visual-testing（视觉测试）

Visual Testing

Overview

Visual regression testing catches unintended UI changes by comparing screenshots. This skill covers three approaches:

1. Chromatic - Cloud-based, unlimited parallelization (recommended)
2. Lost Pixel - Self-hosted, open source alternative
3. Playwright Built-in - Simple snapshot testing

Quick Start: Chromatic (5 Minutes)

1. Install Chromatic

npm install --save-dev chromatic

2. Get Project Token

1. Go to chromatic.com
2. Sign up with GitHub (free tier: 5000 snapshots/month)
3. Create project → Copy project token

3. Run First Build

# Set token
export CHROMATIC_PROJECT_TOKEN=your_token_here

# Run visual tests with Playwright
npx chromatic --playwright

4. Review Changes

• Chromatic UI shows visual diffs
• Approve or reject changes
• Approved changes become new baselines

Chromatic with Playwright

Configuration

// playwright.config.ts
import { defineConfig } from '@playwright/test';

export default defineConfig({
  testDir: './tests/visual',
  use: {
    screenshot: 'on',
  },
  reporter: [
    ['html'],
    ['chromatic', { projectToken: process.env.CHROMATIC_PROJECT_TOKEN }],
  ],
});

Visual Test Example

// tests/visual/homepage.spec.ts
import { test, expect } from '@playwright/test';

test('homepage visual', async ({ page }) => {
  await page.goto('/');

  // Full page screenshot
  await expect(page).toHaveScreenshot('homepage.png');
});

test('dashboard visual', async ({ page }) => {
  await page.goto('/dashboard');

  // Wait for data to load
  await page.waitForSelector('.dashboard-loaded');

  // Component screenshot
  const chart = page.locator('.revenue-chart');
  await expect(chart).toHaveScreenshot('revenue-chart.png');
});

CI Integration (GitHub Actions)

# .github/workflows/visual.yml
name: Visual Tests
on: [push, pull_request]

jobs:
  visual:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0  # Required for Chromatic
      - uses: actions/setup-node@v4
        with:
          node-version: 20
      - run: npm ci
      - run: npx playwright install --with-deps
      - run: npx chromatic --playwright
        env:
          CHROMATIC_PROJECT_TOKEN: ${{ secrets.CHROMATIC_PROJECT_TOKEN }}

Chromatic Features (Free Tier)

Lost Pixel (Self-Hosted)

For confidential projects that can't use cloud services.

Installation

npm install --save-dev lost-pixel

Configuration

// lostpixel.config.ts
import { CustomProjectConfig } from 'lost-pixel';

export const config: CustomProjectConfig = {
  pageShots: {
    pages: [
      { path: '/', name: 'home' },
      { path: '/login', name: 'login' },
      { path: '/dashboard', name: 'dashboard' },
    ],
    baseUrl: 'http://localhost:3000',
  },
  // Store baselines locally
  imagePathBaseline: '.lostpixel/baseline',
  imagePathCurrent: '.lostpixel/current',
  imagePathDifference: '.lostpixel/difference',

  // Fail on difference
  generateOnly: false,
  failOnDifference: true,

  // Comparison settings
  threshold: 0.1,  // 0.1% pixel difference allowed
};

Run Lost Pixel

# Generate baselines (first run)
npx lost-pixel update

# Run comparison
npx lost-pixel

# View differences in .lostpixel/difference/

Docker Setup (for CI)

# Dockerfile.lostpixel
FROM mcr.microsoft.com/playwright:v1.40.0-focal

WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .

CMD ["npx", "lost-pixel"]

# .github/workflows/visual-selfhosted.yml
jobs:
  visual:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: docker build -f Dockerfile.lostpixel -t visual-tests .
      - run: docker run visual-tests

Playwright Built-in Snapshots

Simplest approach, no external services.

Basic Usage

// tests/visual/basic.spec.ts
import { test, expect } from '@playwright/test';

test('visual regression', async ({ page }) => {
  await page.goto('/');

  // Full page
  await expect(page).toHaveScreenshot('homepage.png');

  // With options
  await expect(page).toHaveScreenshot('homepage-full.png', {
    fullPage: true,
    maxDiffPixels: 100,  // Allow 100 pixel difference
  });

  // Specific element
  const header = page.locator('header');
  await expect(header).toHaveScreenshot('header.png');
});

Update Baselines

# Update all baselines
npx playwright test --update-snapshots

# Update specific test
npx playwright test tests/visual/homepage.spec.ts --update-snapshots

Configuration

// playwright.config.ts
export default defineConfig({
  expect: {
    toHaveScreenshot: {
      maxDiffPixels: 100,
      maxDiffPixelRatio: 0.01,  // 1%
      threshold: 0.2,  // Color difference threshold
    },
  },
  snapshotPathTemplate: '{testDir}/__screenshots__/{testFilePath}/{arg}{ext}',
});

Cross-Platform Issues

Problem: Screenshots differ between Mac/Linux/Windows.

Solutions:

1. Use Docker (recommended):

# Run tests in consistent environment
docker run --rm -v $(pwd):/app mcr.microsoft.com/playwright:v1.40.0 \
  npx playwright test --update-snapshots

1. Platform-specific baselines:

// playwright.config.ts
export default defineConfig({
  snapshotPathTemplate: '{testDir}/__screenshots__/{platform}/{testFilePath}/{arg}{ext}',
});

1. Increase threshold:

await expect(page).toHaveScreenshot('page.png', {
  maxDiffPixelRatio: 0.05,  // 5% tolerance
});

Comparison Matrix

Best Practices

1. Stable Selectors

// Wait for dynamic content
await page.waitForSelector('[data-loaded="true"]');
await page.waitForLoadState('networkidle');

// Hide dynamic elements
await page.evaluate(() => {
  document.querySelectorAll('.timestamp').forEach(el => el.style.visibility = 'hidden');
});

await expect(page).toHaveScreenshot('page.png');

2. Consistent Viewport

// playwright.config.ts
export default defineConfig({
  use: {
    viewport: { width: 1280, height: 720 },
  },
});

// Or per-test
test('mobile view', async ({ page }) => {
  await page.setViewportSize({ width: 375, height: 667 });
  await expect(page).toHaveScreenshot('mobile.png');
});

3. Mock Time/Data

// Freeze time
await page.addInitScript(() => {
  Date.now = () => new Date('2024-01-15T10:00:00').getTime();
});

// Mock API responses
await page.route('**/api/user', route => {
  route.fulfill({
    json: { name: 'Test User', avatar: '/default-avatar.png' },
  });
});

await expect(page).toHaveScreenshot('profile.png');

4. Component-Level Testing

// Test individual components
test('button states', async ({ page }) => {
  await page.goto('/storybook/button');

  const button = page.locator('.button');

  // Default state
  await expect(button).toHaveScreenshot('button-default.png');

  // Hover state
  await button.hover();
  await expect(button).toHaveScreenshot('button-hover.png');

  // Active state
  await button.click({ force: true });
  await expect(button).toHaveScreenshot('button-active.png');
});

Workflow: Visual Testing in CI

PR Workflow

1. Developer pushes code
   ↓
2. CI runs visual tests
   ↓
3. Chromatic detects changes
   ↓
4. Review visual diff in Chromatic UI
   ↓
5. Approve → New baseline
   Reject → Fix and re-push
   ↓
6. Merge PR

Baseline Management

# Update baselines locally
npx playwright test --update-snapshots

# Commit baselines to git
git add tests/__screenshots__/
git commit -m "Update visual baselines"

# Or use Chromatic (baselines stored in cloud)
npx chromatic --auto-accept-changes  # Accept all changes

References

• references/chromatic-setup.md - Complete Chromatic configuration
• references/lost-pixel-self-hosted.md - Self-hosted setup guide
• references/playwright-snapshots.md - Built-in snapshot testing

Visual testing catches UI regressions before users do - choose Chromatic for teams, Lost Pixel for confidential projects.