Conformance

Test strategy

edgesharp is tested at three levels to ensure parity with Next.js image optimization.

1. Visual conformance (vs Sharp)

Every resize operation is compared pixel-for-pixel against Sharp (the engine Vercel/Next.js uses). Tests use PSNR (Peak Signal-to-Noise Ratio) with a threshold of 30 dB — below which differences become visible.

What’s tested:

• JPEG decode + Lanczos3 resize at multiple widths (320, 640, 1080px)
• PNG decode + Lanczos3 resize at multiple widths (64, 256, 384px)
• Aspect ratio preservation
• Quality scaling (lower quality = smaller file)
• Edge cases: 1×1 pixel, panorama (10:1), portrait (1:10), grayscale, transparency
• PNG compression ratio within 2x of Sharp

Alpha handling: PSNR is weighted by alpha channel. Fully transparent pixels are skipped since their RGB values are meaningless — different renderers produce different RGB for alpha=0 pixels.

2. Protocol conformance (vs Next.js)

HTTP API tests verify edgesharp matches Next.js’s /_next/image endpoint behavior:

Parameter validation:

• Missing/invalid url → 400
• Protocol-relative URLs (//evil.com) → 400
• Backslash URLs (/\evil.com) → 400
• Absolute URLs (https://...) → 400
• Scheme injection (data:, javascript:) → 400
• URL length > 3072 chars → 400
• Width not in allowed sizes → 400
• Width > 3840 → 400
• Quality outside 1-100 → 400

Content negotiation:

• Accept header parsing matches Next.js priority (AVIF > WebP > JPEG)
• Format falls back gracefully when backend doesn’t support requested format

Security headers:

• Cache-Control: public, max-age=31536000, immutable
• Vary: Accept
• Content-Security-Policy: script-src 'none'; frame-src 'none'; sandbox;
• X-Content-Type-Options: nosniff

Content type safety:

• Safe types (JPEG, PNG, WebP, etc.) are accepted
• Unsafe types (SVG, HTML, JS, PDF) are rejected

3. Size conformance (vs Next.js defaults)

Verifies that edgesharp uses the exact same width breakpoints as Next.js:

• deviceSizes: [640, 750, 828, 1080, 1200, 1920, 2048, 3840]
• imageSizes: [16, 32, 48, 64, 96, 128, 256, 384]
• No overlap between the two sets
• BlurDataURL width = 8px

Running tests

# Visual + sizes (no server needed)
npx vitest run tests/conformance/visual.test.ts tests/conformance/sizes.test.ts

# Protocol (requires wrangler dev running)
IMAGEMODE_TEST_URL=http://localhost:8788 npx vitest run tests/conformance/protocol.test.ts

Bugs found by conformance tests

The conformance suite caught and helped fix three significant bugs during development:

1. JPEG IDCT scaling — custom integer IDCT had wrong constants (PSNR was 12 dB, should be >= 30). Fixed by replacing with reference floating-point IDCT.

2. Alpha premultiplication — resize without premultiplied alpha caused color fringing at transparent edges (PSNR 13 dB on icons). Fixed by premultiplying before resize, unpremultiplying after.

3. PNG compression — Zig’s stdlib flate.Compress was incomplete when this codebase was first written. Fixed by statically linking miniz (public domain C library); kept on Zig 0.16 for consistent output and smaller binary.