Behind the Scenes of a Visual Regression Test

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In our daily routines as frontend engineers, we diligently apply unit, integration, and end-to-end tests to our development processes. The primary goal of these practices is to deliver features at high velocity with confidence in the robustness of our code. It's crucial to ensure that any modifications we introduce do not inadvertently disrupt the app's functionality. However, an often overlooked aspect of software testing is the visual part. Even if your app works correctly from a functional standpoint, what if key visual elements, such as call-to-action buttons, are obscured or entirely missing from the user's view?


In this talk, we will dive into the inner workings of visual regression testing. We'll explore how it functions and what steps are taken between the start and the result of such a test. We will look into the challenges it encounters along the way as well.

This talk has been presented at React Summit 2024, check out the latest edition of this React Conference.

FAQ

Visual Regression Tests are a method of detecting unintended changes to the visual aspects of a website, app, or UI. They are similar to unit or integration tests but focus on visual elements.

Visual Regression Tests work by taking screenshots of the UI before and after changes (called baseline and current shots) and comparing them to identify any visual differences. A machine generates a difference mask to highlight these changes.

Visual Regression Tests are important because they catch unintended visual changes that may not be obvious to the human eye, such as changes in text, button visibility, or layout shifts. These changes can impact user experience and functionality.

Examples include missing buttons due to color changes, text overlap, incorrect line heights, and other layout issues that can affect user experience and functionality.

Common challenges include dealing with animations, network requests, flakiness in test results, and differences in font rendering across browsers. Solutions involve stopping animations, mocking network requests, and running tests in a consistent environment like a CI pipeline.

Flakiness can be handled by stopping animations, mocking network requests, giving wait time, masking problematic areas, and questioning the validity of the tests if they consistently produce different results.

Different browser renderings can be managed by running tests in Docker or on a CI pipeline to ensure consistent results across different environments. This helps mitigate issues caused by different rendering engines.

Thresholds define the amount of visual change that is acceptable without failing the test. It's important to keep thresholds low to avoid unintended changes. Thresholds can be set as percentages or absolute pixel values.

Integration tests can be utilized by taking screenshots at the end of the tests and feeding them into the visual regression testing tool. This approach allows for efficient testing by combining integration and visual regression tests.

There are many tools available for Visual Regression Testing, both open source and paid. Examples include Playwright, Cypress, Storybook, and Ladle. These tools help automate the process and ensure consistent results.

Chris Kalmar
Chris Kalmar
19 min
18 Jun, 2024

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Video Summary and Transcription
Visual Regression Tests are like unit or integration tests but focus on the visual part, allowing developers and QA personnel to identify and address any changes. Challenges in detecting UI changes include elements that are not visible to the human eye and misalignment of elements. Use cases for Visual Regression Tests include testing design system components, responsive designs, and browser renderings. Building a Visual Regression Test Tool involves handling animations, network requests, and flakiness. Docker is the best solution for resolving visual regression issues, and finding the baseline for comparison can be challenging but is handled by the testing tool.

1. Introduction to Visual Regression Tests

Short description:

Visual Regression Tests are a method of detecting unintended changes to your app's UI. They are like unit or integration tests but focus on the visual part. By comparing screenshots of the current and previous versions, a machine can highlight the differences, which may not be noticeable to the human eye. This allows developers and QA personnel to identify and address any changes.

Welcome. Let's talk about Visual Regression Tests. So what are Visual Regression Tests anyway? They're a method of detecting changes to your website, app, UI that were not intended. Think of them as unit or integration tests, but dealing more with the visual part of your app.

There's a long description to that, but that's kind of boring so as we're talking about Visual Regression Tests, let me just show you. So imagine you're working in a company where there is a website, and you have a marketing department, and the marketing department wants to introduce a small change. We have the change implemented by the department, and now we take a screenshot of that page. So this is something that we call a current shot or a change shot. This is after the change. And then we pull up a version from before, what the website looked like before we made the change. So this is called a baseline image. Now if you look at both versions before and after, you might be able to spot the differences, but it's not that easy because the human eye is not good at it. So what we need is a machine to show us a difference mask. If you look at this, it becomes really obvious what changed. Let me pull this up a little bit so you can see now in the top we added a new blog link. The change in the marketing header is there as well, and at the bottom we have as well some marketing copy changed. Not that obvious for the human eye, but it's there. Now the visual regression test would catch that and will tell somebody like the developer or the QA person or custom manager that, hey, there's a change. What do you want to do about it?

Now you might be saying that, well, it's just a small text change, nothing harmful. What can go wrong? It's not that important. In this case, maybe you're right, but let me show you an example where it's not that harmless. But before we get to that, let me quickly introduce myself. Hi, my name is Chris. I'm a full-stack engineer. I love open source and I'm a co-founder at Lost Pixel. My passion for visual regression tests is so big that I started building a product with a friend of mine and we built even a company around it. If you want to catch up with me, you can find me on social media, on X, Twitter, YouTube, LinkedIn. Just look for the handle Chris Calmer. It would be nice to meet you. All right, let me get back to the example that I was talking about before.

2. Challenges of Detecting UI Changes

Short description:

Integration tests can miss UI changes that are not visible to the human eye. For example, a button may disappear due to a color change that blends it with the background. This can lead to negative user experiences and potential losses. Another issue is the misalignment of elements, such as the squished title or overlapping content.

What a shop usually has, right. Somebody made a change and suddenly the sales department gets in panic mode because sales numbers are dropping like crazy. Nobody's buying anything anymore. What is going on? Okay, the engineering team has to check what's up and well, they look at the checks. Everything is passing green. They look deeper, even really close to every single build step. And well, unit test passed. Even the Playwright integration test passed. So, what happens? If you look closely, you'll find out what happened. The buy button is missing.

So how could that happen? We had integration tests there, right. They should have caught that problem that somebody removed the button. The truth is a bit more complicated. The button was never gone because this is what the integration test sees. Playwright or Cypress in this case, sees the button. It's still there and can click it. So your tests are passing. But if you go back, the customer doesn't see the button. After some research, you finally find out what happened. Somebody changed the primary color of the action button in another place. And that caused the button to disappear because now the color of the button blends in perfectly with the background of the page. The button is still there, but you can't see it as a user. Only the machine can do it.

Well, that's really not good. And this will cost you. Let me show you another example. For example, here where the description of the title of the product is re-squished because somebody made changes to the line height somewhere else. And this doesn't look good. Or another case where we have content overlapping. So the image is on top of the description, which makes it really hard to read.

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