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Visual Regression Testing Introduction and Tools

Visual Regression Testing

Visual Testing is the process of testing an application’s graphical user interface (GUI) as appearing to the users. The main purpose of the visual regression testing is to find visual bugs like font-matching, layout, and other rendering issues so that they can be fixed before they are visible to the end-user. Apart from that, visual testing is used to verify the content visible on a page. This type of testing is very important for sites that use graphical functionality like charts, dashboards, etc. because identifying the bugs with traditional automated functional testing tools is very challenging.

The nature of visual testing can be complex, depending upon the number of variables like web browsers, operating systems, screen resolutions, responsive design, etc. Nowadays this complexity of visual testing is manageable with existing open source and commercial solutions, which make it easier to automate.

The Best Visual Regression Testing Tools

Over the last many years, the term ‘visual regression testing is cropping up in web development more and more. It promises to provide tremendous value for those who are fed up with manually checking their sites for style issues. With the growing popularity of visual regression testing, it seems like every month, there are new tools showing up every day which promise to provide an awesome visual regression testing automation. Unfortunately, the truth is that these types of tests are extremely complicated.

Responsive layouts are one of the most difficult aspects to test. It is more tedious to design and test all of a project’s screen elements at every breakpoint than it is to code them in the first place. Visual regression allows you to make visual comparisons between the baseline versions of the site and the version in development. The process of visual regression testing starts with taking the screenshot of the original design and comparing it with the new, looking for the pixel differences. Visual regression comes in various formats, using a variety of technologies and workflows as mentioned below.

  1. BackstopJS

    BackstopJS is an automated visual regression tool for responsive web UI and works by comparing DOM screenshots over time. BackstopJS is used for responsive websites and web-applications and it’s a config-driven automated screenshot test application. The workflow is as simple as mentioned below:

    1. BackstopJS incorporates the reference screenshots of your webpage with multiple screen sizes.
    2. After doing editing of the CSS, BackstopJS is used to find any unwanted CSS regressions.
    3. Resemble.js, CasperJS, PhantomJS, and SlimerJS is used by BackstopJS to report the visual changes of a web application.
  2. Wraith

    Wraith tool created by developers at BBC News is a screenshot comparison tool that uses a headless browser to create screenshots of webpages on different moments in time or in different environments. Then it sorts out the difference between the two images. Thereafter the affected areas are shown in blue. The workflow process is as follows :

    1. Wraith takes the screenshots of the webpages.
    2. It then conducts a comparison task across the pages and images.
    3. Comparing the two images, it finds out the percentage of difference in pixels in a PNG file and a data.txt file.
    4. Then it makes a package of all this to a gallery.html, to have a view for the user.
    5. If the difference in the screenshot is above the specified threshold of the configuration file, the task exits with a system error code (useful for CI).
    6. In the end, the failed result or screenshot will also be highlighted in the gallery.
  3. PhantomCSS

    PhantomCSS is a CasperJS module for automating the process of visual regression testing with PhantomJS 2 or SlimerJS and Resemble.js. Live style guides and responsive layouts are used for testing Web apps. PhantomCSS tool takes screenshots of the webpage which are captured by CasperJS and then it compares them to baseline images using Resemble.js to test for RGB pixel differences. PhantomCSS then generates a difference of image to help in finding the cause.

  4. CasperJS

    CasperJS is basically an advanced tool for navigation scripting & testing utility used for PhantomJS and SlimerJS. It facilitates the process of defining a full range of navigation scenario and provides a set of useful high-level functions and methods for doing common tasks such as:

    • defining & ordering the navigation steps
    • filling forms and then clicking the links
    • capturing the screenshots of a page or an area
    • making the viable assertions on remote DOM
    • logging & events
    • downloading resources (even binary ones)
    • catching errors and reacting accordingly
    • writing functional test suites, exporting results as JUnit XML (xUnit)ML (xUnit)
  5. Spectre

    Spectre is a free web application to find the difference in the screenshots. It is a simple UI for inspecting and browsing differences. Spectre doesn’t run tests. Rather it focuses on providing image comparison capabilities, along with an admin interface for managing the screenshots.

    Normally Spectre is used with a tool like WebdriverIO or PhantomJS. WebdriverIO or PhantomJS would do the work of capturing screenshots, while Spectre would manage to store and process the images.

  6. VisualReview

    VisualReview is a human-friendly tool for testing and reviewing visual regressions. VisualReview’s objective is to provide a productive and human-friendly workflow for testing and reviewing your web application’s layout for any regressions.

    VisualReview function is a server that accepts screenshots of your web application, sent for example, from your selenium or protractor scripts. These shots are then compared to screenshots you uploaded and reviewed earlier. VisualReview will display and highlight any differences between the two and allow you to either accept or reject these changes.

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