OptiPNG: Advanced PNG Optimizer

Current version: OptiPNG 0.4.7 (released 30 Oct 2004)
Download the source code: optipng-0.4.7.tar.gz, optipng-0.4.7.zip
Download the Windows executable: optipng-0.4.7-exe.zip

What's new (see HISTORY for the complete list of changes):

• Fixed reduction bug
  Fixed a palette-to-gray reduction problem that occurred when having more than one alpha value per RGB triple.
• Improved speed
  Reduced the IDAT buffer size, allowing abandoned trials to terminate earlier in some circumstances.
• Improved new error recovery feature
  Fixed image when PLTE is too small, or tRNS is too large.

Previous version: OptiPNG 0.4.6 (released 25 Oct 2004)
Download the source code: optipng-0.4.6.tar.gz, optipng-0.4.6.zip
Download the Windows executable: optipng-0.4.6-exe.zip

What's new:

• Improved security
  Upgraded zlib and libpng to the latest, most secure versions.
• Improved speed
  Trials are ended prematurely, in the moment when IDAT grows beyond the smallest size previously achieved. This has no effect on the output size.

OptiPNG is a PNG optimizer that recompresses the image files to a smaller size, without losing any information.
The idea has been inspired from pngcrush (http://pmt.sourceforge.net/pngcrush), and is explained in detail in the PNG-Tech article "A guide to PNG optimization". The implementation is carried forward in OptiPNG, which offers a faster execution per trial, and a wider search space.

The documentation is included in the downloadable archives, and it is also available here:

• LICENSE
  OptiPNG is open-source software, released under the zlib/libpng license.
• FEATURES
  The complete list of features.
• USAGE
  Program usage.
• CAVEAT
  We cannot, in good conscience, fail to inform the users about the known bugs, problems, or limitations. Caveat Emptore!
• DESIGN
  Internal documentation: design decisions and rationale.
• HISTORY
  The program revision history.
• TODO
  The "to-do" list.

NOTE:
The OptiPNG source distribution contains modified versions of zlib-1.2.2 and libpng-1.0.17. Still, OptiPNG works with the original libraries as well. These libraries are available at their home sites:

• zlib at http://www.zlib.net/
  A notable change: the constant TOO_FAR is set to 32768, for a slight compression improvement; see the PNG-Tech article "TOO_FAR in zlib is not too far...".
• libpng at http://www.libpng.org/pub/png/libpng.html
  The modified version provides a tiny speed improvement. In addition, the unused features are disabled and compiled out, allowing a smaller executable.

OptiPNG Plus

OptiPNG Plus is an enhanced version that provides additional features. This version is not publicly available.