oxipng

oxipng is a multithreaded lossless PNG optimizer. It reduces the file size of PNG images by trying different compression strategies and filter combinations, keeping whichever produces the smallest output — with no change to the image's appearance whatsoever.

It is a Rust reimplementation and significant improvement over the classic optipng tool, with parallelism, better compression ratios, and support for modern PNG features like stripping unnecessary metadata.

Features

• Lossless — pixel data is never altered; only the compressed representation is optimised
• Multithreaded — processes multiple files in parallel and tries compression strategies concurrently
• Better than optipng — consistently produces smaller files than its C predecessor
• Metadata stripping — removes unnecessary chunks (Exif, ICC profiles, text comments) that bloat file size
• Interlace control — convert between interlaced and non-interlaced PNG
• Transparency optimisation — can fix "dirty transparent" pixels to improve compressibility
• Bit depth reduction — optionally reduces colour depth when the image content allows it
• Preserve or strip colour profiles — fine-grained control over which PNG chunks to keep

Installation

cargo install oxipng

Or via your system package manager:

# Debian / Ubuntu
apt install oxipng

# Fedora
dnf install oxipng

# Arch Linux
pacman -S oxipng

# macOS
brew install oxipng

# Nix
nix-env -iA nixpkgs.oxipng

Usage

# Optimise a single file in-place
oxipng image.png

# Optimise multiple files
oxipng *.png

# Optimise recursively in a directory
oxipng --recursive images/

# Set the optimisation level (0–6, default is 2; higher = smaller but slower)
oxipng -o 4 image.png

# Maximum compression (equivalent to -o 6 with extra options — slow but thorough)
oxipng -o max image.png

# Strip all non-critical metadata chunks (recommended for web)
oxipng --strip all image.png

# Strip only safe metadata (preserves colour profile)
oxipng --strip safe image.png

# Write output to a new file instead of overwriting
oxipng image.png --out optimised.png

# Preview how much would be saved without modifying files
oxipng --pretend image.png

# Optimise and fix dirty transparent pixels
oxipng --alpha image.png

Optimisation levels

For most workflows, -o 4 --strip safe is a good balance between compression and speed. For a one-time batch optimisation of an image library, -o max --strip all is worth the wait.

Typical savings

On a typical set of PNG screenshots or UI assets, oxipng at -o 4 commonly achieves 10–30% size reduction with zero visual difference. Files that were already processed by other optimisers can still often be reduced further.

CI / build pipeline integration

oxipng is well-suited to running in CI to keep image assets lean:

# Fail if any PNG can be reduced by more than 1%
oxipng --pretend --quiet images/*.png

Or as a pre-commit step to automatically optimise any PNG added to the repository.