The SmallFry metrics library is based on the optimizer jpeg SmallFry. The SmallFry metric is specially designed for the jpeg algorithm. With it, you can compress JPEG without obvious loss of quality. This works in a similar way to how JPEGmini works, with some borrowed metrics from its SPIE documents. This metric is used in jpeg-recompress. If you want a nice graphical interface and simplified automation of batch processing, take a look at JPEGmini.

Update

In addition to the SmallFry metric, this library contains the SharpenBad and NHW metrics, not jpeg-oriented, but more general. These metrics, though more labor-intensive than SmallFry, but well reflects both the visual quality of the image, and the "badness" of its further processing. The complexity of these metrics is due to the use of the Sharpen filter.

2022
zvezdochiot
https://github.com/zvezdochiot/libsmallfry