# iqa
[](https://crates.io/crates/iqa)
[](https://docs.rs/iqa)
[](https://github.com/justin13888/iqa-rs/actions/workflows/ci.yml)
`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.
## Project status
`iqa` is in maintenance mode and public API will change only on a major release bump. Each metric is validated against its reference implementation before it is marked stable; see [Metrics](#metrics) for the per-metric status.
Every supported metric is **full-reference and image-focused** — there is no
video quality assessment (temporal metrics, VMAF, and the like are out of
scope). If you need a metric that isn't here yet, a GitHub issue or PR is very
welcome.
## Installation
```sh
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](#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:
```toml
[dependencies]
iqa = { version = "1.2", 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`](https://crates.io/crates/image)
crate):
```rust
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}");
// MS-SSIM — pure Rust; multi-scale SSIM, 1.0 = identical.
let msssim = iqa::msssim(&reference, &distorted, iqa::MsssimOptions::default())?;
println!("MS-SSIM: {msssim:.3}");
// IW-SSIM — pure Rust; information-weighted SSIM, 1.0 = identical.
let iwssim = iqa::iwssim(&reference, &distorted, iqa::IwssimOptions::default())?;
println!("IW-SSIM: {iwssim:.3}");
// PSNR-HVS-M — pure Rust; DCT-domain PSNR with HVS masking, higher is better.
let psnr_hvs_m = iqa::psnr_hvs_m(&reference, &distorted, iqa::PsnrHvsOptions::default())?;
println!("PSNR-HVS-M: {psnr_hvs_m:.3} dB");
// CIEDE2000 — pure Rust; mean ΔE₀₀ color difference, 0.0 = identical, lower is better.
let ciede2000 = iqa::ciede2000(&reference, &distorted, iqa::Ciede2000Options::default())?;
println!("CIEDE2000: {ciede2000:.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`](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`](https://crates.io/crates/iqa-cli) crate is a thin
front-end: it decodes two images with the
[`image`](https://crates.io/crates/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`.
```sh
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`,
`iw-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.
| SSIMULACRA2 | [cloudinary/ssimulacra2](https://github.com/cloudinary/ssimulacra2) | Stable and tested |
| Butteraugli | [libjxl](https://github.com/libjxl/libjxl) | Stable and tested |
| DSSIM | Native implementation† | Require testing |
| PSNR | Native implementation | Stable and tested |
| PSNR-HVS-M | [Ponomarenko `psnrhvsm.m`](https://www.ponomarenko.info/psnrhvsm.htm) | Stable and tested |
| SSIM | Native implementation | Stable and tested |
| MS-SSIM | [Wang et al. `msssim.m`](https://ece.uwaterloo.ca/~z70wang/research/ssim/) | Stable and tested |
| IW-SSIM | [Wang & Li `iwssim.m`](https://ece.uwaterloo.ca/~z70wang/research/iwssim/) | Stable and tested |
| CIEDE2000 | Native implementation | Stable and tested |
| LPIPS | [richzhang/PerceptualSimilarity](https://github.com/richzhang/PerceptualSimilarity) | Not planned* |
*: 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](https://github.com/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.
IW-SSIM is cross-validated the same way: `tests/iw_ssim_reference.rs` pins
`iqa::iwssim` against Wang & Li's original `iwssim.m` (atop Simoncelli's
matlabPyrTools) run on the same grayscale fixtures
(`scripts/gen-iwssim-goldens.sh`, via Octave) — matching exactly — with an
independent NumPy reimplementation that reuses the authors' `pyrtools` Laplacian
pyramid (`gen-iwssim-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.
PSNR has a closed form, so it is pinned to the definition itself rather than to
an external tool: `tests/psnr.rs` checks `iqa::psnr` against
`10·log10(MAX²/MSE)` for hand-computed errors at both bit depths, and pins the
Rec.709 luma weights (`0.2126 / 0.7152 / 0.0722`) by distorting every channel at
once. A closed form is an exact oracle, and the suite's *completeness* is proven
by mutation testing — `just mutants-psnr` reports every mutant of `src/psnr.rs`
caught — so a green run leaves no untested path. See
[Mutation testing](#mutation-testing).
SSIM has no simple closed form once the Gaussian window sees structure, so it is
pinned by an **independent second implementation** rather than an external tool:
`tests/ssim_reference.rs` re-derives mean SSIM from the textbook definition —
building the window from `σ = 1.5` and taking `K1`, `K2`, and the 11×11 window
from the spec — and checks `iqa::ssim` against it on structured fixtures, at a
single-window size and a multi-window size, in both modes and at both bit depths.
That covers the variance, covariance, and window-shape terms that the uniform
closed-form test leaves untouched, and `just mutants-ssim` confirms every mutant
of `src/ssim.rs` is caught.
## Cargo features
Each metric is gated behind its own Cargo feature:
| `psnr` | yes | Native Rust; no system dependencies. |
| `ssim` | yes | Native Rust; no system dependencies. |
| `dssim` | yes | Native Rust; structural dissimilarity `(1 - SSIM) / 2`. Enables `ssim`. |
| `ms-ssim` | yes | Native Rust; multi-scale SSIM over an image pyramid. Enables `ssim`. |
| `iw-ssim` | yes | Native Rust; information content weighted SSIM over a Laplacian pyramid. Enables `ssim`. |
| `psnr-hvs-m` | yes | Native Rust; DCT-domain PSNR with a CSF and contrast masking. |
| `ciede2000` | yes | Native Rust; CIEDE2000 (ΔE₀₀) mean color difference over sRGB→CIELAB (D65). |
| `ssimulacra2` | yes | Binds the vendored C++ reference; see the build requirements below. |
| `butteraugli` | yes | Binds vendored libjxl C++; shares the same native build as `ssimulacra2`. |
| `vendored-lcms2` | yes | Builds the `lcms2` dependency from vendored source — no system lib needed. Mutually exclusive with `system-lcms2`. |
| `system-lcms2` | no | Links a system `lcms2` via `pkg-config` instead. Mutually exclusive with `vendored-lcms2` — enable exactly one. |
**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:
```toml
[dependencies]
# Pure-Rust subset only — no C/C++ toolchain required.
iqa = { version = "1.2", 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`:
```toml
[dependencies]
iqa = { version = "1.2", default-features = false, features = ["psnr", "ssim", "ssimulacra2", "system-lcms2"] }
```
```sh
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`):
```sh
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:
```sh
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](https://github.com/evilmartians/lefthook) and activate the pre-commit and pre-push hooks:
```sh
# 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.
### Mutation testing
The metric test suites are validated for *completeness* with
[`cargo-mutants`](https://github.com/sourcefrog/cargo-mutants): it compiles many
deliberately broken copies of a metric and checks the suite rejects each one. A
surviving (`MISSED`) mutant is a hole — a line the tests do not actually pin. The
bar is **zero survivors**, except the genuinely equivalent mutants documented
with their justification in `.cargo/mutants.toml`.
```sh
cargo install cargo-mutants # once
just mutants # PSNR, SSIM, DSSIM
just mutants-sweep # the remaining metrics (slow: rebuilds vendored C/C++)
```
Features are scoped per metric because the crate cannot build with
`--all-features` (the `lcms2` backends are mutually exclusive). `cargo-mutants`
is a developer tool, run on demand rather than in CI, and is not a crate
dependency, so it has no effect on the published crate.
### Releasing
Releases are automated with [release-plz](https://release-plz.dev) and driven by
[Conventional Commits](https://www.conventionalcommits.org): 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.
Now that the API is finalized the crate is `≥ 1.0.0`, so the standard SemVer
mapping applies — a breaking change bumps the *major* slot:
| `feat!:` / `fix!:` / any `BREAKING CHANGE:` | major (`1.2.0 → 2.0.0`) |
| `feat:` | minor (`1.2.0 → 1.3.0`) |
| `fix:` | patch (`1.2.0 → 1.2.1`) |
| `docs:`, `chore:`, `test:`, `refactor:`, … | patch (`1.2.0 → 1.2.1`) |
The `1.0.0` major bump was a deliberate one-time promotion (the `0.x → 1.x` step
is never produced by Conventional Commits alone, since under Cargo's pre-1.0
rules the *minor* slot played the role of "major"). The minor was carried over
unchanged, so `0.2.0` became `1.2.0`.
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](https://crates.io/docs/trusted-publishing) over OIDC, so no
registry token is stored in the repository. The workflow lives in
[`.github/workflows/release-plz.yml`](.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.