An excerpt, from the wiki page:
StuffIt Image Format (SIF)
"
Early in 2005, a new JPEG compression system was released that regularly obtained compression in the order of 25% (meaning a compressed file size 75% of the original file size) without any further loss of image quality and with the ability to rebuild the original file, not just the original image. (ZIP-like programs typically achieve JPEG compression rates in the order of 1 to 3%. Programs that optimize JPEGs without regard for the original file, only the original image, obtain compression rates from 3 to 10% (depending on the efficiency of the original JPEG). Programs that use the rarely implemented arithmetic coding option available to the JPEG standard typically achieve rates around 12%.)
The new technique was implemented as a StuffIt X format option in their StuffIt Deluxe product. They have also proposed a new image format known as SIF, which simply consists of a single JPEG file compressed using this new technique.
Pending filing of their patent, they retain knowledge of the details of this algorithm as a trade secret. Some details are disclosed in: the high JPEG recompression is achieved by undoing the last step of the JPEG compression itself (the Huffman encoding of quantized transform coefficients). Instead, the transform coefficients are compressed by a more efficient algorithm (a predictive model based on the DC coefficients of neighboring blocks). Similar techniques are also applied for other image file formats such as GIF and TIFF and even the MP3 music file format. By means of decomposition, the relatively high compression rates for individual file formats can also be achieved for container file formats such as PDF, PSD and even ZIP."
Attached below is an archive containing, what I believe to be all the appropriate resources needed to create an StuffIt archive, a sample jpeg and a simple batch script to test packing. I haven't experimented with every available switch, so don't take the sample script as being the most optimal method of packing. Theres a plethora of other resources that are created when installing this software but I'm pretty confident that, at least in testing only these files are needed to create a StuffIt archive.
Preliminary tests have shown uninmpressive results, in contrast to a typical compression chain of srep & lolz
Code:
Source: 32 MiB JPEG
StuffIt -> 27.3 MiB
S + L -> 15.1 MiB
It's entirely possible I failed to configure the compressor properly but thought I'd share it regardless in case anyone would like to test it for themselves.