An Introductory Guide to Visual Regression Testing

Find Waldo in the crowd
Introduction to Visual Regression Testing

Even the most experienced developers ask the most daunting question before they push software to production: "did what we changed break anything else?"

This question is essential since the slightest alteration in a single line of code can cripple the functionality of an existing code structure that used to work just fine.

If a bug occurs after changing a code, your application's user experience (UX) can get ruined in mere seconds.

To avoid such unfortunate and unforeseen problems, you must run continuous tests at the end of every system modification.

This process is known as visual regression testing.

In this guide, we'll explore the meaning of visual regression testing, how bugs affect user experience, and how this testing can save you time, money, and hassle.

What is Visual Regression Testing?

Visual regression is a process of testing software functionality by comparing new screenshots with the existing ones.

This comparison identifies any regressions in the UI, which are then fixed before pushing the code to production.

Visual regression testing is done to verify that each UI (user interface) element looks as intended, even after you make any changes or updates in the code script.

The process is quick to catch bugs that aren't spotted by other types of testing, like unit or functional testing. This is because visual regression testing compares the entire UI and not just individual functionality.

Since the whole interface is being checked for changes, no element goes untested. Consequently, this reduces the number of support tickets and improves customer satisfaction.

Sometimes, after making minor changes to your code, you might think that testing your app for bugs can wait. But what if that small change you made to the code script unintentionally moved an element on the UI by a few pixels?

For most people, such a change would go unnoticed. But for people with specific disabilities, this change can be enough to make your app unusable.

This is one example of how bugs can unintentionally ruin your app's user experience.

So take out your virtual flamethrower and keep reading to find out how to get rid of annoying bugs with the help of visual regression testing!

How Do Bugs Affect User Experience?

Bugs can have a significant impact on your app's user experience.

Did you know around 90% of users abandon apps because of poor performance?

This is bad news for businesses since it means that users will either stop using your app or never come back after having a bad experience.

And it's not just users that are affected by bugs.

Bugs cause developers to waste time trying to fix them, which could be spent on developing new features or improving the existing ones.

They can also result in lost sales and revenue if they prevent users from completing a purchase.

So it's essential to catch and fix bugs as quickly as possible before they have a chance to impact your app's user experience.

After all, the visual interface of your web application is the face of your brand. It's the first thing potential customers see, therefore, giving you the perfect opportunity to make a good first impression.

Visual bugs like these can damage the image of your brand and tarnish your reputation for sure. But the worst part is that such errors are not only detrimental to the design of your software's visual interface, but they can also impair the functionality of your key features, directly impacting the UX.

Let's say your end-user is repeatedly trying to click on a button. They can't proceed with their action because a text is overlapping it. Chances are this user will get frustrated and leave your app never to come back again.

Having debilitating effects on the usability of your application, UI bugs cause you to lose customer trust and impede your growth dramatically. However, being highly capable of resolving visual bugs right then and there, visual regression testing can finally save you from such consequences.

So, without further ado, let's delve into how visual regression testing works and what types are available.

How Visual Regression Testing Works

To put it simply, visual regression testing tools aim to ensure that each UI element appears in the right color shade, position, or size and that no component overlaps or hides one another. It makes sure that your application is completely defect-free even after various software modifications.

To achieve this, it captures a screenshot of your web application as a baseline image before it's undergone any changes and takes another screenshot after a code has been altered. If the screenshot comparison doesn't display any unexpected discrepancies and the new state of your software is identical to your existing reference image, then the test is a success.

The Types of Visual Regression Testing

There are several types of visual regression testing:

  1. Manual visual testing:

Performed without the help of any automation tool, it's done manually by developers or designers.

2. Pixel-by-pixel comparison:

It's a type of regression testing in which each individual pixel of the new image is compared with the corresponding pixel of the baseline image. If any difference is found, the test fails.

3. Layout comparison:

In this approach, the position and size of UI elements are compared rather than pixels. So if an element's position or size has changed, the test will fail.

4. Structural comparison:

This method compares the structure of the DOM (Document Object Model) to check if there have been any changes in the HTML markup. If there are, then the test fails.

5. DOM-based comparison:

DOM-based visual regression testing is a combination of both layout and structural comparisons. In this approach, the UI elements are first compared structurally, and then their positions and sizes are checked.

6. Visual AI comparison:

Also known as perceptual diffing, it uses artificial intelligence and machine learning algorithms to compare two images. It doesn't require a baseline image and can be used on dynamic web applications.

Most app owners or developers might want to opt for manual testing if it's just one or two pages that need to be checked for changes. But if you have a massive web application with hundreds of pages, then automated visual regression testing would seal the deal.

And if you're still stuck between manual and automated, jump to the next section to make an informed decision!

The Drawbacks of Manual Testing

If you want to skip automated testing because you think it's expensive and time-consuming, you're wrong.

Here are some drawbacks of using manual testing that you need to keep in mind:

1. Requires Repetition

Visual regression is not a one-time deal. It needs to take place at the end of every development iteration. Whenever you add new functionality to your software, fix a bug, or alter even a single line of code, you have to check and verify that nothing is regressing before continuing to the next step of production.

As an excessively repetitive process, manual regression testing not only takes too long to execute but also takes up the precious time that you could have spent on more complex and fulfilling projects instead of tedious tasks.

Taking all things into account, we think it's safe to say that manual testing can tremendously slow down your release cycle and have crippling effects on the productivity of you and your team.

2. Prone to Human Errors

No matter how much attention you pay, it's always possible to overlook some UI errors or even introduce more visual bugs to your software while trying to fix only one. At the end of the day, we're all human, and we make mistakes.

We might write a faulty test script that could corrupt the rest of the application. Or we might fail to notice the difference between the previous and the modified state of our app since these differences can be too subtle for the human eye to catch. So, manually executed tests may be brittle to changes in the UI and less reliable since test results might not be accurate at all times due to human errors.

3. Too Many Testing Environments

Due to the wide range of browsers, devices, screen resolutions, and operating systems that applications are expected to run on, visual regression testing has become incredibly difficult to handle in recent years.

And if your app or website has a responsive design, it gets even harder since you now have to deal with visual compatibility issues in addition to an unmanageable number of execution environments.

But the worst part is, even if you don't touch a single line of code, there are still third-party upgrades you must factor in. For instance, if a browser suddenly gets updated, it can introduce a bug into your site or application in the blink of an eye.

To detect and eliminate these errors before they make their way into your system, you should constantly be on guard against abrupt changes and continuously run tests in all of these environments, which is quite impossible to manage manually.

Advantages of Automated Visual Testing Tools

Now that you have a clear understanding of all the drawbacks of executing visual testing manually, let's concentrate on the benefits of performing it automatically.

  1. Accelerate the testing process by automating the regression testing suite

Since you have to run regression tests after each release of a new product or its updated version and repeat this process every time you alter a code, the regression testing suite is the first in line for automation.

A regression testing suite is a collection of tests that are run together in order to verify that a software program still works after it has been changed. By automating it, you accelerate the test execution process of your software and shorten the release cycle of your development all at once.

  1. Getting Precise Visual Inspection

Another asset to bear in mind is the precision of visual inspection that automation provides. Thanks to the diligent difference detection capability of machine vision, automated visual tests can detect the slightest changes and bugs that get past even the most advanced QA engineers. And when a project grows in scope and becomes even more complex, the odds of them spotting these bugs and differences get even slimmer.

  1. Helping Alleviate Repetitive Tasks

Automation does not pose a risk to anyone as its role is not to replace humans but, on the contrary, to reduce human efforts.

It's designed to assist developers and testers to achieve maximum efficiency by helping them detect UI bugs earlier in the development lifecycle and conducting tests across multiple environments and screen sizes.

Alleviating the load on developers and QA engineers, automated visual tests let them put their time to better use and concentrate on mission-critical tasks that require their knowledge and talent.

Automating visual testing can be very helpful but how EXACTLY does it work? Let's take a quick look!

How Automated Visual Regression Testing Works

Spotting Waldo in a crowd is like spotting a visual defect in a web app

To make our point stand out, we've decided to explain how automated visual regression testing works with the help of someone that many of us are familiar. That someone is none other than our old friend Waldo, who's always been too hard to find among a crowd. But what does Waldo have to do with automated visual regression testing? Hear us out!

Let's assume Waldo is a bug in your software

Let's say Waldo is a visual bug that you don't want to appear on your application's page. Yet, somehow, he snuck into your software, nowhere to be seen. He's out there somewhere, perhaps hiding behind another UI element, causing it to break and corrupt the way it should look or even function.

With automated visual tests, you no longer have to break a sweat to spot a visual error, even if it's as hard as finding Waldo. “How,” you wonder? Here's a step-by-step explanation of an automated visual testing workflow:

  • The testing tool takes a screenshot of your existing page as a baseline image and then takes another one after any system modifications have been done.
  • To make sure what has changed doesn't impact any other page or component that's not meant to be affected, it then compares these two screenshots to see if there's any difference by highlighting the pixel variations between them. And that is exactly how we spotted Waldo so easily!

Waldo is spotted by Preflight's Visual Assertion

  • After the test comparison, the tool generates a detailed report, showing the discrepancy between the previous and the new state of your application. Based on this report, you then have to decide if the change it displays is an intended alteration or an unexpected visual bug.
  • Once you examine the report and approve of the current screenshot, you can use it as your new and improved baseline image for subsequent test runs.

As you can see, automated visual tests speed up and ease your workflow substantially. However, even though automation is a significant enhancement compared to manual visual testing, we believe that it can be improved even further. And that's why we developed Preflight.

Automating Visual Regression Testing with Preflight

Even when you automate your visual regression testing process, you still need to manually write a test script to be able to run it first. And no matter how much you enjoy or excel at writing test codes, it can eventually turn into a difficult task to carry out due to the repetitive nature of this process. But with Preflight, you don't even have to write a single line of code!

Here are several key benefits of using Preflight:

  1. It's a no-code testing tool, meaning anyone can use it. With no requirement to have any technical background or coding knowledge, it allows even the most technologically-challenged people to conduct tests and check the functionality and the visual experience of their web applications.
  2. Even if you know how to code, Its fully automated no-code-driven tests come in handy whenever you have trouble dealing with huge test scripts that take too long to execute.
  3. Preflight's tests are quick to create and easy to run. All you need to do is install it as an extension and run the test on your product, which it will immediately start recording. Its recorder captures each step of your visual regression test, making it way easier and faster to re-run it if need be.
  4. By performing a quick drag-and-drop action, you can instantly create a visual checkpoint on any component you want to test. This checkpoint, aka assertion, checks if anything has changed visually after a code modification has taken place by comparing the screenshots of your application's previous and current states.
  5. It detects what type of baseline it should use for the specific screen size, saving you from the tedious task of setting it up yourself and from dealing with visual compatibility issues.

Automate Your Visual Regression Tests to Catch Bugs Early in Development

Visual regression testing offers an elaborate set of advantages. It protects your brand's integrity by detecting visual bugs in the early stages of your development lifecycle.

It also provides your visitors with consistent and seamless user experiences, discouraging them from slipping away. This way, it plays a remarkable role in the maximization of your user conversion and retention rates.

Last but not least, visual regression testing – especially when automated – makes life way easier for developers and QA engineers. By executing tests a lot smoother, visual regression lightens the load on those who have to constantly write codes and conduct tests.

If you want to render your application correctly and make it bug-free today, consider taking our software for a test drive. Try Preflight for free now, and ensure a smooth customer experience!