Library for further (lossless) Jpeg-compression

Question

I'm looking for a library specialized at compressing Jpegs even further without changing the image. I've found PackJpeg but it doesn't provide any source code and you're not allowed to use it commercially.

Answer 1

On the PackJPG page, there's a link to the JPEG Open Source Developers Package. It doesn't include PackJPG source code, but basic routines to process and compress JPG data. This has been used to develop rejpeg which is open source and licensed under GPL v2.

Matthias Stirner has made an announcement some days ago that says he is going to make PackJPG open source and is thinking about a dual license (GPL + special permissions for closed source developers).

There is PAQ (open source, GPL license) from Matt Mahoney in several variants that implement JPEG compression (doesn't support progressive images, though). On his page, there's a table comparing different programs.

For a completely free approach, you can use libzpaq together with the JPG compression configuration file.

Quoting the libzpaq license:

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so without restriction. This Software is provided "as is" without warranty.

Answer 2

In researching this same problem, I ran across JPEGmini, which is available as an online service. The claim:

JPEGmini is capable of reducing the file size of standard JPEG photos by up to 80% (5X), while the resulting photos are visually identical to the original photos.

I took the jpg/jpeg lossless image compression test A10 image down from 823KB to 171KB.