added possibility to compare against a previously captured screenshot if using selector

2 files changed, 28 insertions, 8 deletions

diff --git a/tests/README.screenshots.md b/tests/README.screenshots.md
index b0189fe343..39648b1435 100644
--- a/tests/README.screenshots.md
+++ b/tests/README.screenshots.md
@@ -143,7 +143,14 @@ If you want to compare only parts (all content within an element / selector) of
             page.load(url);
         }, done);
     });
-    
+
+If you want to compare a screenshot against an already existing expected screenshot and capture a selector you can do the following:
+
+    it("should load correctly", function (done) {
+        expect.screenshot("screenshot_name").to.be.captureSelector("processed_screenshot_name", "#content", function (page) {
+            page.load(url);
+        }, done);
+    });
 
 ### Manipulating Pages Before Capture
 
diff --git a/tests/lib/screenshot-testing/support/chai-extras.js b/tests/lib/screenshot-testing/support/chai-extras.js
index 1a7da4a7c6..72112cad1b 100644
--- a/tests/lib/screenshot-testing/support/chai-extras.js
+++ b/tests/lib/screenshot-testing/support/chai-extras.js
@@ -140,22 +140,35 @@ function capture(screenName, compareAgainst, selector, pageSetupFn, done) {
     }
 }
 
-chai.Assertion.addChainableMethod('captureSelector', function (selector, pageSetupFn, done) {
-    var screenName = this.__flags['object'];
+chai.Assertion.addChainableMethod('captureSelector', function () {
+    var compareAgainst = this.__flags['object'];
+
+    if (arguments.length == 3) {
+        var screenName  = compareAgainst,
+            selector    = arguments[0],
+            pageSetupFn = arguments[1],
+            done        = arguments[2];
+    } else {
+        var screenName  = app.runner.suite.title + "_" + arguments[0],
+            selector    = arguments[1],
+            pageSetupFn = arguments[2],
+            done        = arguments[3];
+    }
 
-    capture(screenName, screenName, selector, pageSetupFn, done);
+    capture(screenName, compareAgainst, selector, pageSetupFn, done);
 });
 
 chai.Assertion.addChainableMethod('capture', function () {
     var compareAgainst = this.__flags['object'];
+
     if (arguments.length == 2) {
-        var screenName = compareAgainst,
+        var screenName  = compareAgainst,
             pageSetupFn = arguments[0],
-            done = arguments[1];
+            done        = arguments[1];
     } else {
-        var screenName = app.runner.suite.title + "_" + arguments[0],
+        var screenName  = app.runner.suite.title + "_" + arguments[0],
             pageSetupFn = arguments[1],
-            done = arguments[2];
+            done        = arguments[2];
     }
 
     capture(screenName, compareAgainst, null, pageSetupFn, done);