To compare a website before and after a change, use Selenium to capture both versions in the same browser and page state, then use Pillow’s ImageChops.difference() to create a pixel-difference image. Inspect that image to locate changes; a summary score can help triage but cannot tell you whether a difference is a defect.
What you need
- Python, plus Selenium and Pillow installed in the project environment.
- A browser and a compatible Selenium driver, or a configured remote WebDriver.
- A stable test page, reference state and candidate state to compare.
Selenium’s Python WebDriver API documents saving a current-window screenshot as a PNG file or returning its PNG bytes. Pillow provides the image comparison and statistics operations used below. The linked API pages document Selenium 4.50.0 and 4.49.0 methods and Pillow 12.3.0; confirm availability against the releases installed in your project. Selenium WebDriver API, Pillow ImageChops, and Pillow ImageStat.
Capture comparable screenshots
The comparison is meaningful only if the captures represent the same conditions. Keep the route, viewport, scroll position and interaction state consistent. Where practical, also keep browser and operating-system versions, device scale, zoom, locale, color settings, authentication and page data stable. Wait for the content relevant to your test—including fonts or images—to settle before capturing.
- Install the dependencies:
python -m pip install selenium pillow. Install and configure the driver for your chosen browser, or configure your remote WebDriver. - Set the browser window size with Selenium’s
set_window_size(width, height). Use the same dimensions for both captures. - Navigate to the stable test URL and wait for the page condition you are testing. Capture the reference state, then repeat the same steps for the candidate state.
- Save both captures as PNGs, using Selenium’s
save_screenshot()orget_screenshot_as_file(). Selenium also documentsget_screenshot_as_png()for returning bytes andget_screenshot_as_base64()for base64 output.
from pathlib import Path
from selenium import webdriver
from selenium.webdriver.support.ui import WebDriverWait
URL = "https://example.com/your-test-page"
OUT = Path("screenshots")
OUT.mkdir(exist_ok=True)
# Configure the driver for the browser and environment used by your project.
driver = webdriver.Chrome()
try:
driver.set_window_size(1440, 1000)
driver.get(URL)
# Replace this with a condition that represents the page state under test.
WebDriverWait(driver, 20).until(
lambda d: d.execute_script("return document.readyState") == "complete"
)
driver.save_screenshot(str(OUT / "reference.png"))
# Apply the code/content change or load the candidate build, then repeat
# the same navigation and wait conditions before taking candidate.png.
# This example captures the current page again as a runnable starting point.
driver.save_screenshot(str(OUT / "candidate.png"))
finally:
driver.quit()The example’s document-ready wait is only a starting condition; it does not guarantee that client-rendered content, web fonts, lazy images or animations have finished. Replace or extend it with the condition your page and test require. To capture separate builds, point the same procedure at each build while keeping the rest of the environment fixed.
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Generate and inspect a pixel difference
Load the PNGs with Pillow, check that their dimensions match, and convert them to the same compatible mode before comparing. ImageChops.difference(reference, candidate) returns an image containing the absolute pixel-by-pixel difference. Identical pixels appear black; changed areas show non-black values.
from PIL import Image, ImageChops, ImageStat
reference = Image.open("screenshots/reference.png").convert("RGB")
candidate = Image.open("screenshots/candidate.png").convert("RGB")
if reference.size != candidate.size:
raise ValueError(
f"Screenshot sizes differ: {reference.size} vs {candidate.size}"
)
diff = ImageChops.difference(reference, candidate)
diff.save("screenshots/difference.png")
stats = ImageStat.Stat(diff)
print("Per-channel mean difference:", stats.mean)
print("Review screenshots/difference.png for the location and shape of changes.")Open difference.png and compare it with both original captures. The mean returned by ImageStat.Stat(diff).mean is the average arithmetic pixel level for each band. It is a summary, not a universal visual-regression pass percentage: a small changed region can matter greatly even when the overall mean is low, while widespread harmless antialiasing noise can raise it. Choose any threshold to fit your project and retain the diff image for diagnosis. Pillow documents the difference operation and per-band image statistics.
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Choose the right capture and comparison scope
| Choice | Use it for | Consider |
|---|---|---|
| Browser-window screenshot | Page layout and visible viewport content. | Keep window dimensions, scroll position and page state matched. Selenium documents current-window screenshot methods in its WebDriver API. |
| Element screenshot | A focused component, such as a card or navigation area. | The element’s own bounds are captured, but surrounding layout can still affect its rendering. Selenium documents element screenshot output as a PNG file or bytes in its WebElement API. |
| Difference image | Finding where pixels changed and visually diagnosing the change. | Keep it alongside the source captures; it shows locations, not causes or importance. |
| Summary statistic or project threshold | Automated triage across many captures. | Calibrate to the page and test goal; an aggregate can conceal a localized issue. |
| One state or broader coverage | One stable state keeps baseline maintenance smaller; multiple routes, viewports or interactions cover more user paths. | Choose coverage around user-critical behavior and maintain a distinct baseline for each tested condition. |
For noisy areas such as timestamps, ads or rotating content, prefer stable test data. If you mask a region, define it narrowly and only when that content is outside the purpose of the test; masking meaningful content can hide a real regression.
Troubleshoot common comparison problems
- The images have different dimensions: set the same Selenium window size for both captures and confirm the resulting image dimensions before calling the pixel operation.
- The diff contains changes in fonts or images that should be stable: wait on the relevant page condition rather than relying on a short arbitrary sleep. Ensure fonts and images have loaded before capture.
- The diff is noisy between runs: stabilize data and environment settings, including browser, operating system, device scale, locale and rendering configuration. Narrowly mask only known irrelevant regions.
- Pillow raises a mode or comparison error: convert both inputs deliberately to the same supported mode, such as
RGB, and ensure the sizes match. Pillow notes that most channel operations are implemented for 8-bit modes such asLandRGB. - The score looks small but a visible defect exists: inspect the difference image and original captures. The mean aggregates all pixels and can dilute a localized change.
- The score is large due to irrelevant animation or randomized content: freeze or control those inputs for the test, or exclude only narrowly defined regions that are not under test.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com/your-test-page -o shot.webpPython and Node.js equivalents:
import requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://example.com/your-test-page"},
timeout=90,
)
r.raise_for_status()
open("shot.webp", "wb").write(r.content)const q = new URLSearchParams({
access_key: 'YOUR_API_KEY',
url: 'https://example.com/your-test-page'
});
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`Screenshot request failed: ${res.status}`);
await Bun.write('shot.webp', res);Use the same URL and equivalent capture conditions for each version if you plan to compare the resulting images. ScreenshotNeo also provides an MCP server with take_screenshot, get_page_info and capture_pdf tools for AI agents. The Free plan includes 1,000 shots per month with no card; paid plans start at $5 for 3,000 shots. Sign up for free.
Frequently Asked Questions
Can a pixel difference tell me whether a change is a bug?
No. It identifies changed pixels; you need to inspect the captures and judge whether the change is expected and important.
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