Visual Regression Testing

1. Introduction

Visual Regression Testing is a technique used to ensure that changes in code do not adversely affect the user interface of an application. This type of testing compares the visual appearance of a web application before and after code changes, highlighting any unintended visual discrepancies.

2. Key Concepts

Key Definitions

Baseline Image: The original image of the component or page that serves as a reference.
Test Image: The image generated after changes have been made, which will be compared against the baseline.
Diff Image: The image highlighting the differences between the baseline and test images.

3. Step-by-Step Process

Visual Regression Testing Workflow


graph TD;
    A[Start] --> B{Is there a UI change?};
    B -- Yes --> C[Capture New Baseline];
    B -- No --> D[End];
    C --> E[Run Visual Test];
    E --> F[Compare Images];
    F --> G{Are there differences?};
    G -- Yes --> H[Review Changes];
    G -- No --> D;
    H --> I[Update Baseline];
    I --> D;

4. Best Practices

Effective Visual Regression Testing

Use a dedicated visual regression testing tool like BackstopJS, Applitools, or Percy.
Keep your baseline images up to date to avoid false negatives.
Automate visual tests as part of your CI/CD pipeline.
Review diffs carefully to ensure only genuine changes are accepted.

Note: Always consider the context of UI changes. Not all visual changes are errors.

5. FAQ

What tools can I use for visual regression testing?

Some popular tools include BackstopJS, Applitools, Percy, and Cypress with visual testing plugins.

How often should I run visual regression tests?

It's best to run tests on every build or deployment to catch visual changes as early as possible.

Can visual regression testing replace functional testing?

No, visual regression testing should complement functional testing, which verifies the functionality of the application.

What should I do if I see unexpected visual changes?

Investigate the changes carefully. If they are intentional, update the baseline; if not, debug the code changes.