Chromatic detects visual changes by rendering Storybook stories in cloud browsers, capturing screenshots, and comparing each capture with the accepted baseline for that test. It highlights differences for a person to review; the diff signals that pixels changed, not that a bug necessarily occurred.
What Chromatic treats as a visual test
A Storybook story defines a repeatable component state, so Chromatic can use each story as a test case. A test might represent a particular component variant, data state, or interaction result. Chromatic’s Visual tests documentation describes the process as capturing snapshots of tests in a cloud browser environment.
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When a Storybook build is uploaded, Chromatic loads the stories selected for testing in its browser environment. Each capture is associated with its test and build context, including configured browser and viewport information.
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- Load the story. Chromatic opens the test in a cloud browser at its configured device and viewport.
- Wait for the UI. It uses readiness heuristics, including network inactivity, and allows Storybook interaction tests’
playfunctions to complete before capture. - Capture a snapshot. Chromatic takes an image of the rendered UI state.
- Compare with the relevant baseline. The new image is compared with the prior accepted snapshot for that test and context.
- Review the result. Differences are highlighted so a developer can decide whether to accept an intentional change or fix an unintended one.
The baseline is the accepted known-good state for that test. Accepting an intentional visual change advances the baseline for later comparisons; it does not make the visual difference disappear from the review that prompted the decision. See Chromatic’s Quickstart for the initial-baseline and subsequent-build workflow.
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What a visual diff means
A diff means Chromatic found image differences under its comparison settings. It does not decide whether those changes are correct. A changed button color may be intended after a redesign; a shifted layout caused by a regression may need a fix. Review the highlighted area against the product requirement and the change being made before accepting it.
How the diff threshold affects sensitivity
Chromatic’s documented diffThreshold default is .063. This is a tolerance for color difference at corresponding image coordinates, described using YIQ color space; it is not a percentage of the page or of pixels that may change. The exact setting and its interpretation are documented in Chromatic’s threshold guide.
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- A lower threshold makes comparison more sensitive and can flag smaller color differences, but may increase noise from rendering variation.
- A higher threshold can reduce noisy diffs, but may stop subtle visual changes from being flagged.
- Anti-aliased pixels are ignored by default; Chromatic provides a configuration option to include them.
Choose sensitivity in light of the kinds of changes that matter in your UI. Do not treat the default as a universal guarantee that every meaningful change will be caught or every harmless difference suppressed.
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Why browser and viewport context matters
Chromatic compares a browser’s capture with a baseline for that same browser rather than comparing, for example, a Chrome capture directly with a Firefox capture. Browser, device, and operating-system rendering can differ, so each configured browser or viewport context can have its own baseline. This makes a diff answer the practical question “Did this test change in this context?” rather than “Are these two browsers pixel-identical?” Details are in Chromatic’s browser documentation.
Adding browser or viewport coverage can reveal issues specific to those contexts, but it also creates more context-specific snapshots to review and maintain.
How branches and accepted changes affect baselines
Chromatic’s UI Tests compare a branch build against a baseline in that branch’s ancestry. Once a change is accepted, the updated baseline can inform subsequent checks. The Storybook Visual Tests addon supports reviewing tests from Storybook and synchronizing accepted local baselines to the cloud. See the Visual Tests addon documentation for that workflow.
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Keeping captures stable
Chromatic attempts to capture a settled interface, but capture readiness is not the same as deterministic rendering. Its snapshot process pauses CSS animations, transitions, videos, and GIFs to reduce variation. JavaScript-driven animation may continue unless the test controls it; asynchronous data, timestamps, random values, and other changing state can also make otherwise identical runs look different. The capture behavior and caveats are covered in Chromatic’s snapshots documentation.
- Give stories stable data and explicit states rather than relying on live or changing content.
- For animated interfaces, make the tested state deterministic—for example, control the animation or render a stable frame in the test.
- Use interaction tests to reach the intended state, and ensure the story’s
playfunction finishes the relevant interactions before capture. - If a capture differs from Storybook’s Canvas view, inspect the configured viewport, browser, readiness conditions, and runtime state; the capture environment may not reproduce the local view exactly.
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Frequently Asked Questions
Does Chromatic compare a screenshot with the live website?
No. It compares a Storybook test capture with the relevant accepted baseline for that test and context.
Is Chromatic’s default diffThreshold .063 a 6.3% pixel-change allowance?
No. It is a color-difference tolerance, not a proportion of the image.
Will accepting a change fix the code?
No. Acceptance updates the baseline; code changes are needed to correct an unintended regression.
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