     var args = ['-metric', 'mse', orig, compareTo]
     if (!isImageMagick) {
@@ -37,6 +37,20 @@ module.exports = exports = function (proto) {
         options.highlightColor = '"' + options.highlightColor + '"';
       }
 
+      // Support all command line arguments.
+      if (typeof options.flags === 'object') {
+        for (var prop in options.flags) {
+          if (options.flags.hasOwnProperty(prop)) {
+            // Expect each command-line argument to be provided in the exact
+            // same format as they would be used in the CL, e.g. '-compose'
+            // and not 'compose'.
+            console.log(prop, options.flags[prop])
+            args.push(prop);
+            args.push(options.flags[prop]);
+          }
+        }
+      }
+
       if (options.file) {
         if (typeof options.file !== 'string') {
           throw new TypeError('The path for the diff output is invalid');
@@ -56,13 +70,13 @@ module.exports = exports = function (proto) {
         }
         args.push(options.file);
       }
-      
+
       if (typeof options.tolerance != 'undefined') {
         if (typeof options.tolerance !== 'number') {
           throw new TypeError('The tolerance value should be a number');
         }
         tolerance = options.tolerance;
-      } 
+      }
     } else {
       // For ImageMagick diff file is required but we don't care about it, so null it out
       if (isImageMagick) {