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January 17, 2021 10:54
June 11, 2022 20:53

Crunch PNG file optimizationCrunch PNG file optimization Crunch PNG file optimization

GitHub release Linux Python CI macOS Python CI Lints Benchmarks


Crunch is a tool for lossy PNG image file optimization. It combines selective bit depth, color type, and color palette reduction with zopfli DEFLATE compression algorithm encoding using the pngquant and zopflipng PNG optimization tools. This approach leads to a significant file size gain relative to lossless approaches at the expense of a relatively modest decrease in image quality (see example images below).

Continuous benchmark testing is available in our GitHub Actions CI. Please see the benchmarks directory of this repository for details about the benchmarking approach and instructions on how to execute benchmarks locally on the reference images distributed in this repository or with your own image files.

Crunch PNG image optimization is available through the following applications that are distributed in this repository:

  • crunch - a *nix command line executable that can be used on macOS, Linux, and Windows POSIX application deployment environments such as Cygwin or the Windows subsystem for Linux
  • Crunch GUI - a native macOS drag and drop GUI tool
  • Crunch Image(s) service - a macOS right-click menu service for PNG images selected in the Finder

Installation and Usage

Installation and usage documentation links for each of the Crunch applications are available below.

crunch Command Line Executable

The crunch command line executable can be installed with make or manually. Please see the Install documentation.

Enter paths to your PNG image files as arguments to the crunch executable. Please see the Usage documentation.

Crunch macOS GUI Application

The Crunch native macOS GUI application can be installed with Homebrew or the dmg installer that can be downloaded from the repository releases. Please see the Install documentation.

Drag and drop one or more images on the application window to process your PNG files. Please see the Usage documentation.

Crunch Image(s) macOS Right-Click Menu Service

The macOS right-click menu service "Crunch Image(s)" can be installed with make or manually by drag and drop in the macOS Finder. Please see the Install documentation.

Select one or more PNG images in the Finder, right-click, and select the Services > Crunch Image(s) menu item to process your files. Please see the Usage documentation.



The following examples demonstrate the benefits and disadvantages of the current iteration of Crunch's aggressive space saving optimization strategy. The optimized image files are updated at every Crunch release. In many cases, the PNG optimization decreases file size with an imperceptible impact on image quality. In some cases, degradation of image quality is visible. Visual confirmation of image quality is highly recommended with lossy optimization tools in production settings.

Photography Examples

Cat Image

  • Original Size: 583,398 bytes
  • Optimized Size: 196,085 bytes
  • DSSIM similarity score: 0.001383
  • Percent original size: 33.61%

cat example pre optimization


cat example post optimization

Sun's Rays

  • Original Size: 138,272
  • Optimized Size: 66,593
  • DSSIM similarity score: 0.000920
  • Percent original size: 48.16%

sun rays example pre optimization


sun rays example pre optimization

Prairie Image

  • Original Size: 196,794 bytes
  • Optimized Size: 77,965 bytes
  • DSSIM similarity score: 0.002923
  • Percent original size: 39.62%

prarie example pre optimization


prarie example post optimization

Illustration Examples

Robot Image

  • Original Size: 197,193 bytes
  • Optimized Size: 67,596 bytes
  • DSSIM similarity score: 0.003047
  • Percent original size: 34.28%

robot example pre optimization


robot example post optimization

Color Circle Image

  • Original Size: 249,251 bytes
  • Optimized Size: 67,135 bytes
  • DSSIM similarity score: 0.002450
  • Percent original size: 26.93%

colors example pre optimization


colors example post optimization

Flowers Image

  • Original Size: 440,126 bytes
  • Optimized Size: 196,962 bytes
  • DSSIM similarity score: 0.001013
  • Percent original size: 44.75%




All images above were obtained from Pixabay and are dedicated to the public domain under the CC0 Public Domain Dedication.

DSSIM testing was performed with v2.10.0 of the kornelski/dssim tool.

Issue Reporting

Have you identified a problem? Please create a new issue report on the Github issue tracker so that we can address it.


Crunch is licensed under the MIT license.

Embedded Software

pngquant is licensed under the Gnu General Public License, version 3. The pngquant source code is available here.

zopflipng is licensed under the Apache License, Version 2.0. The upstream zopflipng source code is available here. The source for the modified zopflipng fork that is used in this project is available here.

See the document for details and additional licensing information for this project.


Contributions to the project are warmly welcomed. Please suggest enhancements as new issue reports on this repository. Source contributors should fork the git repository and submit changes as a Github pull request.


Crunch is a simple tool that makes excellent, free, open source software built by others more accessible. The hard work on the optimization engines that run Crunch has been performed by:

The fantastic macOS GUI animations were designed by Gary Jacobs.