ScreenshotAPI’s POST /v1/compare endpoint compares a page rendered now with either a second URL or a named saved baseline. It returns a changed-pixel percentage, boxes around changed regions, and a diff image. Use it to flag visual changes for review—not to assume every changed pixel is a defect. The instructions and limits below describe ScreenshotAPI’s documented service behavior; they are not independent test results. ScreenshotAPI.
What the comparison endpoint returns
Send a comparison request to POST /v1/compare and choose one reference mode: against for a second URL rendered now, or baseline for a named image stored earlier. Provide one of these parameters, not both. ScreenshotAPI documents three useful outputs:
- A percentage of pixels that changed.
- Boxes identifying regions where changes were detected.
- A diff image in which changes are tinted and unchanged areas are faded.
Capture parameters apply to both sides of the comparison, helping the images line up. That is useful, but it does not make every difference meaningful: changed content, animation, timing, or other page behavior can produce a diff that still needs human review. The documentation does not set a universally correct change threshold.
Choose a reference: another URL or a named baseline
| Mode | Use it for | What is rendered |
|---|---|---|
against |
A direct comparison, such as preview versus production. | Both URLs are rendered for the comparison. |
baseline |
Checking one page over time against a named saved image. | The current page is rendered and compared with the stored image. |
Use update_baseline when an observed change is intentional and you have decided to accept it. It defaults to false; updating should be a deliberate approval, not an automatic response to any detected difference.
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Build a visual-regression check into CI
- Protect the API key. Put it in your CI platform’s secret store, not directly in a committed pipeline file.
- Capture the candidate. Point the comparison at the preview or staging deployment and select the viewport and other capture parameters your project expects.
- Compare against a persistent reference. For ongoing checks, use a named baseline. ScreenshotAPI’s integration guide advises storing baseline images with the repository because CI artifacts may be temporary.
- Review before deciding. Inspect the changed percentage, region boxes, and diff image. Apply a project-defined threshold or route the result for review; the vendor does not prescribe a universal threshold.
- Accept intentional changes explicitly. After review, update the named baseline when appropriate using
update_baseline.
ScreenshotAPI names GitHub Actions, GitLab CI, and Bitbucket Pipelines as integration targets, and says the endpoint can be called from CI/CD with curl or a script. The exact job configuration depends on your pipeline and how you store baselines.
Keep comparisons meaningful
- Keep capture settings consistent. Matching viewport and other capture parameters reduces differences caused by capturing the two sides under different conditions; the endpoint applies the same capture parameters to both sides.
- Treat the diff as evidence, not a verdict. A changed pixel can reflect a legitimate content or layout update. Review the affected regions before failing a build or replacing a baseline.
- Make the baseline durable. Ensure a named baseline remains available across CI runs; a temporary build artifact is not a reliable long-term reference.
- Budget for renders, not just comparisons. Each rendered side consumes one quota unit, while the comparison operation itself is free. URL-to-URL mode therefore uses two renders; comparing the current page with an existing baseline uses one current render.
ScreenshotAPI’s current documentation lists monthly quotas of 100 renders on Free, 2,000 on Starter, 10,000 on Pro, 25,000 on Team, and 100,000 on Business. Quotas reset at the start of each UTC calendar month, and failed renders receive their reserved unit back. These are changeable product limits; check the current official plan and comparison documentation before estimating ongoing CI use.
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Check whether the hosted renderer can reach the target
A comparison cannot render a URL the hosted service rejects or cannot reach. ScreenshotAPI’s documented restrictions include:
- Only HTTP and HTTPS schemes are accepted.
- Loopback, RFC1918 private, link-local, carrier-grade NAT, and cloud metadata addresses are rejected, as are hostnames resolving to those address ranges.
- URLs containing embedded credentials are rejected.
- Ports other than 80, 443, 8080, and 8443 are rejected.
This can prevent a privately networked staging site from being captured through the hosted endpoint. Confirm that the preview URL is reachable within these rules before making the visual check a required CI gate.
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Troubleshoot common comparison problems
| Symptom | Likely cause | What to check |
|---|---|---|
| The request is rejected when choosing a reference. | Both against and baseline were supplied, or neither was supplied. |
Send exactly one reference mode. |
| The target cannot be rendered. | The URL uses an unsupported scheme, a blocked address or port, or embedded credentials; a private deployment may also be inaccessible to the hosted renderer. | Use an allowed HTTP/HTTPS URL on a supported port without embedded credentials, and verify the destination is reachable under the service’s restrictions. |
| The diff shows extensive changes that do not appear to be a regression. | The pages may differ in content or state, or the expected change may be intentional. | Inspect the diff image and region boxes; verify capture settings and page state, then decide whether to accept the change. |
| A CI run cannot find its reference on a later build. | The baseline may have been kept only as a temporary CI artifact. | Use a persistent named baseline and retain it across runs; the integration guide recommends repository storage for baseline images. |
| Builds fail on small visual changes. | The project may be using a threshold that is too strict for its pages or workflow. | Set a team-defined threshold and review policy. ScreenshotAPI’s docs do not specify a universally appropriate value. |
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