iqa

iqa provides a single, ergonomic API over the patchwork of visual quality assessment metrics available in the Rust ecosystem. It wraps existing crates where they exist and fills in the gaps where they don't, so you can compute PSNR, SSIMULACRA2, and friends without juggling a different type, color space, and pixel format for each one.

Installation

cargo add iqa

This pulls in every metric. Some (such as ssimulacra2) compile vendored C/C++ and therefore need a C++ toolchain — but no system libraries: lcms2 is vendored and built from source by default. See Cargo features for the details. For a pure-Rust build with no C/C++ toolchain at all, disable the defaults and take just the metrics you need:

[dependencies]
iqa = { version = "0.1", default-features = false, features = ["psnr"] }

Quick start

iqa consumes a tightly packed, row-major sample buffer and deliberately leaves image decoding to you (here, the image crate):

use iqa::{Image, PsnrOptions, SsimOptions};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Decode however you like, then hand iqa the raw samples.
    let reference = image::open("reference.png")?.to_rgb8();
    let distorted = image::open("distorted.jpg")?.to_rgb8();

    let (w, h) = reference.dimensions();
    let reference = Image::srgb8(w, h, reference.into_raw())?;
    let (w, h) = distorted.dimensions();
    let distorted = Image::srgb8(w, h, distorted.into_raw())?;

    // PSNR — pure Rust, always available.
    let psnr = iqa::psnr(&reference, &distorted, PsnrOptions::default())?;
    println!("PSNR:        {psnr:.3} dB");

    // SSIM — pure Rust; 1.0 = identical.
    let ssim = iqa::ssim(&reference, &distorted, SsimOptions::default())?;
    println!("SSIM:        {ssim:.3}");

    // DSSIM — pure Rust; (1 - SSIM)/2, so 0.0 = identical and lower is better.
    let dssim = iqa::dssim(&reference, &distorted, iqa::DssimOptions::default())?;
    println!("DSSIM:       {dssim:.3}");

    // SSIMULACRA2 — requires the `ssimulacra2` feature; 100 = identical.
    let ssimulacra2 = iqa::ssimulacra2(&reference, &distorted)?;
    println!("SSIMULACRA2: {ssimulacra2:.3}");

    // Butteraugli — requires the `butteraugli` feature; 0 = identical, lower is better.
    let butteraugli = iqa::butteraugli(&reference, &distorted, iqa::ButteraugliOptions::default())?;
    println!("Butteraugli: {butteraugli:.3}");

    Ok(())
}

Both inputs must share one pixel format, so a color-space, channel, or bit-depth mismatch is a compile error rather than a meaningless score. See examples/compare.rs for the full pipeline — run it with cargo run --release --example compare -- reference.png distorted.jpg.

Command-line interface

The companion iqa-cli crate is a thin front-end: it decodes two images with the image crate, computes the requested metrics, and prints them as a JSON object keyed by metric name. It is versioned and released in lockstep with iqa.

cargo install iqa-cli
iqa-cli --reference ref.png --distorted out.png --metric ssimulacra2,psnr,ssim,butteraugli
# -> {"butteraugli":0.83,"psnr":38.114,"ssim":0.992,"ssimulacra2":87.421}

Every metric in the iqa crate is exposed: psnr, ssim, dssim, ms-ssim, psnr-hvs-m, ciede2000, ssimulacra2, and butteraugli. With no --metric, every available metric is computed; --list-metrics prints the full set and each metric's direction. Non-finite scores (the PSNR of two pixel-identical images is +inf) are emitted as JSON null.

Metrics

The table below tracks the planned metric set — the nine-metric full-reference core that covers essentially every published JXL/AVIF codec comparison from the last two years, plus LPIPS as a learned reference metric.

The Implementation column points at the implementation iqa is built on. We prefer porting or binding the upstream source implementation over reusing a Rust-specific reimplementation: cross-compilation is a requirement, but portability beyond that is not, so staying close to the reference keeps results faithful.

*: LPIPS is Python implementation only.

†: DSSIM is implemented as the classic structural-dissimilarity transform (1 - SSIM) / 2 over the native SSIM (Wang et al. 2004). This is intentionally not kornelski/dssim's multi-scale L*a*b* metric: that implementation is AGPL-licensed and defined only by its source, so it cannot be reproduced under this crate's permissive (MIT OR Apache-2.0) license. No numeric parity with it is implied.

Status legend:

• Planned — selected for implementation, not yet started.
• Require testing — implemented, but not yet validated against reference outputs.
• Stable and tested — implemented and verified against the reference implementation.

SSIMULACRA2 is cross-validated numerically: tests/ssimulacra2_reference.rs checks iqa::ssimulacra2 against the scores the reference ssimulacra2 tool — built from the exact cloudinary source we vendor (SSIMULACRA 2.1) — produces on the same pixels, including a deliberately non-vector-aligned fixture that exercises the SIMD row-padding handling. scripts/gen-ssimulacra2-goldens.sh rebuilds those reference values from source. (The oracle must be 2.1: the metric was retuned from 2.0 in April 2023, so an older self-contained libjxl build would score the same pixels very differently.)

Butteraugli is cross-validated numerically: tests/butteraugli_reference.rs checks iqa::butteraugli against the exact distances libjxl's own butteraugli_main (v0.8.2, the vendored version) produces on the same pixels — matching to the reference tool's printed precision. scripts/gen-butteraugli-goldens.sh rebuilds those reference values from source.

MS-SSIM is cross-validated the same way: tests/ms_ssim_reference.rs pins iqa::msssim against Wang, Simoncelli & Bovik's original msssim.m run on the same grayscale fixtures (scripts/gen-msssim-goldens.sh, via Octave) — matching exactly — with an independent NumPy reimplementation (gen-msssim-goldens.py) as a no-Octave cross-check.

PSNR-HVS-M is cross-validated the same way: tests/psnr_hvs_m_reference.rs pins iqa::psnr_hvs_m against Ponomarenko's original psnrhvsm.m run on the same grayscale fixtures (scripts/gen-psnrhvs-goldens.sh, via Octave) — matching exactly — with an independent NumPy reimplementation (gen-psnrhvs-goldens.py) as a no-Octave cross-check.

Cargo features

Each metric is gated behind its own Cargo feature:

Every metric is enabled by default for convenience — cargo add iqa gets you the full set. Some metrics (such as ssimulacra2) bind native C/C++ code and therefore need a C++ toolchain and system libraries to build.

For leaner builds, disable the default features and opt into exactly the metrics you need:

[dependencies]
# Pure-Rust subset only — no C/C++ toolchain required.
iqa = { version = "0.1", default-features = false, features = ["psnr"] }

Building the C++ metrics (ssimulacra2, butteraugli)

SSIMULACRA2 and Butteraugli are bound via FFI to their original libjxl-derived C++ rather than reimplemented, and share one native build. Enabling either (including via the default feature set) needs a native build environment:

1. A C++ toolchain. build.rs compiles the reference with the cc crate, which uses c++ by default (override with the CXX environment variable).

That is the only requirement. libjxl's color management depends on lcms2, which iqa vendors and builds from source by default (the vendored-lcms2 feature) — there is no system library to install.

If you would rather link a system lcms2 (for example, as a distro packager), turn the vendored feature off and enable system-lcms2, which locates the library via pkg-config:

[dependencies]
iqa = { version = "0.1", default-features = false, features = ["psnr", "ssim", "ssimulacra2", "system-lcms2"] }

sudo apt install liblcms2-dev   # Debian / Ubuntu
brew install little-cms2        # macOS / Homebrew

When you depend on iqa from crates.io the vendored C/C++ sources (including lcms2) are packaged inside the published crate, so there are no submodules to fetch.

Building from a git checkout (contributors) additionally needs those sources, which live under third_party/ as git submodules (ssimulacra2, highway, and lcms2):

git submodule update --init --recursive

Butteraugli's extra libjxl sources are hand-vendored directly under third_party/butteraugli/ (see its README), so they need no submodule fetch.

Either way, build or test with the feature enabled:

cargo test --features ssimulacra2,butteraugli

Development

It is important that development velocity is maintained regardless of project complexity. Unit tests for all contributions are expected, especially for platform-specific behaviours!

Setup

Install lefthook and activate the pre-commit and pre-push hooks:

# macOS / Homebrew
brew install lefthook

# Linux (Homebrew on Linux)
brew install lefthook

# via cargo
cargo install lefthook

# then install the hooks
lefthook install

The pre-commit hook runs cargo fmt --check and cargo clippy. The pre-push hook runs the full test suite.

Checking size

just size builds the library under every unique feature combination and prints the compiled footprint of each, plus the published-crate size. It also enforces that the C++ FFI compiles away cleanly: every pure-Rust combo must link zero native code and pull neither cc nor pkg-config into its build graph. A violation exits non-zero. The same check runs weekly (and on any PR that touches Cargo.toml, build.rs, third_party/, or the script) via the Size workflow, which posts the size table to the run summary.

Releasing

Releases are automated with release-plz and driven by Conventional Commits: the commit messages landed on master decide the next version number and fill in CHANGELOG.md.

How release-plz picks the next version

release-plz scans every commit landed since the last release tag, maps each to a bump from its Conventional Commit type, and applies the largest bump any one of them implies. The version in the open release PR therefore reflects everything accumulated on master since the last release, and rises as more commits land.

While the crate is pre-1.0 (0.y.z) the bumps are deliberately conservative — following Cargo's compatibility rules, the minor slot plays the role of "major", so a breaking change moves 0.1.z → 0.2.0 rather than 1.0.0:

The practical consequence: while the crate is still 0.1.z, a plain feat: does not reach 0.2.0 — only a breaking-change commit (feat!: or a BREAKING CHANGE: footer) bumps the minor. This is why 0.1.1 shipped three new metrics as a patch rather than a minor release.

To cut a release:

1. Land changes on master with Conventional Commit messages (feat:, fix:, refactor!:, ...).
2. release-plz maintains an open release PR that bumps the version in Cargo.toml, updates CHANGELOG.md, and refreshes Cargo.lock. Review it as you would any other PR.
3. Merge the release PR. Merging it is the whole release: release-plz publishes the crate to crates.io, pushes a vX.Y.Z git tag, and creates the matching GitHub release.

Publishing authenticates with crates.io trusted publishing over OIDC, so no registry token is stored in the repository. The workflow lives in .github/workflows/release-plz.yml.

0.1.0 was published by hand to bootstrap the crate — trusted publishing can only be configured once a crate already exists on crates.io. Every release after it is fully automated by merging the release PR.