To compare website screenshots with Pixelmatch in Node.js, decode both PNG files into pixel buffers, make sure they have identical dimensions, pass their pixel data to pixelmatch, and check the returned mismatch count against a tolerance you choose. If you want to inspect changes, write the output buffer as a diff image. Pixelmatch’s documented Node.js example uses pngjs for PNG input and output. Pixelmatch’s README describes the result as the number of mismatched pixels.
Install Pixelmatch and pngjs
In your Node.js project, install the comparison library and the PNG reader/writer:
npm install pixelmatch pngjsSave the following as compare.js. It uses ES module imports; if your project uses CommonJS, see the note below.
Compare two PNG screenshots and save a diff
import fs from 'node:fs';
import pixelmatch from 'pixelmatch';
import { PNG } from 'pngjs';
const baseline = PNG.sync.read(fs.readFileSync('baseline.png'));
const actual = PNG.sync.read(fs.readFileSync('actual.png'));
if (baseline.width !== actual.width || baseline.height !== actual.height) {
throw new Error(
`Screenshots must have equal dimensions: baseline is ${baseline.width}x${baseline.height}, actual is ${actual.width}x${actual.height}`
);
}
const diff = new PNG({ width: baseline.width, height: baseline.height });
const mismatchedPixels = pixelmatch(
baseline.data,
actual.data,
diff.data,
baseline.width,
baseline.height,
{ threshold: 0.1 }
);
fs.writeFileSync('diff.png', PNG.sync.write(diff));
console.log(`Mismatched pixels: ${mismatchedPixels}`);
const allowedMismatches = 100;
if (mismatchedPixels > allowedMismatches) {
console.error(`Visual comparison failed: more than ${allowedMismatches} pixels differ`);
process.exitCode = 1;
}Run it with node compare.js. The example expects baseline.png and actual.png in the current directory, writes diff.png, prints the count, and exits unsuccessfully when the count exceeds the example tolerance. Set allowedMismatches to a value that suits your own test; 100 is an illustrative choice, not a Pixelmatch recommendation.
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For the ES module syntax, set "type": "module" in your project’s package.json, or use a .mjs filename. In a CommonJS project, load Pixelmatch with dynamic import because it is an ES module:
const pixelmatch = (await import('pixelmatch')).default;
const { PNG } = require('pngjs');Use those declarations in place of the corresponding imports and run the script in an environment that supports top-level await, or put the code inside an async function.
What Pixelmatch compares—and what it returns
Pixelmatch compares raw image pixel data rather than opening screenshot filenames itself. In the example, pngjs decodes each PNG, and baseline.data and actual.data provide the pixel buffers. Both source images must have the same width and height. When you request a diff image, its buffer must use those same dimensions too.
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The function returns an integer: the number of pixels it classifies as mismatched under the selected options. That number is not automatically a test verdict. Decide what your project will tolerate, then fail the test when the returned count crosses that limit. Pixelmatch also allows null as the output argument when you need only the count and do not need a diff image.
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Set the comparison threshold
The threshold option ranges from 0 to 1; smaller values make the comparison more sensitive. The documented default is 0.1. Choose deliberately: changing sensitivity changes which pixel differences contribute to the mismatch count, so a tolerance should be calibrated for your test rather than copied as a universal pass rule.
Decide how to handle anti-aliased pixels
includeAA defaults to false, so anti-aliased pixels are detected and ignored. Set includeAA: true if you want those pixels counted as differences. This can make comparisons stricter around rendered edges, but it may also make small rendering variations more visible to the test.
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Customize the diff image
Pixelmatch exposes options for the diff’s presentation: alpha adjusts the blending of unchanged pixels; aaColor, diffColor, and diffColorAlt set colors for anti-aliased and differing pixels; and diffMask renders the diff over transparency instead of the original image. These affect how the output is shown, not your project’s decision about how many mismatches to permit.
Count all mismatches or focus on a region
The current Pixelmatch README documents windowSize. Its default is Infinity, which reports mismatches across the full image. A finite window size instead reports the maximum mismatch count in any sliding square of that size. This can help identify concentrated changes rather than treating scattered differences over the whole screenshot as equivalent. Check the README for the release you have installed before relying on this option; it is not established here for every older release.
Keep screenshot comparisons meaningful
Pixelmatch reports pixel differences; it does not decide whether a page was captured under comparable conditions. For a useful baseline comparison, keep the capture setup consistent: viewport dimensions, device scale, browser and rendering environment, page state, and timing can all affect a screenshot. This is testing practice, not a guarantee that two captures will be identical. If a diff is noisy, inspect the images and the capture conditions before simply raising the allowed mismatch count.
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Quick manual comparison from the command line
For a one-off PNG comparison, Pixelmatch also provides a command-line interface:
pixelmatch image1.png image2.png output.png 0.1Use the library in a Node.js test when you need the mismatch count, a project-specific pass/fail rule, or integration with a capture pipeline. The CLI is a simpler route for quickly generating an image diff.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshooting
- Dimension mismatch: Pixelmatch requires both input images to have equal dimensions, and a requested output image must match them. Capture both screenshots at the same size, or intentionally resize/crop them before comparison.
- Cannot decode the input: The example uses
PNG.sync.read, so the files must be readable PNG images. Confirm the paths and file contents; do not pass a JPEG or WebP file to the PNG decoder. - Import or module syntax error: The sample uses ES modules. Mark the package as
"type": "module"or use.mjs; for CommonJS, dynamically import Pixelmatch as shown above. - The test fails on tiny rendering changes: Review the
threshold, anti-aliasing behavior, and allowed mismatch count. Adjust only after deciding which visual differences your test should ignore. - The count passes but the page looks wrong: A numeric tolerance can hide a meaningful localized change. Save and inspect
diff.png; consider whether a finitewindowSizeis appropriate for localized mismatch checks in the installed version. - No diff image is needed: Pass
nullas Pixelmatch’s output argument and omit the PNG encoding/write step; the function still returns the mismatch count.
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Frequently Asked Questions
Does Pixelmatch compare JPEG or WebP files directly?
The workflow here uses pngjs’s PNG decoder, so convert other formats to PNG or use a decoder that produces the pixel data Pixelmatch accepts.
Is Pixelmatch’s default threshold a pass/fail tolerance?
No. The default comparison threshold is 0.1; the allowed mismatch count in a test is a separate rule you choose.
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