Visual Regression Testing

Visual Regression Testing compares screenshots of an application's UI before and after code changes. It detects unintended visual differences, ensuring that updates haven't introduced visual defects or broken existing UI elements.

Detailed explanation

Visual Regression Testing, also known as visual testing or snapshot testing, is a crucial technique in software quality assurance that focuses on identifying unintended visual changes in an application's user interface (UI). Unlike functional testing, which verifies the correctness of application logic and data, visual regression testing ensures that the UI remains consistent and visually appealing across different environments, browsers, and devices after code modifications.

The core principle behind visual regression testing is to capture baseline screenshots of the UI in a known good state. These screenshots serve as the "golden standard" or "reference images." Subsequently, after any code changes, new screenshots are taken and compared against the baseline images. Any discrepancies between the new and baseline screenshots are flagged as potential visual regressions, indicating that the UI has been altered in an unexpected or undesirable way.

Practical Implementation

The typical workflow for visual regression testing involves the following steps:

  1. Establish a Baseline: The initial step is to capture screenshots of the application's UI in a stable and known-good state. These screenshots are stored as the baseline or reference images. It's crucial to ensure that the baseline images accurately represent the desired visual appearance of the application.

  2. Implement Code Changes: Developers make changes to the codebase, which may include UI modifications, bug fixes, or feature enhancements.

  3. Capture New Screenshots: After the code changes are implemented, new screenshots of the UI are captured. These screenshots should cover the same areas and components as the baseline images.

  4. Compare Screenshots: The newly captured screenshots are compared against the baseline images using specialized visual comparison tools. These tools employ image differencing algorithms to identify pixel-by-pixel differences between the images.

  5. Analyze and Resolve Differences: Any detected differences are flagged as potential visual regressions. QA engineers or developers then need to analyze these differences to determine whether they are intentional and acceptable or represent genuine visual defects. Intentional changes might be due to planned UI updates or feature enhancements. Unintentional changes could indicate bugs, styling issues, or layout problems.

  6. Update Baseline (If Necessary): If the detected visual changes are intentional and represent the desired new state of the UI, the baseline images should be updated to reflect these changes. This ensures that future tests compare against the updated UI.

Best Practices

  • Isolate UI Components: To improve the accuracy and efficiency of visual regression tests, it's best to isolate individual UI components or sections of the page for testing. This reduces the scope of each test and makes it easier to identify the source of any visual regressions. Component libraries like Storybook can be very useful for this.

  • Use Stable Selectors: Ensure that the selectors used to target UI elements for screenshot capture are stable and consistent. Avoid using dynamic or auto-generated selectors that may change with each build, as this can lead to false positives.

  • Handle Dynamic Content: Dynamic content, such as dates, times, or user-specific data, can cause variations in screenshots and lead to false positives. Consider masking or replacing dynamic content with static placeholders during testing.

  • Cross-Browser and Cross-Device Testing: Perform visual regression testing across different browsers (e.g., Chrome, Firefox, Safari) and devices (e.g., desktops, tablets, mobile phones) to ensure that the UI renders correctly in various environments.

  • Integrate with CI/CD: Integrate visual regression testing into your continuous integration and continuous delivery (CI/CD) pipeline to automatically detect visual regressions with each build. This helps to catch visual defects early in the development process and prevent them from reaching production.

Common Tools

Several tools are available for performing visual regression testing, each with its own strengths and weaknesses. Some popular options include:

  • Applitools: A cloud-based visual testing platform that provides advanced image comparison algorithms and AI-powered analysis to identify visual regressions accurately. It integrates with various testing frameworks and CI/CD tools.

  • Percy: A visual review platform that automates visual regression testing and provides a collaborative workflow for reviewing and approving visual changes. It integrates with GitHub, GitLab, and other version control systems.

  • BackstopJS: An open-source visual regression testing tool that compares screenshots of web pages and generates reports highlighting any visual differences. It supports various browsers and testing frameworks.

  • Happo: A visual testing tool designed for React, React Native, and other JavaScript-based UI frameworks. It provides a simple and intuitive API for capturing and comparing screenshots.

  • Jest Image Snapshot: A Jest matcher that allows you to easily create and compare image snapshots in your Jest unit tests. It's a lightweight and convenient option for testing individual UI components.

Example using BackstopJS

First, install BackstopJS:

npm install -g backstopjs

Then, initialize a BackstopJS project:

backstop init

This creates a backstop.json configuration file. Edit this file to define your test scenarios. For example:

{
  "id": "my-project",
  "viewports": [
    {
      "name": "desktop",
      "width": 1920,
      "height": 1080
    }
  ],
  "scenarios": [
    {
      "label": "Homepage",
      "url": "http://localhost:3000",
      "delay": 1000, // Wait for 1 second to allow content to load
      "selectors": [
        ".main-content" // Only capture the main content area
      ],
      "misMatchThreshold" : 0.1,
      "requireSameDimensions": false
    }
  ],
  "paths": {
    "bitmaps_reference": "backstop_data/bitmaps_reference",
    "bitmaps_test": "backstop_data/bitmaps_test",
    "html_report": "backstop_data/html_report",
    "ci_report": "backstop_data/ci_report"
  },
  "report": ["browser"],
  "engine": "puppeteer",
  "engineOptions": {
    "args": ["--no-sandbox"]
  },
  "asyncCaptureLimit": 5,
  "asyncCompareLimit": 50,
  "debug": false,
  "debugWindow": false
}

Now, capture the baseline screenshots:

backstop reference

After making code changes, run the tests:

backstop test

BackstopJS will compare the new screenshots with the baseline images and generate a report highlighting any visual differences.

Visual regression testing is an essential practice for maintaining the visual integrity of software applications. By automating the process of detecting visual regressions, it helps to ensure that UI changes are intentional and do not introduce unintended visual defects. This leads to a better user experience and reduces the risk of visual bugs reaching production.

Further reading

Related Terms

A/B Testing

Acceptance Testing

Accessibility Tester