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Introduction to Visual Regression Testing

By Shreya Bose, Technical Content Writer at BrowserStack -

Table of Contents

What is Visual Regression Testing?

Regression Testing is used to verify that any system changes do not interfere with existing features and/or code structure. There’s a reason regression tests are part of almost every test suite in software development. It is common for devs to change or add a section of code, and having it unintentionally disrupt something that was previously working just fine.

Visual Regression Testing applies the same logic but confines testing to the visual aspects of the software. In other words, it checks that code changes do not break any aspect of the software’s visual interface.

A visual regression test checks what the user will see after any code changes have been executed by comparing screenshots taken before and after code changes. This is why visual regression tests are also sometimes called visual snapshot tests.

Use Case Scenario for Visual Bugs

Let’s say a user opened a web app and tried to click a button, but couldn’t do so because an ad covers 3/4th of the button. The user is naturally irritated and wonders how such an obvious issue with user experience got past the app developers. They might even delete the app because it actively prevents them from gaining any benefits from it.

This particular issue is an example of a visual bug. Essentially, it is an error that affects the users’ visual experience of using a particular software. Visual bugs are most apparent to users because it is usually the first thing they see when accessing a website or app.

Visual testing is integral to resolving such bugs. In the world we live in, there are hundreds, possibly thousands of device-browser-operating system combinations that enact “code-to-pixel” conversations in their own way. This is why the same software can look different (or be scrambled) on different devices. Differences in screen sizes and resolutions must also be considered in this regard, which is why responsive design should be a mandatory part of any development project.

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How Visual Regression Testing Works?

Visual tests generate, analyze, and compare browser snapshots to detect if any pixels have changed. These pixel differences are called visual diffs (sometimes called perceptual diffs, pdiffs, CSS diffs, UI diffs).

For visual testing, one needs –

  • A test runner to write and run tests.
  • A browser automation framework to replicate user actions

Developers create code that replicates user functions, (typing text into a field, for example). At appropriate points in the code, they place commands to capture a screenshot. The first time the test code is executed, an initial set of screenshots is recorded – to act as a baseline against which all further changes will be compared.

After setting the baseline, the QA runs the test code in the background. Anytime a change is identified, a screenshot is captured. Each screenshot is compared to the baseline image corresponding to that particular section of the code and the software. If differences occur between the images, the test is considered failed.

Once the test code runs completely, a report is generated automatically. A reviewer then reviews all the images that have been diagnosed as changed from their baseline. Some testing tools generate reports highlighting the differences between baseline and final images, as detected after the test execution.

If these image differences are caused by bugs, developers can fix them and rerun the test to check if the fixes actually worked. Suppose subsequent changes in the UI cause discrepancies. In that case, developers will have to review the screenshot and update baseline images against which visual tests can be run in the future.

Introduction to Visual Regression Testing

How to Implement Visual Regression Testing?

It is best to automate visual regression testing and incorporate it into the CI/CD pipeline. This saves time, reduces the possibility of human error, and ensures maintenance of the software’s visual appeal.

  • Create Testing Scenarios. Define what is to be captured in screenshots and at what point in the test they will be captured. Ensure that these scenarios include a variety of user interactions, as that is what software will have to handle in the real world.
  • Use an automated visual testing tool to compare recent screenshots (captured after code changes have been implemented to software) with ones captured earlier. The tool will generate a report detailing all differences detected between two sets of screenshots.
  • One or more reviewers check and report if changes introduced have led to expected results or if disruptions have occurred.
  • If any bugs are detected, fix them (or send them to the relevant devs to be fixed). Once that is done, update the new screenshot as a baseline for future visual regression tests.

Tools for Visual Regression Testing

As previously mentioned, visual testing requires a test runner – to write and run tests. Test runners enable testers or developers to create code that reproduces user actions. Each test carries assertions defining a condition that will either pass or fail. Organize tests into blocks and put these blocks into one file that will be used to test a specific module or software function.

Once tests are written, use a tool like Selenium or Cypress to interact with the browser. They support visual tests and allow for the generation of screenshots of web pages.

Finally, one must have a tool for managing the testing process. Percy by BrowserStack is one of the best-known tools for automating visual testing. It captures screenshots, compares them against the baseline images, and highlights visual changes. With increased visual coverage, teams can deploy code changes with confidence with every commit.

With Percy, testers can increase visual coverage across the entire UI and eliminate the risk of shipping visual bugs. They can avoid false positives and get quick, deterministic results with reliable regression testing. They can release software faster with DOM snapshotting and advanced parallelization capabilities designed to execute complex test suites at scale.

Percy’s SDKs enable users to easily install them and add snapshots. Percy also integrates with CI/CD pipelines and source code management systems to add visual tests to each code commit. After tests are initiated, Percy grabs screenshots, identifies visual changes, and informs the testing team of all changes. Then, testers can easily review visual diffs to decide if the changes are legitimate or require fixing.

Visual Regression Testing makes life infinitely easier for developers on any project. It allows them to make sweeping code changes, with the assurance that there is a process in place that will capture any resultant bugs.

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