Synthetic fixtures

Synthetic is the first choice: it controls the property under test, reproduces exactly, and keeps private material out of commits and screenshots.

Contract

• Name the property and failure the fixture is meant to exercise before generating it.
• Seed every stochastic generator and pin material tool versions when output bytes matter.
• Store the generator beside the fixture or generate during tests. Record the command, seed, dimensions/duration/schema, SHA-256, and expected assertions.
• Verify properties programmatically: image mode/profile/EXIF/orientation, audio codec/rate/channels/loudness, video codec/rate/timestamps, or data encoding/schema. A file that merely opens is not a valid fixture.
• Include edge fixtures deliberately: empty, one-unit, maximum practical size, malformed, unusual Unicode, mismatched metadata, and boundary values relevant to the code.
• Strip accidental metadata and inspect generated files before committing them.

Use a project fixtures/ directory and descriptive names such as gradient-p3-16bit.png or silence-mono-48k-3s.wav.

Recipes

• Images: Pillow, ImageMagick, or SVG for exact test cards, alpha ramps, color casts, masks, resolution wedges, and seeded noise.
• Audio: SoX or FFmpeg for tones, sweeps, silence, clipped samples, seeded noise, channel layouts, and sample-rate mismatches.
• Video: FFmpeg test sources with burned-in frame numbers for sync, rate, color, and timestamp tests.
• Structured data: small seeded generators covering empty values, normalization forms, emoji, long names, case collisions, and invalid records.

When synthetic media cannot represent the required real-world messiness, ask for approved material or use clearly licensed public material. Store source URL, author, license, retrieval date, and any transformations. Do not assume that visible-on-the-web means reusable.

Pair every durable fixture with a verifier that fails when the property drifts. See file-batch-safety before generating or renaming a large fixture corpus.