Screenshots and snapshots

toHaveScreenshot, toMatchSnapshot and page.screenshot() work with Playwright's API and file paths. The pixels are Pyro's: pages are rendered by its own CPU renderer, not by Chromium. Baselines from a Playwright suite will not match, so re-baseline once when you migrate.

toHaveScreenshot

test('home page', async ({ page }) => {
  await page.goto('/');
  await expect(page).toHaveScreenshot('home.png');
  await expect(page).toHaveScreenshot('home-full.png', { fullPage: true });
  await expect(page.getByRole('navigation')).toHaveScreenshot('nav.png', { maxDiffPixels: 20 });
});

The page (or the locator's element) is rendered and compared inside the browser. The test process sends the baseline once per connection; later comparisons against the same baseline do not resend it.

Options

Option	Default	Meaning
threshold	0	How far each color channel of a pixel may differ (0 to 1) and still match. 0 means exact.
maxDiffPixels	0	Differing pixels allowed.
maxDiffPixelRatio	none	Differing pixels allowed, as a fraction of the image. With maxDiffPixels too, the stricter limit applies.
clip	none	{ x, y, width, height } in CSS pixels.
fullPage	false	The whole document, not only the viewport.
timeout	expect timeout

Defaults for threshold, maxDiffPixels and maxDiffPixelRatio can be set in the config:

export default defineConfig({
  expect: { toHaveScreenshot: { maxDiffPixelRatio: 0.01 } },
});

threshold here is a per-channel tolerance. Playwright's threshold (default 0.2) is a perceptual color distance. Both are 0 to 1, but the same number does not mean the same thing.

Other options are errors

animations, caret, mask, maskColor, omitBackground, scale and stylePath are in the types but the browser rejects them:

LP.run: step 0: unknown field `animations`, expected one of `name`, `baseline`, `sceneHash`, `threshold`, ...

Remove them when you migrate. Pyro renders a still frame, so there is no animation or caret to disable.

expect(...).not.toHaveScreenshot() is not supported.

Names and paths

A name must end in .png (Playwright requires this too); otherwise the assertion throws. A list of strings (['nav', 'open.png']) makes sub-directories. Without a name, the name is the test's title path plus a counter, as in Playwright.

The baseline path follows Playwright's default template:

{snapshotDir}/{testFileDir}/{testFileName}-snapshots/{arg}{-projectName}{-snapshotSuffix}{ext}

So expect(page).toHaveScreenshot('home.png') in e2e/home.spec.ts, project chromium, on Linux, uses e2e/home.spec.ts-snapshots/home-chromium-linux.png. Without a project name the -chromium part is left out. The suffix is the platform (darwin, linux); set testInfo.snapshotSuffix = '' to drop it.

snapshotPathTemplate (config or project) and expect.toHaveScreenshot.pathTemplate change the template, with Playwright's tokens: {testDir}, {snapshotDir}, {testFileDir}, {testFileName}, {testFilePath}, {testName}, {arg}, {ext}, {platform}, {projectName}, {snapshotSuffix}. Paths resolve against the config file's directory.

Beside each baseline home-chromium-linux.png the runner writes home-chromium-linux.json. It holds the hash of the render's display list. When a later render has the same hash, the assertion passes without rasterizing or comparing pixels. Commit the .json files with the .png files. Without one, the comparison runs on pixels.

Failures

On a mismatch the assertion fails with the number of differing pixels and writes three images into the test's output directory (test-results/<test>/ by default):

test-results/e2e-home.spec.ts-home-page/home-actual.png
test-results/e2e-home.spec.ts-home-page/home-expected.png
test-results/e2e-home.spec.ts-home-page/home-diff.png

A missing baseline is written, and the test fails at its end with A snapshot doesn't exist at <path>, writing actual. That test is not retried, as in Playwright. The next run compares against the new file.

Update modes

-u / --update-snapshots [mode] on lightningpanda test, the config's updateSnapshots, or $LP_UPDATE_SNAPSHOTS choose what is written:

Mode	Missing baseline	Mismatch
missing (default without -u)	Written; the test fails	Fails
changed (default with -u)	Written; passes	Rewritten; passes
all	Written; passes	Rewritten; passes. Matching files are rewritten too.
none	Fails: A snapshot doesn't exist at <path>.	Fails

bunx lightningpanda test -u            # changed
bunx lightningpanda test -u all        # rewrite every baseline
bunx lightningpanda test -u none       # CI: never write

With plain bun test, -u means changed. These modes also apply to toMatchSnapshot and toMatchAriaSnapshot files.

Re-baselining on migration

Chromium and Pyro draw text, anti-aliasing, form controls and some layout details differently. The renderer is described in Rendering: screenshots and PDF. Expect every Chromium baseline to fail.

1. Remove the old baseline images, or keep them on a branch for reference.
2. Run bunx lightningpanda test -u all on the platform where CI runs. Fonts come from the operating system, and the file name carries the platform, so macOS and Linux need their own baselines.
3. Look at the new images. A rendering problem in Pyro would otherwise become the new truth.
4. Commit the .png and .json files.
5. In CI, run with -u none or without -u, so a missing baseline fails.

If a test's screenshot depends on text wrapping at hyphens next to digits (IDs, dates), use: { lightningpanda: { args: ['--hyphen-digit-breaks', 'on'] } } wraps like Chrome does. It is off by default.

toMatchSnapshot

test('export', async ({ page }) => {
  await page.goto('/report');
  const csv = await page.getByTestId('csv').textContent();
  expect(csv).toMatchSnapshot('report.csv');
  expect(await page.screenshot()).toMatchSnapshot('report.png', { maxDiffPixels: 10 });
});

expect(value).toMatchSnapshot(name?, options?) compares a string or a Buffer with a file at Playwright's snapshot path. It is synchronous, as in Playwright.

• PNG files compare by pixel, with threshold (default 0.2, Playwright's perceptual distance), maxDiffPixels and maxDiffPixelRatio. Other files compare byte for byte; a text mismatch prints a line diff.
• Without a name, the extension comes from the content: .txt for strings, .png or .jpg from the bytes, else .dat.
• On a mismatch, -actual and -expected files are written to the test's output directory.
• Defaults come from expect.toMatchSnapshot in the config.
• .not works against an existing file; a missing file with .not fails.

testInfo.snapshotPath(name, { kind: 'snapshot' | 'screenshot' | 'aria' }) returns the path for a name, as in Playwright.

page.screenshot and locator.screenshot

const png = await page.screenshot({ fullPage: true });
await page.locator('#chart').screenshot({ path: 'out/chart.png' });

page.screenshot() takes path, fullPage, clip, type (png or jpeg, also from the path's extension) and quality. Other options (mask, animations, omitBackground, scale, caret, style) are ignored. locator.screenshot() clips the page screenshot to the element's bounding box.

Both need the browser's rendering support, which the npm package's builds include.

page.pdf

page.pdf(options) passes its options to CDP Page.printToPDF unchanged and writes path if given. Use CDP's option names: paperWidth and paperHeight in inches, marginTop / marginBottom / marginLeft / marginRight in inches, landscape, scale, printBackground, pageRanges. Playwright's format: 'A4' and margin: { top: '1cm' } are not translated and have no effect. See Screenshots and PDFs.