Some .NET samples demonstrating how to use the Selenium WebDriver to perform BDD tests and compare screenshots with PhantomJS
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#Visual Regression Testing (Vizregress) Spot visual regression in your web application by comparing screen-shots against an approved set of screen-shots.

This is my research project where I use TDD (Test Driven Development) to support visual regression testing.

I use SpecFlow as my BDD (Behavior Driven Development) tool to perform browser based tests.

##About ###Background experience I have used the Selenium WebDriver to run Chrome and FireFox browser automation tests to identify regression.

The integration tests are executed on Jenkins which we use as our Continuous Delivery platform.
After successful integration tests a staging release is made which is reviewed by the QA team.

###The Problem One thing that we kept missing in terms of regression was spotting visual elements that change.
A simple CSS change can be catastrophic for another view/page that has some Css hacks. The Selenium WebDriver will continue to perform and run tests despite the page looking visually incorrect because the css selectors will remain the same.

This is a research project using AForge.Net Framework. The idea to a create a set of expected images for different parts of an application that can be compared in a test.

#Project Breakdown ##Vizregress Contains the core image utilities based on AForge.Net Framework for determining whether or not images are equal.

This is the assembly that you'll consume in your test application.

###Exclude regions/zones You can add regions/zones to indicate parts of the application you want to ignore during image comparison. This could be to excluded zones that are locale specific or contain dynamic text such as date times.

The RGB region is configurable defaulting to FFFFD800.


#####Github Stats The zone below (yellow rectangle at top) ignores the github statistics bar.

#####Stockport Kitchens Carousel The zone below (yellow rectangle at top) ignores the Stockport Kitchens carousel.

The carousel formed an interesting problem. It transitions between a set a of images and the transition has fade effect.
The easiest option was to exclude this section but it does pose an interesting challenge: How do we test that the carousel images are correct? It needs to be more predictable?

###Match Regions and Zones

You can identify and compare regions within an image.


Below is the original article

I have chosen that I want to extract the donkey image as the zone of interest

so you'll get

###Images currently have to be saved as png files This is the way it is for the moment.

###Differing images Your default browser size may differ from mine so you might have failing tests due to mismatching image sizes.

You'll have to take this into consideration when designing your automation platform.

  • Browser size must be constant
  • Browser version must be consistent
  • The images will need to be stored independently across versions
  • Consistent operating system.

You could solve this by allowing your developers to provision VM's that are used by your build environment.

##Vizregress.Tests Contains tests for Vizregress.Tests using NUnit.

##Vizregress.BDD.Examples Contains example Selenium tests using the WebDriver to assert DOM properties.
It takes screen-shots and compares them to some expected embedded resources.

I have used SpecFlow to create features so you'll need to download the SpecFlow with NUnit extension to run them.

Take this project and modify, make your own and grow :) I have used to create some tests against

It's meant to be your starting point; or an example of how to use Vizregress.

###Image naming conventions Images are currently stored as embedded resources at Vizregress.BDD.Images and structured using the following naming convention:

  • Site name
    • Browser
      • Page.png
      • Page.en-GB.png


  • Example
  • PhantomJS
    • Home.png
    • Contact.png

If a page variant Page.en-US.png isn't found then it'll fall back to Page.png.

###Images are exported on failure If a failure occurs the actual image is written to the executing domain base directory with naming convention of foo.actual.png, foo.expected.png and foo.difference.png.

These paths should be detailed in the assertion message like:

Expected Image: foo.expected.png But was: foo.actual.png Difference:foo.difference.png

foo.difference.png will give you an idea of which zones to check. You could try inverting the image to see if it's clearer to find the difference.


###Pages must be predictable I found that components such as a carousel and Facebook/Twitter feeds can cause unpredictability leading to more sections being excluded.

###Consistent Platform

It's important that a consistent platform is used. I found that using Plantomjs with the same OS on different computers can still have anti-aliasing differences.

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