fast-ssim2

Rust implementation of the SSIMULACRA2 perceptual image quality metric.

Attribution

This crate is a fork of rust-av/ssimulacra2. Thank you to the rust-av team for creating the original Rust implementation - their clean, well-structured code made it straightforward to build upon.

What's different

The upstream crate provides a correct, readable scalar implementation. This fork experiments with SIMD acceleration and adds some API conveniences:

• SIMD-accelerated blur and XYB conversion (via wide crate and x86 intrinsics)
• Precomputed reference API for batch comparisons (Ssimulacra2Reference)
• imgref type support via ToLinearRgb trait

These changes come with trade-offs: higher MSRV (1.89.0 vs 1.65.0), more complex code, and platform-specific behavior. If you prefer simplicity and stability, the upstream crate may be a better fit.

Contributing back

We'd be happy to upstream any of these changes if the rust-av team is interested. The SIMD work is substantial and may not align with their goals, but we're open to collaboration.

Benchmarks

Full SSIMULACRA2 computation (AMD Ryzen, x86_64):

Run your own:

cargo run --release --features "simd unsafe-simd" --example benchmark_unsafe_simd

Installation

[dependencies]
fast-ssim2 = "0.6"

Features

• simd (default): Safe SIMD via the wide crate
• unsafe-simd (default): x86_64 intrinsics (faster on supported hardware)
• imgref: Support for imgref image types
• rayon: Parallel computation

Usage

With imgref

use fast_ssim2::compute_ssimulacra2;
use imgref::ImgVec;

let source: ImgVec<[u8; 3]> = /* your image */;
let distorted: ImgVec<[u8; 3]> = /* distorted version */;

let score = compute_ssimulacra2(source.as_ref(), distorted.as_ref())?;
// 100 = identical, lower = more different

With yuvxyb types

use fast_ssim2::{compute_frame_ssimulacra2, Rgb, TransferCharacteristic, ColorPrimaries};

let source = Rgb::new(data, width, height,
    TransferCharacteristic::SRGB, ColorPrimaries::BT709)?;
let distorted = Rgb::new(/* ... */)?;

let score = compute_frame_ssimulacra2(source, distorted)?;

Precomputed reference (for batch comparisons)

use fast_ssim2::Ssimulacra2Reference;

let reference = Ssimulacra2Reference::new(source)?;

for distorted in variants {
    let score = reference.compare(distorted)?;
}

Score interpretation

License

BSD-2-Clause (same as upstream)

Notes

Developed with assistance from Claude (Anthropic). Tested against the C++ reference implementation.