pixelhog 1.0.0

Rust-accelerated pixelmatch and SSIM for PNG bytes

pixelhog

Fast visual regression primitives for Python, implemented in Rust.

pixelhog compares screenshots in two complementary ways:

• diff: exact pixel-level differences (with anti-alias handling), optional diff image output
• ssim: perceptual similarity score in [0.0, 1.0]

Install (local dev)

uv venv .venv --python 3.12
source .venv/bin/activate
uv pip install -U pip maturin
maturin develop --release

Quickstart

from pixelhog import diff, ssim, compare

# 1) Pixel-level diff + diff PNG
# returns: (diff_png_bytes, diff_count, width, height)
diff_png, diff_count, width, height = diff(baseline_png, current_png)

# 2) Perceptual similarity
score = ssim(baseline_png, current_png)

# 3) One-call gate (count + SSIM), no diff image encoding overhead
# returns: (diff_count, ssim, width, height, maybe_diff_png)
diff_count, score, width, height, _ = compare(
    baseline_png,
    current_png,
    return_diff=False,
)

API at a glance

Function	Input	Output	Use when
diff	PNG bytes	(diff_png, diff_count, width, height)	You need a visual diff artifact
diff_count	PNG bytes	(diff_count, width, height)	You only need the mismatch count
ssim	PNG bytes	float	You need perceptual similarity
compare	PNG bytes	(diff_count, ssim, width, height, optional_diff_png)	You want both metrics in one call
diff_rgba	RGBA bytes + sizes	(diff_rgba, diff_count, width, height)	You already decoded images in Python/Rust
diff_count_rgba	RGBA bytes + sizes	(diff_count, width, height)	Count-only on pre-decoded buffers
ssim_rgba	RGBA bytes + sizes	float	SSIM on pre-decoded buffers
compare_rgba	RGBA bytes + sizes	(diff_count, ssim, width, height, optional_diff_rgba)	Combined metrics on pre-decoded buffers
diff_batch	list[(baseline_png, current_png)]	list[diff result]	Run many diffs in one call
diff_count_batch	list[(baseline_png, current_png)]	list[count result]	Batch count-only checks
ssim_batch	list[(baseline_png, current_png)]	list[float]	Batch SSIM checks
compare_batch	list[(baseline_png, current_png)]	list[compare result]	Batch combined checks

Behavior

• High-level APIs accept PNG bytes and decode internally.
• Smaller images are padded to the larger dimensions with transparent pixels.
• diff always generates a diff image.
• diff_count and compare(..., return_diff=False) skip diff-image generation.
• SSIM uses 11x11 uniform windows with reflect padding.
• For images smaller than 11x11, SSIM falls back to global SSIM.
• No SSIM visualization image is produced.

Correctness and tests

The test suite is designed to validate both algorithm fidelity and practical product behavior.

• Rust unit/integration tests cover:
  • identical/completely different/partial-diff images
  • threshold behavior
  • different-size padding behavior
  • SSIM behavior (identical, slight change, large change, small-image fallback)
• Canonical pixelmatch fixture tests use the official Mapbox test set:
  • 8 fixture pairs with exact expected mismatch counts
  • expected diff image comparison against golden outputs
  • decoded RGBA byte equality checks to ensure pixel-perfect output matching
• Python integration tests cover:
  • high-level API contracts and error behavior
  • tall-page and subtle-change scenarios
  • cross-validation against a pure-Python reference implementation
    • pixel diff counts must match exactly
    • SSIM must stay within tolerance

Run the full correctness suite:

# Rust core only
cargo test -p pixelhog

# Full suite including Python integration tests
cargo test
uv run --python 3.12 --with maturin --with pytest --with pillow bash -lc \
  "maturin develop --release && pytest -q"

Benchmarks

The repo includes both Criterion benches and pipeline breakdown tools.

• cargo bench runs Criterion API benchmarks (PNG-bytes entry points).
• Breakdown binaries in examples/ measure where time goes:
  • breakdown.rs: decode vs core diff vs encode vs API call
  • ssim_breakdown.rs: decode/pad vs core SSIM vs API call
  • combined_estimate.rs: separate calls vs combined single-decode flow

Run:

cargo bench -p pixelhog
cargo run -p pixelhog --release --example breakdown
cargo run -p pixelhog --release --example ssim_breakdown
cargo run -p pixelhog --release --example combined_estimate

For screenshot-style workloads, the practical guidance is:

• diff_count is cheaper than diff when you do not need an artifact.
• compare(..., return_diff=False) avoids duplicate decode work when you need both diff-count and SSIM.

Development

# Rust tests (includes canonical Mapbox fixture tests)
cargo test

# Python extension + tests
uv run --python 3.12 --with maturin --with pytest --with pillow bash -lc \
  "maturin develop --release && pytest -q"

# Lint/format/type-check
uv run --python 3.12 --with ruff ruff format --check .
uv run --python 3.12 --with ruff ruff check .
uv run --python 3.12 --with ty --with pytest --with pillow ty check . --python .venv

License

This repository is MIT licensed. See LICENSE.

Algorithm attribution for pixelmatch is documented in THIRD_PARTY_NOTICES.md.