Visual Regression Testing: A Comprehensive Guide

Introduction

• Definition of Visual regression testing

  Visual regression testing is a regression testing technique that compares the visual appearance of a web application or user interface against a known baseline(original snapshot or UI Design) to detect unintended changes in layout, design, or functionality.

• Importance in modern web development

  Code changes on UI can alter the looks of the application with unintended changes, to spot this early in the cycle, visual regression tests help in debugging these changes and helping the developer to fix them.

  In the era of rapid development, we need to also give user the best User Experience, and having consistent and functional UI is one of the core pillars of user engagement. Thus, having Visual Regression helps to keep application UI and Functionality intact for customer.

Fundamentals of Visual Regression Testing

• A. Basic concepts

  • Capturing new screenshots against baseline

  • Checking inconsistencies in layout

  • Finding hidden elements affected in DOM

• B. How it differs from functional testing

  • Visual regression testing focuses on detecting unintended changes in the visual appearance of a user interface, while functional testing verifies that the application's features work correctly according to specifications.

  • Visual regression testing compares screenshots against a baseline, whereas functional testing checks the behaviour and logic of the application.

The Visual Regression Testing Process

For the purpose of this Demo we will use Playwright, you can use your favourite tool (We also mentioned some alternatives below)

• A. Capturing baseline screenshots

  • The command below will help us to get the snapshot for baseline

  • npx playwright test —update-snapshots

  • Baseline snapshot

    

• B. Running comparison tests

  • Now we will change font for this web app, let’s see if our visual regression test can detect this.

  • The below code helps us to compare the baseline screenshot

    import { test, expect } from '@playwright/test';

    test.beforeEach(async ({ page }) => {
      await page.goto('<http://localhost:5173>');
    });

    test('Screenshot', async ({ page }) => {
      expect(await page.screenshot()).toMatchSnapshot('screenshot.png');
    });

• C. Analyzing and reviewing results

  • playwright test

  • Changes and detected

    

  • Changes in the browser

    

• D. Updating baselines

  • Let’s change the fonts back to normal and check

  • The test passes

    

Tools

A. Popular visual regression testing tools

• Aye Spy: can do 40 comparisons in 60 secs

• Hermoine.js: User friendly, and test fails are minimised by re-executing failed test events

• Needle: Supported by selenium, straight forward documentation and beginner friendly

• BackstopJS: A testing tool that can be seamlessly integrated with CI/CD also has a lot of customisation

• Vizregress: Vizregress analyzes and compares every pixel to mark regressions.

Best Practices

• A. Establishing a robust testing strategy

  • Planning which elements of your application need visual regression testing

  • Frequency of tests

  • Comprehensive coverage without overcomplicating the testing process

• B. Handling dynamic content

  • Best practices might include masking or ignoring areas with dynamic content, using placeholder data, or implementing ways to stabilize dynamic elements during testing

• C. Managing false positives

  • Setting appropriate threshold levels for pixel differences

  • Implementing smart comparison algorithms

  • Regularly reviewing and updating baselines

• D. Version control for baseline images

  • Storing them in version control systems allows for better tracking of changes over time

  • Maintaining a history of how the UI has evolved

Challenges in Visual Regression Testing

• A. Cross-browser compatibility

  • Different browsers may render web elements slightly differently

  • Keeping up with frequent browser updates

• B. Responsive design testing

  • Dynamic layouts and breakpoints can cause elements to shift or resize

  • Tested across multiple screen sizes and resolutions

• C. Performance considerations

  • Resource-intensive, especially for large applications

  • Running tests across multiple browsers and device sizes can significantly increase test execution time

  • Storage requirements for baseline images and diff results can grow rapidly

Lessons learned and success stories

• Reddit: r/SoftwareTesting When to do Visual Regression testing?

  • Learn how people are using these Visual Testing Strategies in their own way and implementing in projects. And also how frequently they’re using for better performance and value from these tests

• Checkout how Visual Comparison testing can help with Testing Graphs: DevAssure Blog: Visual Testing Selenium

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Future Trends with AI and outcomes

• Intelligent comparison algorithms

• Reduce false positives

• Improve self healing capabilities

• Adapting to minor UI changes without human intervention

• AI Powered Analytics

Conclusion also a TLDR;

• Visual regression testing has become an essential practice in modern web development, helping teams maintain consistent and functional user interfaces in an era of rapid development. By comparing the visual appearance of web applications against known baselines, this technique effectively detects unintended changes in layout, design, and functionality.

• The process, as demonstrated using Playwright, involves capturing baseline screenshots, running comparison tests, analyzing results, and updating baselines when necessary. Various tools like Aye Spy, Hermione.js, Needle, BackstopJS, and Vizregress offer different features to suit diverse testing needs.

• Best practices for visual regression testing include establishing a robust testing strategy, effectively handling dynamic content, managing false positives, and implementing version control for baseline images. These practices help teams overcome challenges such as cross-browser compatibility issues, responsive design testing, and performance considerations.

• Looking ahead, the integration of AI and machine learning in visual regression testing promises exciting advancements. Intelligent comparison algorithms, reduced false positives, improved self-healing capabilities, and AI-powered analytics are set to enhance the efficiency and effectiveness of visual testing processes.

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